FALL 2023
VOLUME LIX

THE QUARTERLY JOURNAL OF THE GEMOLOGICAL INSTITUTE OF AMERICA

Study of Chromophores in Yellow Sapphire
Colors and Optical Effects of Oregon Sunstone
Analysis of Etch Pits in Ukrainian Beryl
Iolite from British Columbia

Fall 2023
VOLUME 59, No. 3

EDITORIAL
267
p. 284

Cause of Color in Yellow Sapphire, Plus Characterization of Oregon Sunstone,
Ukrainian Beryl, and British Columbian Iolite
Duncan Pay

FEATURE ARTICLES
268

Yellow Sapphire: Natural, Heat-Treated, Beryllium-Diffused, and Synthetic
John L. Emmett, Ungkhana Atikarnsakul, Jennifer Stone-Sundberg, andSupharart Sangsawong
Investigates the various chromophores responsible for color in four types of yellow sapphire.

298

Special Colors and Optical Effects of Oregon Sunstone:
Absorption, Scattering, Pleochroism, and Color Zoning
Shiyun Jin, Aaron C. Palke, Nathan D. Renfro, and Ziyin Sun
Details the colors and optical effects caused by metallic copper inclusions in this gemquality feldspar.

p. 320324

Etch Pits in Heliodor and Green Beryl from the Volyn Pegmatites,
Northwest Ukraine: A Diagnostic Feature
Gerhard Franz, Oleksii A. Vyshnevskyi, Volodymyr M. Khomenko, Peter Lyckberg, andUlrich Gernert
Examines dissolution features of heliodor and green beryl from a Ukrainian pegmatite field
and offers criteria to distinguish them from samples found in other localities.

340

Iolite from the Thor-Odin Dome, British Columbia, Canada:
Geology, Chemical Composition, Inclusions, and Cause of Chatoyancy
Philippe M. Belley
Reports on the geological setting and gemological properties of violet-blue gem cordierite
from new deposits located in British Columbia.

p. 348REGULAR FEATURES
322

2023 Challenge Winners

356

Lab Notes
Etch features reveal morphology of diamond • Diamond with unusual phosphorescence• Type IIa diamond with unusual red fluorescence distribution • CVD “etch” channel• CVD-grown diamond with few diagnostic features • Solid laboratory-grown singlecrystal diamond ring • Traditional Bombay pearl bunch • Flame-like surface structure innacreous atypical bead cultured pearl • Pearls in traditional Indian nose rings • Nebulalike inclusion in ruby beryllium-diffused to heal fractures • Translucent ruby filled withzinc glass

p. 364370

380

Diamond Reflections
Explores eclogitic diamonds, which make up a significant portion of the gem diamondmarket, and what they reveal about Earth’s dynamic processes.

p. 380388

p. 407

G&G Micro-World
Breyite in diamond • Spray of columbite crystals in topaz • Inclusion resembling a gadain diamond • Iridescent inclusion in brown diamond • Diamond with twinning wisps
resembling a whale • Red herring in red garnet • Heliodor with large schorl inclusion• Rainy lakeside city in peridot • Zigzag fingerprint in Sri Lankan sapphire • Fissure withmoiré pattern in spinel • Large stellate dislocation in spinel • Tourmaline in emerald• Turquoise planet Earth • Quarterly Crystal: Dravite in fluorapatite

p. 378

Gem News International
Burmese “chameleon” amber • Iridescent andradite from Inner Mongolia • Zigzag growthline inclusions in aquamarine • Blackish green omphacite jade from Guatemala • Shell
blister on Isognomon isognomon shell • Basalt-related gems from southeastern Vietnam• Survey of colored stone preferences • Mantle eclogite xenolith with inclusion-bearing
diamond • Environmental, social, and governance assessment of marine pearl farming
operation • GIA Alumni Collective • Gübelin Gem Museum • Chicago Responsible
Jewelry Conference • Turquoise United conference • Erratum

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About the Cover“The Rose,” Darryl Alexander’s 52 ct sunstone carving, displays three distinct areas of color: reddish orange, yellow,
and green. Carved from rough uncovered in the Sunstone Butte mine in Harney County, Oregon, the piece exhibits theschiller effect, also known as aventurescence, caused by reflective copper platelets. Schiller and other optical effects ofOregon sunstone are explored in a study featured in this issue. Photo by Robert Weldon; courtesy of Darryl Alexander.
Printing is by L+L Printers, Carlsbad, CA.
GIA World Headquarters The Robert Mouawad Campus 5345 Armada Drive Carlsbad, CA 92008 USA© 2023 Gemological Institute of America
All rights reserved.
ISSN 0016-626X

Cause of Color in Yellow Sapphire,
Plus Characterization of Oregon Sunstone,
Ukrainian Beryl, and British Columbian Iolite
Welcome to the Fall 2023 issue of Gems & Gemology! This issue delivers four feature articles
covering a range of colored stone topics, from a color study of yellow sapphire to a new source ofviolet-blue iolite in Canada.
In our lead article, Dr. John Emmett and coauthors investigate the various chromophores
responsible for color in four types of yellow sapphire—natural, heat-treated, beryllium-diffused,and synthetic. Thesechromophores include Fe3+, “…investigate the various chromophores responsiblea trapped hole associatedfor color in four types of yellow sapphire—natural,with iron (h•-Fe3+), and aheat-treated, beryllium-diffused, and synthetic.”
combination of the two, allof which are involved in the coloration of natural and treated yellow sapphire. Synthetic yellow
sapphire, on the other hand, is usually colored by the Ni3+ chromophore or by a combination of Ni3+ and Cr3+.
Next, Dr. Shiyun Jin and colleagues share their research on the special colors and optical effects caused by metallic copper
inclusions in Oregon sunstone. Using samples from different mines in Oregon, they explore absorption and scattering,pleochroism, and color zoning in this gem-quality feldspar known for its wide range of beautiful colors.
A team led by Dr. Gerhard Franz reports on the morphological characteristics of green beryl and the yellow beryl variety
heliodor from the Volyn pegmatite field in Ukraine. The authors observe an abundance of characteristic etch pits
throughout the samples and offer details on distinguishing Volyn green beryl and heliodor from those found in otherlocalities.
Dr. Philippe Belley contributes our final feature article, an examination of violet-blue iolite from new deposits in the ThorOdin dome in British Columbia, Canada. Various samples are analyzed to reveal the nature of their chemical composition,mineral and fluid inclusions, and chatoyancy. The abundance of cordierite-rich rocks in the Thor-Odin dome indicatespotential for future iolite discoveries in the area.
G&G’s regular columns contain exciting gemological findings from all over the world. Lab Notes details recent noteworthy
submissions to GIA’s laboratories, including a 4.04 ct ring fashioned from a single-crystal CVD-grown diamond, anacreous atypical bead cultured pearl featuring a flame-like surface structure, and a heart-shaped ruby filled with zinc glass.
Observations of fascinating internal features are captured in the Micro-World section, including a rare iridescent ferropericlase in brown diamond, bladed columbite crystals in topaz, and zigzag-patterned fingerprints in blue sapphire. Diamond
Reflections returns in this issue, focusing on diamonds formed within eclogitic mantle host rocks. Finally, Gem NewsInternational offers brief studies of Burmese chameleon amber, Guatemalan omphacite jade, and other gem materials, aswell as coverage of the recent Turquoise United conference and the Chicago Responsible Jewelry Conference.
Congratulations to this year’s G&G Challenge participants who received a perfect score! See a list of the top scorers on
page 322. Thank you to those who put their knowledge to the test.
We hope you enjoy the latest edition of Gems & Gemology!

Duncan Pay | Editor-in-Chief | dpay@gia.edu

EDITORIALGEMS & GEMOLOGYFALL 2023

267

FEATURE ARTICLESYELLOW SAPPHIRE: NATURAL, HEAT-TREATED,BERYLLIUM-DIFFUSED, AND SYNTHETICJohn L. Emmett, Ungkhana Atikarnsakul, Jennifer Stone-Sundberg, and Supharart Sangsawong

Natural yellow sapphires are colored by one of two entirely different chromophores or by a combination of thetwo. These two chromophores are iron (Fe3+) and a trapped hole paired with iron (h•-Fe3+). The color saturation of
the Fe3+ chromophore, as previously documented, does not linearly depend on its areal density, unlike the otherfive chromophores in natural corundum. It exhibits a complex dependence on areal density, a relationship that is
explored here. Low-iron-content natural yellow sapphires are colored solely by the h•-Fe3+ chromophore. The yellow
sapphires from Sri Lanka are colored in this way. Some of the basalt-hosted high-iron sapphires from Australia andThailand are colored by h•-Fe3+ in addition to Fe3+. Natural sapphires that are colorless or weakly yellow often
develop strong yellow coloration via heat treatment. They depend on formation of the h•-Fe3+ chromophore for thatchange. In these natural stones that respond to heat treatment, one of two different internal chemistries is present,
which must be altered to bring about the color enhancement. These two chemistries, which require two different
heat treatment processes, are presented. The diffusion of beryllium into various types of sapphire can shift theirchemistry from donor- to acceptor-dominated, forming the trapped hole, h•, which pairs with iron to produce theintense yellow coloration. Although crystals with the h•-Fe3+ chromophore were grown as a part of our study, synthetic yellow sapphires are not often colored by the same chromophores of natural sapphires. They are usually colored by the Ni3+ chromophore or by Ni3+ and Cr3+. Somewhat surprisingly, the color saturation of Czochralski-grownnickel-doped sapphire can sometimes be enhanced by heat treatment.

T

he complexities of yellow sapphire have been
discussed in some detail for many years (e.g.,
Schmetzer et al., 1983; Nassau and Valente,1987). A clear understanding of the nature of themultiple types of yellow sapphire was inhibited by
the lack of understanding of the individual chromophores and their individual absorption spectra.
The chromophores that cause yellow color in natural
corundum have been recently elucidated (Emmett etal., 2017a; Dubinsky et al., 2020), allowing the moredetailed study of yellow sapphire and its heat treatment, presented here. This work intends to provide
a clear basis for future study of yellow sapphire andits individual color stabilities.
Yellow sapphires are found in a wide range of deposits worldwide. They are colored by two entirely
different chromophores: the iron (Fe3+) chromophore

See end of article for About the Authors and Acknowledgments.
GEMS & GEMOLOGY, Vol. 59, No. 3, pp. 268–297,
http://dx.doi.org/10.5741/GEMS.59.3.268© 2023 Gemological Institute of America

268

CHROMOPHORES IN YELLOW SAPPHIRE

and the trapped hole paired with iron (h•-Fe3+)1 (figure1). These two chromophores represent the two extremes of natural chromophore strength in corundum. In faceted stones up to several carats, a few
thousand ppma of Fe3+ are required for good coloration, while only a few ppma of h•-Fe3+ produce
similar levels of color saturation (Dubinsky et al.,2020). It is not unusual for high-iron yellow sapphires
from basalt-hosted deposits to be colored by both
chromophores. In the past, the combination of thesetwo chromophores in the same stone at dramaticallydifferent concentrations has impeded a complete understanding of the cause of color.
The h•-Fe3+ chromophore arises in natural stones
that are acceptor dominated, meaning the divalentcation concentration exceeds the sum of the tetravalent cation concentration and the excess divalentcation concentration is not charge compensated by

The symbol h• is the symbol for an electron hole in Kröger-Vink
notation, and h•-Fe3+ means that the hole has paired with an Fe3+
ion. See box B.
1

GEMS & GEMOLOGYFALL 2023

Figure 1. These exceptional natural yellow
sapphires from Madagascar (2.1–2.6 ct) represent
both the Fe3+ chromophore and the trappedhole paired with Fe3+.
Photo by Ronnakorn
Manorotkul/Lotus
Gemology; courtesy of
GemFever.

H+ or oxygen vacancies. Because heating in an oxygen atmosphere can diffuse hydrogen out of the stoneor oxidize the oxygen vacancies, colorless or weakly
yellow stones can often be heat treated to create richyellow gems.
The total amount of iron in natural corundum
ranges from a few ppma to nearly 5000 ppma. Iron innatural corundum can exist in two valence states: Fe2+
and Fe3+. The cation site in corundum is trivalent

In Brief• Natural yellow sapphires are colored by the iron (Fe3+)
chromophore or the trapped hole paired with iron(h•-Fe3+) chromophore, or a combination of the two.
• Yellow coloration in colorless or weakly yellow sapphire can be developed via heat treatment in an oxygen atmosphere to form the h•-Fe3+ chromophore
under two different conditions, depending on if thestone contains hydrogen or not.

the crystal must remain electrically neutral. Oxygen
vacancies and H+ only exist in natural corundum at
low concentrations (a few ppma). Thus, any significant amount of Fe2+ can only exist if it is paired with
Ti4+ or Si4+. Concentrations of Ti4+ and Si4+ rarely exceed a few hundred ppma in natural corundum, and
it is unclear how much of that is actually in solution
in the lattice. Some portion of the Si4+ and Ti4+ will
also have to preferentially charge compensate Mg2+ if
present, as its level lies below that of Fe2+ in the bandgap. Only Fe2+ charge compensated by H+ ions or byoxygen vacancies would be expected to exhibit an Fe2+
spectrum. Such spectra are expected to be in the nearinfrared region and be very weak. If charge compensated by Ti4+ or Si4+, extremely strong pair spectra will
result (e.g., the Fe2+-Ti4+ pair resulting in the blue colorof sapphire).

MATERIALS AND METHODS(Al3+). When iron alone occupies this site, it will be
primarily trivalent. For Fe2+ to exist in natural corundum, it must be charge compensated by a donor suchas Si4+, Ti4+, H+, or one-half of an oxygen vacancy, as

The corundum samples in this study represent manydifferent deposits around the world and were chosenfrom the Crystal Chemistry collection and GIA’s colored stone reference collection in Bangkok. Samples
with three- and four-digit designations are from the
Crystal Chemistry collection, while those with nineto twelve-digit designations are from GIA’s coloredstone reference collection. Samples used to characterize the Fe3+ and h•-Fe3+ chromophores originated from
Myanmar, Thailand, Cambodia, Sri Lanka, Australia,
Nigeria, Cameroon, Rwanda, Nigeria, Kenya, and theUnited States (Montana). Additionally, several synthetic crystals were grown using the Czochralski
method by Saint-Gobain Crystals and Detectors andby Teledyne FLIR Scientific Materials. A yellow syn-

CHROMOPHORES IN YELLOW SAPPHIREGEMS & GEMOLOGY• The addition of beryllium by diffusion in an oxidizing
atmosphere can shift internal chemistry from donor- to
acceptor-dominated, resulting in the formation of additional trapped holes (h•) to pair with available Fe3+ andthus a stronger yellow coloration.
• Unlike natural stones, synthetic yellow sapphires areusually colored by the Ni3+ chromophore or by Ni3+
and Cr3+.

FALL 2023

269

thetic nickel-doped sapphire grown using theVerneuil method was purchased from RusGems in
Bangkok. All wafer samples were crystallographicallyoriented using a crystal alignment instrument withthe c-axis either perpendicular or parallel to the planeof the wafer (Thomas et al., 2014). Trace element concentrations were determined either by laser ablation–inductively coupled plasma–mass spectrometry(LA-ICP-MS) using the current standards at GIA in
Bangkok (Stone-Sundberg et al., 2017), by secondary
ion mass spectrometry (SIMS) against ion implant
standards at Caltech (Stone-Sundberg et al., 2020), or
by both. In addition, ultraviolet/visible/near-infrared(UV-Vis-NIR) spectra of 102 of these wafer samples
were measured, from which 24 were chosen for thisstudy of the 450 nm Fe3+ band, as they represent equal
concentration intervals of iron from most to least
concentrated. Because the trace element concentrations in natural sapphires are often not uniform, the24 samples used for the study of the Fe3+ chromophore
were measured at four points on each side of the sample within the optical aperture of the sample plate.
We required that the standard deviation of these eightdata points be less than 6%. These sample selection
criteria greatly minimized spatial concentration variations. The iron concentration in these samples covered the 56.5–4730 ppma region. The 450 nm Fe3+
absorption cross section2 was too weak to accurately
fit the curve below this lower value.
In addition, large samples (25–50 g each) of roughstones in the 3.5–7.0 mm range were used for many
individual heat treatment experiments to test for acceptor domination before spectroscopic samples werechosen for these types of material. These stones werefrom the Rock Creek, Dry Cottonwood Creek, and Eldorado Bar deposits in Montana; Sri Lanka; theChimwadzulu Hill deposit in Malawi; several Madagascar deposits; the Kings Plain deposit in New South
Wales, Australia; and the Songea deposit in Tanzania.
Optical absorption measurements were obtainedwith Hitachi 2900 and 2910 spectrophotometers at
Crystal Chemistry and with a Hitachi 2910 at GIA
in Bangkok. The Hitachi 2910 units were reengineered by Tim Thomas (Applied Materials, Portland,
Oregon) to have a rotatable polarized optical beam sothat both E⊥c and E||c spectra could be measured on
wafers with the c-axis in the plane of the wafer.
These instruments measure the spectra to a resolu-

tion of 1.5 nm. The optical absorption data was corrected for the multiple reflections between the two
optical surfaces of the sample wafers (see box A).
Fourier-transform infrared (FTIR) spectra were
measured using Thermo Nicolet 6700 and iS50 FTIR
spectrometers operating with a Pike 4× beam condenser accessory at a resolution of 2 cm–1. The FTIR
spectrometer was also connected to an air compressor
with a purge gas generator (Parker Balston model 7552) to minimize water and carbon dioxide contamination of the sample and the air in the sample chamber,as these compounds absorb light and obscure spectral
peaks of interest. This helps to minimize moistureand carbon dioxide features, improve signal-to-noise
ratio, and achieve a flat baseline. Spectra were collected from each sample through a hole of size chosen
to accommodate a particular sample in the aluminum
sample plate from the same location where the opticalabsorption spectra were collected.
The low-temperature heat treatment experimentsof wafer samples in air were conducted at GIA in
Bangkok using a Thermolyne FB1400 benchtop
1100°C muffle furnace manufactured by Thermo Scientific. The high-temperature (>1100°C) experiments
in controlled atmospheres were conducted by Crystal
Chemistry and by Columbia Gem House (Vancouver,
Washington) with modified Thermal Technology
1000A graphite furnaces, using Coors Tek 998 alumina muffle tubes to allow the use of carefully controlled atmospheres.
The beryllium diffusion experiments were conducted at Crystal Chemistry in the Thermal Technology furnace described above. Diffusion experiments
were run at 1780° or 1800°C in pure oxygen for 33
hours. This procedure is discussed at length in Emmett et al. (2003).
Color coordinates (Berns, 2000) were calculated
from the sample’s transmission spectrum and thecharacteristics of the light source. Calculations of thecolor coordinates for a given color temperature were
performed via Thermo Scientific’s GRAMS/AI spectroscopy software, a general-purpose spectra manipulation code that includes color coordinate
calculations. Among the many color coordinate systems, the CIE 1976 L*a*b* system was chosen for its
approximately uniform color space, and we selecteda color temperature of 5000K.

RESULTS2

The terms absorbance, absorption coefficient, absorption cross section, ion density, and areal defect density are defined in box A.

270

CHROMOPHORES IN YELLOW SAPPHIRE

The Fe3+ Chromophore. Fe3+ exists in essentially all
natural sapphire. The resulting stones can be color-

GEMS & GEMOLOGYFALL 2023

BOX A: PRESENTING DATA
It is perhaps unfortunate, but like many other measurements that have been devised, there are multiple sets ofunits used to quantify the optical absorption features of
gemstones, minerals, and other materials.

Thus, data presented in the literature in units of A are
unambiguous.

Absorption Coefficient
Two different absorption coefficients are currently present in the literature. As a very general statement, physicists and materials scientists studying the absorption
strength of ions in crystals and glasses tend to use:

I0

I

I/I0 = 10 –A = 10 – ad

Figure A-1. Illustration of incident light being transmitted through a sample of a given thickness.

Assuming zero reflection losses from the two sample
surfaces in figure A-1, the Beer-Lambert law can be expressed using several different units:(A-1)

Where:
I0 = incident light intensity
I = transmitted light intensity
T = transmission in percent
A = decadic true absorbance as measured by thespectrophotometer (a unitless ratio)=
decadic
absorption coefficient in cm–1ad = sample thickness in cm
ε = molar absorptivity in liter mole–1 cm–1, also called
the extinction coefficientc = chromophore concentration in moles per liter
α = naperian true absorption coefficient in cm–1
C = chromophore concentration in absorbers per cm3
σ = true absorption cross section in cm2
e = base of the natural logarithm = 2.71828…

Current commercial spectroscopic instruments measure
the absorbance, A, in decadic units defined as:

CHROMOPHORES IN YELLOW SAPPHIRE

(A-2)

(A-4)

These two representations are very simply related by:
α = 2.303a(A-5)

While we have used a and α to clearly show the difference, α is often used in the literature for either absorption coefficient.
With the rise of multidisciplinary sciences and therapid spread of materials science, the usage has become
very mixed. In fact, one can often find both conventions
in papers presented at the same technical meeting. It is
important to note that just because the term absorption
coefficient is used with the symbol α, it is not clear
which system is in use unless the simple equation isstated. Therefore, it is very important for scientists topresent the simple equation that defines their units inboth published papers and presentations.

Absorption Cross Section and Extinction Coefficient
Now suppose we were to compare different chromophores in corundum, or the same chromophore in different hosts, in terms of their absorption strength andthe depth of color they produce. If we divide the absorption coefficient by the concentration, C, of the chromophore, we can define an absorption cross section
spectrum, σ, which is independent of concentration or
sample thickness, as shown in equation A-1. The absorption cross section is calculated from α or A as:
σ = α/C = 2.303A/dC

Absorbance

I/I0 = 10 –A(A-3)

This is the set of units we have used in this article andin Emmett et al. (2017a) and Dubinsky et al. (2020). It isalso the system of units used in GIA’s colored stone reference collection database. Chemists and mineralogists(e.g., Rossman, 1988) have generally expressed absorption coefficient as:dI= T =10 –A =10 – ad =10 –εcd =e –αd =e – σCd
I0 100

I/I0 = 10 –A = e –αd(A-6)

The absorption cross section spectrum depends only
on the characteristics of an individual chromophore ion(e.g., Cr3+) and the particular host (i.e., corundum), and it
is measured in square centimeters (cm2). The unit ofsquare centimeters denotes an area, and one can think

GEMS & GEMOLOGYFALL 2023

271

of the cross section as the “area” of an individual ion as
seen by the photons in a light beam. However, it does
not represent the physical size of the ion. This absorption cross section is used for comparisons in this article
and in Dubinsky et al. (2020).
In chemistry and mineralogy, epsilon (ε) is often used
for the extinction coefficient, which is the same concept
but in a different set of units. From equation A-1, we have:A = εcd(A-7)

As shown above, the units of ε are liter mole–1 cm–1. The
extinction coefficient for corundum is easily calculated
from σ by the following from equation A-1:
10 –εcd = e –σCd

(A-8)

Thus:

and simplest but takes up more space on the graph. Unit
3 is very common in the scientific literature, but it is formally incorrect since the numbers shown on the plot are
0 to 4. As unit 2 is the clearest, we have chosen to use it
instead of unit 1 in this article with the hope of eliminating ambiguity.

Concentration Units
Generally, there are two major ion concentration units
in the literature: ions/cm3 and moles/liter. The literatureoften uses ppma, which is ions/cm3 divided by 106 (1ppma in corundum is 1.178 × 1017 ions/cm3). Moles/literis easily converted to ions/cm3. One mole/liter equals
Avogadro’s number divided by 1000 cm3, or 6.0221 × 1020
ions/cm3. For corundum, 1 mole/liter equals 5111 ppma.

Areal Density
ε = (2.615 × 10 )σ
20

(A-9)

Figure A-2 compares the numerical values of the extinction coefficient and the absorption cross section for
the Cr3+ ion in corundum. The term extinction coefficient is also called the molar absorptivity in some publications, but the units are the same.

Units Used on Plots
A variety of descriptions are used on spectra plots when
the axis values are far larger or smaller than can be easily
represented without exponents. In figure A-2, the valuesplotted on the vertical axis range from 0 to 4 × 10–19 cm2.
There are several ways in which these units have been
represented in the literature:
1. Absorption Cross Section × 1019 cm2
2. Absorption Cross Section in units of 10–19 cm2
3 Absorption Cross Section × 10–19 cm2

The areal density (ρA) is simply the product of the ion
concentration, in ions/cm3 times the sample thickness,d, in cm. Thus, the units are ions/cm2. Under the color
circles in this paper, we present the areal density in
ppma-cm. This factor is:
ppma-cm = Cd/(1.178 × 1017 ions/cm3)(A-10)

where 1.178 × 1017 is the number of ions in 1 cubic centimeter of Al2O3 determined by:
1 ppma = 5Aρ/M = 1.178 × 1017 ions/cm3(A-11)

where A is Avogadro’s number (6.022 × 1023
molecules/mole), ρ is the density of corundum (taken as
3.99 grams/cm3), and M is the molecular weight ofcorundum (101.961 grams/mole).

Correction for Surface Reflection Losses

In previous publications we have used unit 1, whichis quantitatively correct but confusing to readers unfamiliar with exponential notation (Emmett et al., 2017a;
Dubinsky et al., 2020). Unit 2 is probably the clearest

To determine the true absorbance A as defined in equation A-1 above, the instrument-measured absorbance,
must first be corrected for the loss from multiple reflections between the two polished surfaces of the sample.
This is done by summing all the reflections between thetwo surfaces (T.S. Hemphill, pers. comm., 2011) andless, yellow, green, or blue, depending on the relative
concentrations of the various trace elements present.
This study examined 102 samples, from which 24 of
the 102 data sets were chosen to represent the entire
set based upon their evenly spaced representation of
the iron concentrations for the 102 samples. Yellow
sapphires colored primarily by iron are often foundin basalt-hosted blue sapphire deposits. A single crystal will often contain both deep blue and deep yellow

regions in varying proportions. Although bicolorstones are occasionally faceted, these will usually be
cut as either blue or yellow gems, depending on the
predominant color. Several of the high-iron samples
from this study were bicolor stones containing welldefined blue and yellow regions. These samples were
fabricated with their planes either perpendicular orparallel to the c-axis, depending on which orientation
most clearly separated the blue and yellow regions.

272

CHROMOPHORES IN YELLOW SAPPHIREGEMS & GEMOLOGYFALL 2023

UV-VIS-NIR SPECTRA
4.0

3.5

90
80

3.0

70
2.5
60
2.0

50
40

1.5

30
1.0
20
0.5

0
200

EXTINCTION COEFFICIENT (liter mole –1 cm–1)

ABSORPTION CROSS SECTION IN UNITS OF 10 –19 cm2

100

E⊥c
Eǀǀc

Figure A-2. UV-Vis-NIR
plot of both the absorption cross section and theextinction coefficient fororiented spectra of
Cr 3+:Al2O3.

10

300

400

500

600

700

800

900

1000

1100

WAVELENGTH (nm)using a three-term Sellmeier equation for corundum to determine the sample’s refractive index as a function of wavelength (Tatian, 1984). The index input data to the Sellmeierequation is a combination of all relatively recent data fromthe suppliers of high-purity synthetic corundum material
used to fabricate high-purity optical elements. The Sellmeier equation fit to these data was performed for us byJohn Trenholme (formerly with Lawrence Livermore National Laboratory) using TableCurve 2D software. Correcting for this loss, we have the true absorbance, A. This
correction is important because it is greatest where the absorption is least, and it is just this region of the absorption

spectrum that transmits the most intense light and thus
contributes most to the color. This reflection correction
has been made for all spectra presented in this article andfor all spectra used in deriving these results. It is also important to remember that very strong absorption peaks,
if very narrow, will have little impact on color.
Since commercial instrumentation is now available
to measure the refractive index of flat samples at multiple wavelengths, it is suggested that these technologies be acquired by major gemological laboratories to
add to their databases and to provide for correction of
optical spectra of other gem minerals.

Figure 2 shows a selection of yellow wafers for spectroscopy. Figure 3 shows a selection of the bicolor
wafers.
Often in the study of corundum chromophores, aneffective procedure has been to grow a crystal containing only one chromophore. However, that procedure was not successful for Fe3+ because of
decomposition and vaporization of the iron oxides at
the Czochralski growth temperature. Thus, 23 of the

24 natural samples chosen for this study were selected from a wide range of deposits worldwide.
Figure 4 shows both the E⊥c and E||c absorption
spectra of sample 1245, from the Garba Tula depositin Kenya. This sample contains 3126 ppma iron.
There are three absorption bands from Fe3+ and Fe3+
pairs in the visible region of the spectrum (Ferguson
and Fielding, 1971, 1972; Krebs and Maisch, 1971) at
450, 530, and 700 nm. Of these, the 450 nm band is

CHROMOPHORES IN YELLOW SAPPHIREGEMS & GEMOLOGYFALL 2023

273

Figure 2. A selection of the 102 wafers studied for thisreport. The three pale yellow wafers are colored by
Fe3+ only at a level around 2000 ppma, while the morecolor-saturated wafer with a blue edge is colored by amuch higher concentration of Fe3+ (around 3500ppma). The deep yellow wafer is colored by Fe3+ andby h•-Fe3+. Photo by John L. Emmett.

Figure 3. A selection of bicolor wafers. The weakcolor saturation in the yellow areas of some samples
results from fabricating them thin enough that bothyellow and blue areas were well within the dynamic
range of the UV-Vis-NIR spectrophotometers. Photo
by John L. Emmett.

by far the dominant cause of color. Because it absorbs
in the blue region of the spectrum, the transmittedcolor is yellow. The 530 and 700 nm bands are broadand very weak. Taken together, this combination has
little impact on color. The 450 nm E||c band is nearly
identical to the E⊥c band, but the peak amplitude is

UV-VIS-NIR SPECTRA
20

E⊥c
Eǀǀc

ABSORPTION COEFFICIENT (cm–1)

18
16

Figure 4. UV-Vis-NIR
absorption coefficient
spectrum of sample
1245, a bicolor sapphirefrom Kenya’s Garba
Tula deposit. The spectrum is taken in the yellow region. The ironconcentration in the yellow region is 3126 ± 65
ppma, and the sample
thickness is 0.99 mm.

14
12
10
8
6
4
2
0
200

300

400

500

600

700

800

900

1000

1100

WAVELENGTH (nm)274

CHROMOPHORES IN YELLOW SAPPHIREGEMS & GEMOLOGYFALL 2023

ABSORPTION CROSS SECTION IN UNITS OF 10 –20 cm2

UV-VIS SPECTRA
10
9

E⊥c
4730 ppma
2960 ppma1320 ppma
580 ppma

8
7Figure 5. UV-Visible absorption cross section
spectra. The E⊥c absorption cross section of
the 450 nm absorption
band of Fe3+ in corundum increases in amplitude and in width(FWHM) with increasing concentration (Dubinsky et al., 2020).

6
5
4
3
2
1
0
300

350

400

450

500

550

600

650

700

WAVELENGTH (nm)about 16% less. Thus, we have focused our study ofthe iron chromophore on the 450 nm E⊥c band.
In the course of previous work on the Fe3+ chromophore (Dubinsky et al., 2020), two unique features
were observed with the 450 nm absorption band,
which is the primary determinant of color. First, thepeak absorption cross section increases with ironconcentration; second, the full width at half maximum (FWHM) of the band increases by about a factorof two from the lowest to the highest iron concentrations (figure 5; see also figure 7 of Dubinsky et al.,2020). Such increases in both peak absorption crosssection and width of the 450 nm absorption band
more than linearly increase the strength of the chromophore with concentration. Other chromophores
in natural corundum do not show these changes andthus exhibit a constant strength with concentration.
The concentration of Fe3+ in natural corundum
ranges from <5 ppma to at least 4730 ppma. Since the450 nm Fe3+ absorption cross section is low (~10–20
cm2), optical measurements were limited to samples
whose concentration was above 50 ppma. For example, the peak absorption coefficient at 450 nm ofsample 1272 with 79.2 ppma and a thickness of 3.415
mm is 0.042 cm–1. The peak absorbance as measured
by the spectrophotometer is therefore 0.0062. At 10–
20% of this peak value we see instrumental noiseproblems. Thus, spectrophotometer noise and sample thickness determined the minimum concentration used. Therefore, for this study we were limited

to using samples with Fe3+ concentrations that
ranged from 56.5 to 4730 ppma.
Of the samples used for this study, 24 were chosen,covering the range from 56.5 to 4730 ppma iron for theiron chromophore data presented here. The E⊥c spectra of these samples were accurately curve fit to makethe best determination of the 450 nm band magnitude
and shape. Curve fitting was performed withGRAMS/AI (Thermo Scientific) using a linear baseline
plus four Lorentzian bands (as suggested by Dr. Ren
Lu of the Gemmological Institute, China Universityof Geosciences in Wuhan). The usual variety of otherband shapes were evaluated for the quality of the fit,
and the Lorentzian bands produced the best match for
four bands or fewer.
Determining the baseline for the 450 nm E⊥c bandpresented some difficulty, as it was overlapped tosome degree by the 388 nm Fe3+ band and by the 580
nm E⊥c Fe2+-Ti4+ band in the blue sapphires. Because
these interferences vary widely in magnitude
throughout the large number of samples used, it wasnecessary to modify the spectra of some of the deep
blue sapphire samples so that the 450 nm band did
not fall on a steeply rising portion of the Fe2+-Ti4+band. The modification was achieved by subtracting
successively larger portions of an Fe2+-Ti4+ spectrum
from the sample spectrum until the minima on eitherside of the 450 nm peak were of the same amplitude.
The spectrum used for this subtraction is the absorption cross section determined for the Fe2+-Ti4+ pair

CHROMOPHORES IN YELLOW SAPPHIREGEMS & GEMOLOGYFALL 2023

275

VIS SPECTRA
0.30

ABSORPTION COEFFICIENT (cm–1)

E⊥cExp. data final baseline
Fitted peak baselined
Peak 1
Peak 2
Peak 3
Peak 40.25

0.20

Figure 6. Visible absorption spectra of sample
1017 from Myanmar,
containing 447 ppma
iron. The experimental
data of the 450 nm E⊥cpeak is accurately fitted
by four Lorentzian
curves. This provides anaccurate measure of theabsorption band interms of peak absorption coefficient, FWHM,
and band shape.

0.15

0.10

0.05

0
400

410

420

430

440

450

460

470

480

490

500

WAVELENGTH (nm)(Dubinsky et al., 2020), multiplied by appropriate concentrations.
Even though the 450 nm band is quite narrow,the use of a linear baseline creates some error because the background decreases toward the 450 nm
peak from longer wavelengths and shorter wave-

lengths. This error is estimated as ≤10% of the peak
amplitude. A typical curve fit is shown in figure 6
for a sample containing 447 ppma iron. Note thetypical quality of the fit by comparing the experimental data final baseline with the fitted peak baselined in figure 6.

ABSORPTION CROSS SECTION IN UNITS OF 10 –20 cm2

PEAK A BSORPTION CROSS SECTION OF THE Fe3+ 450 nm BAND
10

E⊥c

Figure 7A. Peak absorption cross section of the
Fe3+ 450 nm band, withiron concentration in
ppma. The plot shows
the increase in absorbance cross section
when Fe3+ concentration
increases beyond approximately 700 ppma.
The red data point identifies the only synthetic
sample in this plot.

1

0.1
10

100

1000

10000

Fe CONCENTRATION (ppma)

276

CHROMOPHORES IN YELLOW SAPPHIREGEMS & GEMOLOGYFALL 2023

PEAK A BSORPTION COEFFICIENT OF THE Fe3+ 450 nm BAND
100

E⊥c

ABSORPTION COEFFICIENT (cm–1)

10

Figure 7B. The absorption coefficient is plotted as a function of theiron concentration. The
red data point identifies
the only synthetic sample in this plot.

1

0.1

0.01
10

100

1000

10000

Fe CONCENTRATION (ppma)

Figure 7A shows the 450 nm Fe3+ band peak absorption cross section data as a function of iron concentration of the 24 curve-fit samples.
From this plot, we can see that the absorption
cross section is essentially constant as the iron concentration increases from low values up to ~700
ppma. Above 700 ppma, it increases to a value at
4730 ppma iron, which is about 4.5 times the low
concentration value. The slope of the curve over theregion from 900 to 4730 ppma iron is approximatedby the proportionality given in equation 1, where σis the absorption cross section:σ ∝ [Fe] 0.75 ± 0.08

CHROMOPHORES IN YELLOW SAPPHIRE(1)

In figure 7A, the lowest concentration measured
was of a synthetic iron-doped crystal grown by SaintGobain Crystals and Detectors. The cross section ofthis sample (shown in red) is noticeably smaller thanthat of the three natural samples containing less than
100 ppma iron. This may imply that the clusteringof iron in corundum is dependent on the thermal history of the sample.
The data of figure 7A could also be presented as
the 450 nm peak absorption coefficient versus the
Fe3+ concentration. This allows an estimate of the
Fe3+ concentration if the peak absorption coefficient
has been measured, or vice versa. The plotting of datain this fashion is shown in figure 7B.

GEMS & GEMOLOGYFALL 2023

277

450 nm Fe3+ BAND SHAPE COMPARISON FOR MIN AND M AX CONCENTRATIONS
1.1

NORMALIZED ABSORPTION COEFFICIENT

1.0

E⊥c
Sample 1121 (4730 ppma)
Sample 947 (56.5 ppma)

0.9
0.8

Figure 8. The E⊥c 450
nm Fe3+ absorption
coefficient plots show
the band shapes for the
sample with the lowest
Fe3+ concentration andthe sample with thehighest Fe3+ concentration studied. Peak values
are normalized to 1.0.

0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
400

410

420

430

440

450

460

470

480

490

500

WAVELENGTH (nm)As indicated earlier, in addition to the absorption
cross section increase with concentration, the
FWHM of the 450 nm peak also increases substantially with concentration. The strength of the Fe3+
chromophore is determined by both the peak absorption cross section and the FWHM of the 450 nm
band. Figure 8 compares the curve shape of the lowest- and highest-concentration samples. The peakvalue of each spectrum is normalized to 1.0.
As shown in figure 8, the peak of the low-Fe3+
sample is at 449.5 nm while that of the high-Fe3+
sample is at 451.5 nm, a shift of about 2 nm. The
shape of the band on the left side (short-wavelengthside) at the half maximum wavelength doesn'tchange with iron concentration. However, the rightside (long-wavelength side) of the peak changes significantly. The position of the half maximum increases significantly with concentration from about
453 to about 466 nm, resulting in a doubling of the
FWHM from about 13 nm to 26 nm going from 56.5
ppma to 4730 ppma iron.
The shifting of the long-wavelength half maximum to longer wavelengths (lower energies) withconcentration is an indication of the energy per ion
reduction as a result of pairing or more complex cluster formation.
Figure 9 shows the normalized spectra as in figure
8, but with eight of the 24 curve-fit samples pre-278

CHROMOPHORES IN YELLOW SAPPHIRE

sented. Note that the half maximum on the shortwavelength side of the peak is nearly constant at 440± 0.7 nm. Expanding the region of the graph aroundthe short-wavelength half maximum shows that the
spread is not systematic with concentration, but apparently nearly random. Thus, much of the spread isprobably a result of slightly different calibrations of
the three Hitachi spectrophotometers over several
years. The two traces in figure 8 (which represent the
Fe3+ concentration extremes) were measured on thesame instrument by the same operator. The shortwavelength half maximum points are at 439.3 and440.0 nm, a difference of only 0.7 nm.
The long-wavelength half maximum increases
from 453.5 nm at the lowest concentration to 467.2
nm at the highest. With this data on all 24 curve-fit
samples, we can plot a curve, shown in figure 10, of
the long-wavelength half maximum (left verticalaxis) as a function of Fe3+ concentration. By taking
the short-wavelength half maximum (right verticalaxis) as 440 nm, figure 10 also presents the FWHM.
With the quantitative data presented for the amplitude and width of the 450 nm band of Fe3+, we cannow examine the dependence of the color on theconcentration produced by this chromophore. The
color produced by most corundum chromophores
depends solely on the areal density (ρA, in ions percm2) of the chromophore. The areal density is simply

GEMS & GEMOLOGYFALL 2023

450 nm Fe3+ BANDWIDTH COMPARISONS
1.1

E⊥c

NORMALIZED ABSORPTION COEFFICIENT

1.0

56.5 ppma
194 ppma
447 ppma
802 ppma1020 ppma
1596 ppma2482 ppma
3800 ppma
4730 ppma

0.9
0.8
0.7

Figure 9. This visible
absorption coefficientplot shows eight of the
24 curve-fit E⊥c band
shapes covering therange from 56.5 to 4730
ppma Fe3+. Peak values
are normalized to 1.0.

0.6
0.5
0.4
0.3
0.2
0.1
0
400

410

420

430

440

450

460

470

480

490

500

WAVELENGTH (nm)the product of the chromophore concentration (C,in ions per cm3) and the sample’s path length (d, incm). The areal density is the number of chromophore ions in the sample per cm2 as measured by

470

30

468

28

466

26

464

24

462

22

460

20

458

18

456

16

454

14

452

12

450
10

100

FWHM (nm)WAVELENGTH (nm)LONG-WAVELENGTH HALF M AXIMUM LEFT, FWHM RIGHT, OF THE 450 nm BAND

Figure 10. This plot
presents the long-wavelength half maximum
on the left axis and the
FWHM on the right axis
as a function of Fe3+
concentration for the
450 nm E⊥c band.

10
10000

1000

Fe CONCENTRATION (ppma)

CHROMOPHORES IN YELLOW SAPPHIREGEMS & GEMOLOGYFALL 2023

279

Sample no.:

1059

949

1121

Origin:Malawi

Kenya

Subera, Australia

Sample thickness:

0.095 cm

0.157 cm

0.141 cm

Figure 11. These threecolor circles show theincrease in chromophore strength with Fe3+
concentration at constant areal density.
Fe 3+ concentration (C ):1020 ppma

2960 ppma

Scaled thickness (d ):0.652 cm

0.225 cm

0.141 cm

Areal density ( ρA ):665 ppma-cm

665 ppma-cm

665 ppma-cm

a light beam traversing the sample, as shown inequation 2:
ρA = Cd(2)

For the chromium chromophore, a sample with
500 ppma Cr3+ and a thickness of 1 cm has the sameabsorbance and color as a sample with 1000 ppma
and a thickness of 0.5 cm, because it has the same
areal density. The Fe3+ chromophore does not share
this simple relationship because, as shown in figure
7A, the absorption cross section is not independentof concentration.
In addition to the peak cross section of the 450 nm
Fe3+ band being dependent on concentration, theband’s FWHM increases with concentration (see figure10). Thus, the magnitude and shape of the bandchange with concentration. Since the strength of the
Fe3+ chromophore depends on both factors, it exhibits
a complex dependence on Fe3+ concentration. For
these reasons, a color calculation for this chromophore
requires both the concentration and the sample thickness. The concentration determines the peak crosssection and the band shape. A series of band shapes fora range of concentrations is presented in figure 9 andin appendix 1 (https://www.gia.edu/doc/fall-2023yellow-sapphire-chromophores-appendix1.pdf).
To illustrate these relationships, we present threecolor circles in figure 11. These three color circles,
calculated for a 5000K illumination and a 2° angle ofobservation, have the same areal density of 665
ppma-cm. For a chromophore such as Cr3+, all threecolor circles would be identical. As we have shown,however, the strength of the Fe3+ chromophore in-

280

CHROMOPHORES IN YELLOW SAPPHIRE

4730 ppma

creases with the density of Fe3+ ions in the sample,
even at constant areal density.
Sample 1121 in figure 11 has the highest Fe3+ density (4730 ppma) and a strong yellow coloration, eventhough it is only 0.141 cm thick. The areal densityof the sample is 665 ppma-cm. Sample 949 has alower density at 2960 ppma; to calculate the color,the thickness has been increased to 0.225 cm, producing a matching areal density of 665 ppma-cm.
Note that the color saturation is significantly reduced. Finally, sample 1059 has a relatively low ironconcentration of 1020 ppma. For the color circle, its
thickness has been increased to 0.652 cm to provide
a matching areal density of 665 ppma-cm. Again,
there is a significant decrease in color saturation. For
these reasons, strongly yellow sapphires colored only
by iron usually have iron concentrations above 3000
ppma.
As demonstrated above, there is no single curve
shape that characterizes the 450 nm Fe3+ absorption
band. Thus, the effect of iron concentration on thecolor of ruby or blue sapphire must be calculated separately for each concentration of either Cr3+ or Fe2+Ti4+, and Fe3+. The most straightforward calculation
would be to simply add in one of the curves presented in appendix 1 to either of the other two chromophores’ spectra for each iron concentration.
The weakest chromophore in natural corundumis Fe3+. Yet some very deeply colored yellow sapphires are found in basalt-hosted corundum deposits.
These deeply yellow sapphires are colored by the
combination of the Fe3+ chromophore and the h•-Fe3+
chromophore, and these will be discussed later.

GEMS & GEMOLOGYFALL 2023

UV-VIS-NIR SPECTRA

ABSORPTION CROSS SECTION IN UNITS OF 10 –17 cm2

3.0

E⊥c
Eǀǀc
2.5

2.0

Figure 12. Oriented UVVis-NIR absorption
cross section plots of
the h•-Fe3+ chromophorein sapphire (Dubinsky
et al., 2020).

1.5

1.0

0.5

0
200

300

400

500

600

700

800

900

1000

1100

WAVELENGTH (nm)The h•-Fe3+ Chromophore and Heat Treatment.3 Traceelements in the corundum lattice that have a valence
less than the 3+ of aluminum are termed acceptors,
while those with a valence of greater than 3+ are
termed donors (Smyth, 2000). If the sapphire is acceptor dominated, we have the following relationship:4[acceptors] – [donors] = [Mg2+ + Ni2+] – [Si4+ + Ti4+] > 0(3)where square brackets indicate concentrations in ppma
or ions/cm3. Since solids must be electrically neutral,
acceptor excess will be charge compensated by eitherhydrogen ions, oxygen vacancies (Vo2+), or trapped holes(h•) with a 1+ charge, as shown here:

While equations 3 and 4 look simple, determining
acceptor domination is not. Based on calibrated SIMS
measurements at the California Institute of Technology, Division of Geological and Planetary Sciences(Stone-Sundberg et al., 2017, 2020), we have learned
that silicon (Si4+), a donor, is about as common in natural corundum as magnesium (Emmett et al., 2017b).
Unfortunately, only a very small portion of the research samples at Crystal Chemistry and in GIA’s colored stone reference collection in Bangkok have been
analyzed for silicon by SIMS. The majority of oursamples have only been analyzed by quadrupole LA-

Whether in nature or in the laboratory, the elimination of the hydrogen ion and/or oxygen vacancies
that are a part of the charge compensation of the acceptor excess results in an increase of the trappedhole concentration to maintain charge neutrality.
The trapped hole in corundum is an oxygen ion witha charge of –1 rather than the normal –2. The O1– ion
paired with Fe3+ is the absorbing specie that creates
the yellow coloration, not the divalent ions (figure
12; see also Dubinsky et al., 2020).

As discussed in Dubinsky et al. (2020), h• will pair with either Cr3+ orFe3+. If both are present, it will preferentially pair with Cr3+, which lies
above Fe3+ in the band gap. The absorption spectra of h•-Cr3+ is very
different from the h•-Fe3+ spectra, even with the excess Cr3+ spectra
subtracted out. In this study, we discuss only stones with yellow coloration that contain Fe3+ but not Cr3+.
4
Nickel is included in these equations because it was discovered insignificant concentrations in various types of natural sapphire by author JS-S. For example, concentrations ranging from 1 to 15 ppma
were found in the blue sapphires from Yogo Gulch, Montana. The valence of nickel in natural corundum depends on whether the sum ofconcentrations of all aliovalent elements results in acceptor or donor
domination. Acceptor domination would make nickel 3+, while
donor domination would make it 2+. Nickel as a chromophore incorundum will be addressed later in this article.

CHROMOPHORES IN YELLOW SAPPHIREGEMS & GEMOLOGY[Mg2+ + Ni2+] – [Si4+ + Ti4+] – [H+ + 2Vo2+ + h+] = 0(4)

3

FALL 2023

281

BOX B: KRÖGER-VINK NOTATION

Figure 13. Sapphires from Rock Creek, Montana. The
wafers in the top row are cut perpendicular to the c-axis
and the wafers in the bottom row are cut parallel to thec-axis. These stones have been heat treated at 1200°C
in oxygen. Photo by John L. Emmett.

ICP-MS, which cannot accurately measure silicon incorundum due to close-in-mass interferences, andthus we cannot demonstrate acceptor domination.
Even with measurements for all four trace elements by SIMS, we often cannot determine whether
or not a sample is acceptor dominated. This is because the standard deviation (SD) of the acceptor excess is the square root of the sum of the squares of theSDs for the concentrations of each of the four traceelements: magnesium, nickel, titanium, and silicon.
Thus, the standard deviation of the acceptor excess
can be large and can exceed the value of [h•] itself,
making the determination of acceptor domination
impossible. It is also important to note that our chemical analyses are only of several spots on the surface
of fabricated wafer samples. We have learned from examination of many heat-treated yellow sapphires thatthe acceptor domination in a given sample is often
spatially nonuniform, and thus our analyses may nothave covered the acceptor-dominated region. Spatial
nonuniformity in natural corundum is more typicalof aliovalent ions than isovalent ions.
This spatial nonuniformity is typical of sapphires
from Madagascar, Malawi, and most of the Montana
deposits, as well as many of the stones from Sri Lanka.
Examples of the nonuniformity of Rock Creek stones
are given in Schmetzer and Schwarz (2007) and also
shown in figure 13. The yellow region in these stones
appears as a smooth spot when viewed down the caxis. Observed along an a-axis in this figure, it can be

282

CHROMOPHORES IN YELLOW SAPPHIRE

Defect chemistry is the study of point defects in imperfect crystals. These include vacancies, interstitial atoms
or ions, trace elements, electrons, or holes or any otherdeviation from the structure of a perfect crystal. Even absolutely pure crystals contain point defects at any temperature above absolute zero.
The presence of defects is described as “disorder” in a
crystal. Modern technological uses of crystalline materials
in lasers, semiconductors, lithium batteries, fuel cells, andthe like depend on controlled disorder. Allochromatic
gems are gems that are colored by impurities. If they werecompletely pure, they would be colorless. The color of all
allochromatic gems is produced by defects. Thus, all color
in corundum is caused by defects.
Writing the equations for chemical reactions among
defects in a crystal is also called defect chemistry. In addition to the normal rules for writing chemical reactions
in a liquid or gas, the reactions in a crystal have additional constraints. The set of rules for such reactions is
as follows:
1. Mass is conserved: The mass on both sides of achemical reaction must be equal.
2. Charge is conserved: The bulk of an ideal crystal
is electrically neutral. Thus, charged defects must
be created or destroyed in combinations that areelectrically neutral. Atoms or molecules added toor removed from a crystal must be in electricallyneutral combinations.
3. Site balance is conserved: Cation or anion sites canonly be created or destroyed in ratios corresponding to the stoichiometry of a perfect crystal.
These are also electrically neutral combinations.
This is the single most important difference between chemical reactions in a crystal and those ina liquid or gas.
The most commonly used formalism for writing
chemical reactions with the rules above is the KrögerVink notation. The notations are as follows:
1. The main symbol is the species, neutral or ionized, indicated by its usual chemical symbol. Va-

seen that the acceptor-dominated region is far more
spatially complex.
Figure 14 shows natural yellow sapphires from Sri
Lanka colored by this chromophore. The iron concentration in these stones typically ranges from 30 to 250
ppma and is therefore far too small to contributemeaningfully to the coloration as an Fe3+ chromophore.
To understand how trapped holes are formed, it is
helpful to express the chemical reaction equations inKröger-Vink notation (see box B). For simplicity, we

GEMS & GEMOLOGYFALL 2023

cancies are denoted by V. The symbol V for vanadium is usually shown in a different font.
2. A subscript indicates a lattice site labeled by its occupant in a perfect crystal. An interstitial site ismarked by the subscript i next to a chemical symbol.
3. A superscript indicates the charge of the specie relative to the charge of that site in a perfect crystal. Oneor more dots (•) are used for extra positive charge(s).
One or more slashes ( / ) are used for extra negative
charge(s). An x is used for zero charge difference.
Some examples of this notation for corundum are:
• Cation and anions on proper sites

x
AlAl

Oox

• Ionized cation and anion vacancies

VAl///

Vo••• Ionized cation and anion interstitials

•••
i

Al

Oi

• Divalent cation impurity on cation site/
MgAl

• Trivalent anion impurity on anion siteNo/

• Divalent anion impurity on anion site

Sox

• Monovalent anion impurity on anion site Fo•• Electrons in the latticee/• Electron holes in the lattice

h•

MgO +

Chemical reactions in corundum are written using
these symbols. Isovalent ions, which are those ions withthe same valence as aluminum or oxygen, can be incorporated as follows:
Cr2O3
Fe2O3
Ga2O3

Al2O3
Al2O3
Al2O3

x
2CrAl
+ 3Oox
x
Al

(B-1)

x
o

2Fe + 3O

(B-2)

x
2GaAl
+ 3Oox(B-3)

These equations show metal oxides being incorporated

will assume that the only acceptor is magnesium andthat the acceptor excess (Mg /excess) is simply written
as Mg /.
Acceptor-dominated natural sapphire is often
formed in a wet environment. Under these conditions, hydrogen provides the charge compensation,
as shown here, and no trapped holes are formed:
Al2O3
1
1
/
MgO + H2O
MgAl
+ Oox + (OH)o•2
2

CHROMOPHORES IN YELLOW SAPPHIRE(B-4)

Aliovalent ions, which have a valence different fromthat of aluminum or oxygen, can be incorporated into a
corundum lattice by the creation of additional defects.
For example, charge compensation of divalent ions canbe achieved by incorporation of additional oxygen andby the formation of holes as shown here:

x
Al

Cr

Al2O3 x 2 ///1
Oo + VAl + 2h•
O2(g)
2
3

//

• Tetravalent cation impurity on cation site Ti•Al• Trivalent cation impurity on cation site

into an aluminum oxide (Al2O3) crystal. No point defects
other than the metals themselves are being incorporated,because these trivalent metal ions can have the same valence as aluminum.
When corundum crystals are heated in an oxygen atmosphere, oxygen ions take oxygen sites on the surfaceof the crystal, resulting in the de facto formation of aluminum vacancies on the surface and trapped holes forcharge compensation. Oxidation in the bulk of the crystal is by inward diffusion of the resulting aluminum vacancies and holes. The diffusion coefficient of these
defects is several orders of magnitude greater than the
diffusion of oxygen ions. Therefore, the following equation is a good description of the process:

(5)

Al2O3
1
3
/
MgAl
O2(g)
+ Oox + h• (B-5)
42

In nature, when aliovalent ions are incorporated they
are often charge compensated by other aliovalent ions.
Here the magnesium is charge compensated by co-incorporation of titanium:
MgO + TiO2

Al2O3

/
•
MgAl
+ TiAl+ 3Oox(B-6)

The use of Kröger-Vink notation eliminates several
other conceptual reaction possibilities because of the necessity of both charge and site balance. This formalism
was proposed in Kröger and Vink (1956) and discussed ineven greater detail in Kröger (1974). An excellent reference from which to learn this approach is Smyth (2000).

However, if the stones are later equilibrated in a
warm, dry environment, part or all of the hydrogen may
diffuse out of the stones, creating h•, as shown here:
/
MgAl
+

heat
1 x
3
1
/
Oo + (OH)o•MgAl+ Oox + h• + H2(g)
2
2
2

(6)

Since nearly all natural corundum contains significant Fe3+, the trapped hole will associate with theFe3+, forming the h•-Fe3+ pair and creating the yellow

GEMS & GEMOLOGYFALL 2023

283

coloration (see footnote 3). These conditions are common in sapphire from Sri Lanka and the Rock Creek
deposit in Montana, where a range of colorless to
weakly yellow stones occurs naturally. When theprocess shown in equation 6 occurs in nature, yellow
sapphires are formed, as shown in figure 14.
We usually do not think of hydrogen having a rolein the visual color of gem corundum, as we assume
the concentrations are very low (Beran and Rossman,2006), a few ppma at most. Because their absorption
cross sections are low (1–2 × 10–19 cm2), a change of
only a few ppma in the valence of the transition
metal chromophores will make little or no differencein the apparent color. However, the h•-Fe3+ pair has
cross sections of 1–2 × 10–17 cm2, which means that a
few ppma change can induce or eliminate intense
coloration.
The natural process shown in equation 6 can also
be conducted in the laboratory. This is demonstrated
in Atikarnsakul and Emmett (2021). If hydrogen-containing acceptor-dominated corundum is heated in
an atmosphere that does not contain hydrogen or
water vapor, the hydrogen will diffuse out of thestone. For time scales of one hour to several hours
and temperatures in the 1000°–1400°C range, hydrogen will be removed from samples whose thickness
ranges from a few millimeters to a centimeter.
Diffusion is described by the proportionality inthe following equation:x ∝ √(Dt)(7)

which states that the diffusion distance, x, in cm isdirectly proportional to the square root of the diffusion coefficient, D, in cm2/sec, multiplied by thetime, t, in seconds (Borg and Dienes, 1988).

The diffusion coefficient for hydrogen in aluminum oxide has been studied by many authors(Fowler et al., 1977; Kronenberg et al., 2000; Fukatsu
et al., 2003; Belonoshko et al., 2004; Serra et al., 2005;
Doremus, 2006; Van Orman and Crispin, 2010), withlittle agreement in the values they determined. Apparently, the diffusion coefficient is sensitive to impurities or structural issues in the crystal, but thatremains to be quantified. For our rough estimates of
the time at temperature required to remove the hydrogen from our samples, the hydrogen diffusion coefficient chosen is that given by Belonoshko et al.
(2004), which at 1200°C is D = 1.3 × 10–6 cm2/sec.
From this value we can calculate the time required
at 1200°C to remove ~90% of the hydrogen from agiven size and shape of sample (Shewmon, 1989). For
a 2 mm thick sample plate whose lateral dimension
is large compared with 2 mm, the time is ~15 minutes. For sample plates that have a lateral dimensionthat is only a few to several times their thickness,
the times would be somewhat less. For larger samples suitable for faceting, the times are significantly
longer. For example, removing 90% of the hydrogenfrom a 1 cm diameter sphere at 1200°C would require heating for approximately 1.5 hours.
From our review of the literature, there is little
consensus in the determination of the hydrogen diffusion coefficient in relatively pure sapphire crystals,
and no information for natural crystals with therange and variety of trace elements found in gem sapphire. The uncertainty is more than one order ofmagnitude. According to equation 7, the uncertaintyof the time required is then also an order of magnitude. Given the sensitivity of the yellow h•-Fe3+ coloration to hydrogen content, that chromophore
might be used as a basis for experiments determining

Figure 14. Sri Lankan
yellow sapphires ranging from 1.0 to 2.5 ct.
The seven on the left are
natural, and the four on
the right are heat
treated. Photo by Ronnakorn Manorotkul/
Lotus Gemology; courtesy of Chaiyut
Rungutaitawornsuk.

284

CHROMOPHORES IN YELLOW SAPPHIREGEMS & GEMOLOGYFALL 2023

5 mm

5 mm

Figure 15. Left: Pale yellow natural sapphires from Sri Lanka. Right: The same sapphires after heat treatment at
1400°C, in oxygen, for five hours to remove hydrogen. Photos by John L. Emmett.

Figure 16. A: FTIR spectra of sample 2802 from Sri
Lanka. B: UV-Vis-NIR spectra of the same sample. C:
The color alteration of the sample before and after
heat treatment. The outlined area indicates theanalysis area where the FTIR and UV-Vis-NIR spectra were collected. Thickness 2.811 mm, 194 ± 26
ppma iron. Photos by Sasithorn Engniwat.

CHROMOPHORES IN YELLOW SAPPHIRE

ABSORPTION COEFFICIENT (cm–1)

a 3161 cm–1 OH peak or 3161 cm–1 OH series in FTIR
spectra, which indicates they are acceptor dominated. The connection of the 3161 cm–1 series withFTIR SPECTRA
1.8

A

1.6

Unheated
900˚C, 6 hrs in air
1550˚C, 6 hrs in oxygen

1.4
1.2
1.0
0.8
3200

3400

3000

2800

2600

2400

WAVENUMBER (cm–1)
ABSORPTION COEFFICIENT (cm–1)

the hydrogen diffusion coefficient in natural or synthetic gem sapphire.
Acceptor domination of weakly colored sapphires
from a single deposit ranges from 0 to 90% of thesamples: 0% of the samples from Songea in Tanzania, ~20% from Chimwadzulu Hill in Malawi, ~50%
from Sri Lanka, and ~90% from Rock Creek in Montana. Thus, it is important to initially test a large lotof small rough to determine the percentage that develop yellow coloration, in order to determine how
many wafers must be fabricated to yield a few thatturn yellow for the experiments and spectroscopy.
This is the approach we have taken for this work, as
presented below.
As seen in figure 15 (left), Sri Lankan yellow sapphires were initially a pale yellow color before heat
treatment. Figure 15 (right) shows the same samples
after heat treatment to quantitatively remove all hydrogen. More than half of the heated stones developed
stronger yellow coloration, and many of them show
the enhanced color following straight or angular
growth structures. Compare the stones in figures 14
and 15. Thus far, it appears that nonuniform coloration is characteristic of this chromophore in natural corundum.
Figure 16 shows the detail of Sri Lankan sample
2802 undergoing this hydrogen removal process.
Many natural Sri Lankan sapphires typically display

UV-VIS-NIR SPECTRA
14

B

12

E⊥c
Unheated
900˚C, 6 hrs in air
1550˚C, 6 hrs in oxygen

10
8
6
4
2
0
200

300

400

500

600

700

800

900

1000

1100

WAVELENGTH (nm)C

Unheated

GEMS & GEMOLOGY

Heated at 900˚C for
6 hours in air

Heated at 1550˚C for
6 hours in oxygen

FALL 2023

285

5 mm

5 mm

Figure 17. Left: Run-of-mine samples from Rock Creek, Montana, selected only for size (3.0–5.5 mm), before heat
treatment. Right: The same samples are shown after heat treatment at 1200°C in oxygen for 10 hours. Photos byJohn L. Emmett.

high magnesium content (acceptor domination) was
first described by Smith and van der Bogert (2006).
Increased yellow coloration after heating (figure16C) is directly related to the reduction of the amplitude of the OH-related peak at 3161 cm–1 (figure16A) (Atikarnsakul and Emmett, 2021) and increased strength of the h•-Fe3+ absorption band in
UV-Vis-NIR spectra (figure 16B). Interestingly, thenatural characteristic 3161 cm–1 feature in certain
samples could be occasionally transformed to the
OH broadband series in the 3000 cm–1 region with abroadband at 2625 cm–1, as shown in figure 16A
when heated at 900°C in air. When heat treated at
1550°C in pure oxygen, the hydrogen was completely
removed, as shown by the disappearance of the OH
absorption features from the FTIR spectra. With theremoval of the hydrogen, the h•-Fe3+ absorption spectrum and thus the yellow coloration increased.
As mined, sapphire from Rock Creek in Montana
has little coloration, which indicates that the acceptor and donor concentrations are quite similar. In addition, about 8% of the stones show some yellow
coloration, although only a few would cut a fine yellow gem. We also present hydrogen extraction results
for this material, as it shows a different broadband
OH absorption spectrum: the wide band 3000 cm–1
series. Interestingly, a few samples have both the3000 cm–1 series and the 3161 cm–1 series, but thus
far none have shown only the 3161 cm–1 series. Whenhydrogen is diffused into magnesium-doped synthetic sapphire, the OH spectrum is a broadband at
3000 cm–1 (Fukatsu et al., 2003). Similarly, when hydrogen is diffused into synthetic sapphire containing
Ni3+ or Co3+, the reduction of these dopants to the divalent state is accompanied by the formation of thebroad OH band at 3000 cm–1 (Müller and Günthard,

286

CHROMOPHORES IN YELLOW SAPPHIRE

1966). These facts, combined with the conversion of
the 3161 cm–1 band to the 3000 cm–1 band in figure
16A, emphasize how little we understand OH incorundum. It would be worthwhile to study theseOH features in a wide variety of acceptor-dominatedcorundum to determine the correlation of the various OH bands with the trace element chemistry and,perhaps, the formation temperature.
Figure 17 (left) shows run-of-mine samples ofRock Creek sapphire selected only for size (3.0–5.5mm). Figure 17 (right) shows the same samples after
heating for 10 hours at 1200°C in oxygen. Nearly
90% of the stones now show yellow coloration insome portion.
The development of yellow color zones in Rock
Creek sapphires is similar to that seen in Sri Lankan
sapphire, but the yellow region is often limited to asmaller portion of the stone, as shown in figure 13.
Figure 18A shows the FTIR spectra of Rock Creek
sample 1021 before and after heat treatment at 1500°C
for five hours in pure oxygen. The heat treatment
process has fully removed all the hydrogen from thesample.
Figure 18C shows the before and after photos ofsample 1021. Heat treatment creates the strongly
colored yellow spot that appears in the sample. Notethat the optical measurement area is only centeredon half of the yellow region (figure 18C, middle). A
second sample plate was made so that optical measurements could also be centered on the yellow region, as shown in figure 18C, right.
Figure 18B shows the UV-Vis-NIR spectrum before
and after heat treatment and in the second measurement area after heat treatment. The results revealed
that the h•-Fe3+ spectrum increased significantly after
heat treatment and was more obvious when the

GEMS & GEMOLOGYFALL 2023

analysis area was centered to be exclusively in theyellow area. By subtracting varying amounts of theh•-Fe3+ absorption spectrum from the spectrum in thesecond measurement area, we can determine that its
concentration is approximately 6 ppma, the highest
value measured thus far.

Yellow sapphires can also be created by heat treatment of an entirely different type of acceptor-dominated stone that does not contain hydrogen. If
acceptor-dominated stones are formed at very low
oxygen fugacity, the resulting reaction is:
MgO

ABSORPTION COEFFICIENT (cm–1)

Figure 18. A: FTIR spectra of sample 1021 from Rock
Creek. B: UV-Vis-NIR spectra of the same sample. C:
The color alteration of the sample before and after
heat treatment, and another measurement area after
heat treatment. The outlined area indicates theanalysis area where the FTIR and UV-Vis-NIR spectra
were collected. Thickness 1.710 mm, 978 ± 111 ppma
iron. Photos by Sasithorn Engniwat.

FTIR SPECTRA
1.5

A

Unheated
1500˚C, 5 hrs in oxygen (partially on spot)
1550˚C, 5 hrs in oxygen (centered on spot)

1.4

/
MgAl
+ Oox +

1 ••
Vo
2

(8)

where the charge compensation is by oxygen vacancies. The result of this reaction is a colorless stone.
If such stones are then heat treated at high temperature in an oxygen atmosphere, the oxidation reaction
can be described as:
Al2O3 x 2 ///1
Oo + VAl + 2h•
O2(g)
2
3

(9)

The oxidation is not caused by the inward diffusionof oxygen but by the inward diffusion of aluminum
vacancies and holes formed on the surface. The combined reaction (equation 8 plus ½ equation 9) is:

1.3
3025
1.2

2422

2984

2626

1.1
1.0
3200

3400

3000

2800

2600

2400

WAVENUMBER (cm–1)
ABSORPTION COEFFICIENT (cm–1)

Al2O3

UV-VIS-NIR SPECTRA
30

B

E⊥c
Unheated
1500˚C, 5 hrs in oxygen(partially on spot)
1550˚C, 5 hrs in oxygen(centered on spot)

25
20
15
10
5
0
200

C

Unheated

300

400

500

600

700

800

900

1000

1100

WAVELENGTH (nm)Heated at 1500˚C for 5
hours in oxygen withmeasurement area partiallyon the yellow spot

CHROMOPHORES IN YELLOW SAPPHIRE

Heated at 1500˚C for 5
hours in oxygen withmeasurement area centeredon the yellow spot/
MgAl
+ Oox +

1 •• 1 x 1 /// •3/
Vo + Oo + VAl + h = MgAl
+ Oox + h• (10)2232This reaction shows how acceptor-dominated, hydrogen-free colorless stones can be heat treated inoxygen to create the h•-Fe3+ chromophore and thus
the yellow coloration. The diffusion coefficient for
the inward diffusion of aluminum vacancies andholes is much lower than the outward diffusion ofhydrogen, so this reaction requires much higher temperatures and much longer times.
At present we know of three sources for sapphire
formed in this way. Pale to colorless stones are foundat the Eldorado Bar deposit on the Missouri River in
Montana, at the Chimwadzulu Hill deposit inMalawi, and at deposits in Madagascar. In each case,only a fraction (5–30%) of the run-of-mine stones exhibit this chemistry. To study the formation of the h•Fe3+ chromophore in this entirely different chemistry,
the following experiments were conducted. A large
600 g parcel of Missouri River sapphire was processed
through a precision splitter to make twelve 50 g test
lots that were nearly identical. There were no identifiable yellow samples in the entire 600 g parcel.
Twenty-five grams (one-half) of one of the test lots areshown in figure 19A. One each of the 50 g sublots
were processed in pure oxygen in each of the following conditions: 1600°C for 10 hours (19B), 1700°C for
10 hours (19C), 1800°C for 10 hours (19D), and1800°C for 31.6 hours (19E). The experiments at

GEMS & GEMOLOGYFALL 2023

287

lower temperatures (1200°C and 1400°C) did not produce any yellow stones. After heat treatment, all theyellow sapphires in the 50 g samples were sorted out
and photographed separately, as shown in figure 19,
B–E.
The stones in figure 19E represent about 23% byweight of the 50 g lot. Plotting the data from theseexperiments indicates that increasing both the temperature and time should further increase the yieldof yellow sapphire and the degree of color saturation.
Comparing the heat treatment conditions of figure 17 (right) and figure 19E shows the dramatic difference between acceptor-dominated stones charge
compensated with hydrogen and those charge compensated with oxygen vacancies. Sapphires from
Chimwadzulu Hill in Malawi responded to hightemperature heat treatment in a similar way, with
about 19% developing the yellow coloration.
If the corundum has excess acceptors and isgrown or later equilibrated in a high-oxygen-fugac-

ity natural environment, we have the following
chemical reaction:
Al2O3
1
3
/
MgO + O2(g)MgAl+ Oox + h•
4
2

(11)

which is the same as the after-heating result shownin equation 10. The interesting question is whether
this condition occurs in nature to produce natural
yellow sapphires. To answer this question, we needto find natural yellow sapphires whose coloration isdue wholly or in part to the h•-Fe3+ pair yet show no
OH absorption features in the FTIR spectrum. Suchexamples may be found in sapphires that are weakly
or strongly yellow colored by Fe3+.
Finding such samples is difficult for several reasons. The 3000 cm–1 broadband usually has a much
lower peak absorption coefficient than the 3161 cm–1
band, as it is much broader. Thus, determining that
there is zero OH absorption was difficult, as most of

B

A

5 mm

5 mm

D

C

5 mm

5 mm

E

Figure 19. A: One-half ofa 50 g lot from a 600 g
parcel of Missouri Riversapphire, selected only
for size (3.5–5.5 mm),before heat treatment.
B: Yellow stones produced from another 50 g
lot after heat treatment
at 1600°C in oxygen for
10 hours. C: Yellow
stones produced fromanother 50 g lot after
heat treatment at
1700°C in oxygen for 10hours. D: Yellow stones
produced from another
50 g lot after heat treatment at 1800°C in oxygen for 10 hours. E:
Yellow stones produced
from another 50 g lot
after heat treatment at
1800°C in oxygen for
31.6 hours. Photos byJohn L. Emmett.

5 mm

288

CHROMOPHORES IN YELLOW SAPPHIREGEMS & GEMOLOGYFALL 2023

FTIR SPECTRUM
ABSORPTION COEFFICIENT (cm–1)

3.8
3.7

A

3.6
3.5
3.4

2925
2852

3.3
3.2
3.1
3400

3300

3200

3100

3000

2900

2800

2700

2600

2500

2400

WAVENUMBER (cm–1)

UV-VIS-NIR SPECTRUM
ABSORPTION COEFFICIENT (cm–1)

30
25

B

E⊥c

Figure 20. A: FTIR spectrum of sample 4713
from Chanthaburi, Thailand. B: UV-Vis-NIR
spectrum of the same
sample measured in theyellow region. E⊥c, thickness 0.465 mm, 3889 ±229 ppma iron. Photo by
John L. Emmett.

20
15
10
5
0
200

300

400

500

600

700

800

900

1000

1100

WAVELENGTH (nm)our research samples were rather thin. Additionally,
weakly colored yellow sapphires that may also showthese two characteristics are not often the focus ofsample collection for research, as opposed to morestrongly colored stones.
At present, our best example of such a sample isyellow sapphire 4713 from Chanthaburi, Thailand,
which shows these two characteristics. As shown infigure 20, there is no indication of the 3161 cm–1 series
or the 3000 cm–1 broadband series in this FTIR spectrum, but the h•-Fe3+ pair absorption in the UV-VisNIR spectrum is clear from the increasing absorption
coefficient from 600 to 500 nm. This sample is weakly
colored, as it is only 0.465 mm thick. Two small peaks
centered at around 2925 and 2852 cm–1 are observedin the FTIR spectrum, but they are not a part of the3000 cm–1 broadband series. They are typically seen
overlying various other mid-infrared spectral features
as a result of oil contamination on the surface of thestone or other materials present in open fractures.
These peaks are related to the C-H stretching (2950–
2850 cm–1) of organic matter (Tanykova et al., 2021),
and the peak positions were the same as those of oilimpregnated emerald (Johnson et al., 1999).
Presently, our only example of the equation 11
phenomenon is sample 4713. Definitive proof thatnatural stones are indeed formed in this way will require the collection of substantially more samples
from a wider list of deposits. In addition, samples
should be fabricated into significantly thicker wafers
to facilitate the accurate measurement of their FTIR
spectra.

CHROMOPHORES IN YELLOW SAPPHIREGEMS & GEMOLOGY

Combination of the Fe3+ and h•-Fe3+ Chromophores.
Untreated high-Fe3+ yellow sapphires from basalthosted deposits such as Australia and Thailand often
have a quite uniform yellow color. The color iscaused mainly by the Fe3+ chromophore, but some-

FALL 2023

289

UV-VIS-NIR SPECTRUM
25

E⊥c

ABSORPTION COEFFICIENT (cm–1)

20

Figure 21. UV-Vis-NIR
spectrum of sample 941
from Bang Kha Cha,
Thailand. E⊥c, iron =3093 ppma, areal density = 4979 ppma-mm,
and t = 1.61 mm. Photo
by John L. Emmett.

15

10

5

0
200

300

400

500

600

700

800

900

1000

1100

WAVELENGTH (nm)times it is substantially modified by the presence ofthe h•-Fe3+ chromophore as well.

Natural yellow sapphires of high iron concentration (>2800 ppma Fe3+) usually exhibit a rather uni-

UV-VIS-NIR SPECTRA
25

E⊥c
As measured
Minus 3.2 ppma h•-Fe3+

ABSORPTION COEFFICIENT (cm–1)

20

Figure 22. UV-Vis-NIR
spectra of sample 940
from Bang Ka Cha,
Thailand. E⊥c, iron =3251 ppma, areal density = 4649 ppma-mm,
and t = 1.430 mm. Photo
by John L. Emmett.

15

10

5

0
200

300

400

500

600

700

800

900

1000

1100

WAVELENGTH (nm)290

CHROMOPHORES IN YELLOW SAPPHIREGEMS & GEMOLOGYFALL 2023

form yellow coloration. This is because Fe3+ is soluble in corundum, easily replacing Al3+ because it has
the same valence and a similar ionic radius. Unlike
the chromophores containing aliovalent ions, Fe3+solubility is not dependent on the presence of another ion. Perhaps more interesting is that thesehigh-Fe3+ natural sapphires often incorporate the h•Fe3+ chromophore as well. It is not unusual to findyellow sapphires colored by Fe3+ only and colored byboth high Fe3+ concentration and the h•-Fe3+ chromophore in the same deposit. The Subera deposit in
New South Wales, Australia, and the Khao Ploi
Waen/Bang Kha Cha deposit in Thailand are two examples of this phenomenon.
Figures 21 and 22 compare two similar samples
from Bang Kha Cha. Sample 941 (figure 21) is coloredby Fe3+ only, while sample 940 (figure 22) is coloredby both chromophores. Both samples exhibit a weak
but broad OH band at 3000 cm–1. Sample 940 has apeak absorption of about 0.04 cm–1, while sample 941
has a peak absorption of about 0.07 cm–1.
As discussed earlier, when comparing colorcaused by the Fe3+ chromophore only, both the Fe3+
concentration and its areal density must be taken
into account. Samples 940 and 941 differ in Fe3+ concentration by only 5.1% and in areal density by only
7.1%. Therefore, the 450 nm absorption bands of Fe3+of the two samples are nearly equal. These samples
offer a direct comparison of high-iron corundum withand without the h•-Fe3+ chromophore.
The dramatic difference in the depth and characterof the color between sample 940 and sample 941 is theresult of the additional ~3.2 ppma of the h•-Fe3+ chromophore. When examining the spectra of yellow sapphire samples with high concentrations of Fe3+, it isvery easy to discern which contain only Fe3+ andwhich contain some addition of the h•-Fe3+ chromophore. In figure 21 there is a distinct minimum between the Fe3+ 450 nm band and the weak, broad Fe3+band at 530 nm (Borg and Dienes, 1988). Dependingon the Fe3+ concentration, the minimum between
these two usually occurs between 490 and 502 nm.
For sample 941, just 0.25 ppma of the h•-Fe3+ pair eliminates the minimum. The 3.2 ppma of the h•-Fe3+
chromophore in sample 940 is determined by subtracting incremental amounts of it from the spectrumof sample 940 until the minimum at 500 nm is restored. Typical concentrations of 1 ppma or more of
the h•-Fe3+ chromophore result in a linear increase in
the absorption coefficient from 600 to 500 nm, which
is very easy to visually recognize. That sample 940
shows a well-saturated color and yet is only 1.43 mm

thick attests to the extraordinary strength of the h•Fe3+ chromophore.
The question naturally arises as to whether thesehigh-iron stones, which also exhibit the h•-Fe3+ chromophore, respond to heat treatment. Figure 23 shows
the result of some experiments. Figure 23A gives the
FTIR spectra from sample 5270. This natural stone
from Australia exhibits the 3000 cm–1 OH broadband
series, not the 3161 cm–1 series. This OH absorption
has been eliminated by heating at 900°C for six hours

CHROMOPHORES IN YELLOW SAPPHIREGEMS & GEMOLOGY

Figure 23. A: FTIR spectra of Australian sample 5270.
B: UV-Vis-NIR spectra of the same sample. C: Thecolor alteration of the sample after heat treatment.
The outlined area indicates the analysis area where
the FTIR and UV-Vis-NIR spectra were collected.
Thickness 1.240 mm, 3557 ± 627 ppma iron. Photos
by Sasithorn Engniwat.

ABSORPTION COEFFICIENT (cm–1)

FTIR SPECTRA
2.0

A

Unheated
900˚C, 6 hrs in air
1550˚C, 6 hrs in oxygen

1.8

1.6

1.4

1.2
3200

3400

3000

2800

2600

2400

WAVENUMBER (cm )ABSORPTION COEFFICIENT (cm–1)–1

VIS-NIR SPECTRA

B

14

E⊥c
Unheated
900˚C, 6 hrs in air
1550˚C, 6 hrs in oxygen

12
10
8
6
4
495 nm minimum

2
0
400

500

C

Unheated

600

700

800

900

1000

1100

WAVELENGTH (nm)Heated at 900˚C for
6 hours in air

Heated at 1550˚C for
6 hours in oxygen

FALL 2023

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The combination of high concentrations of both
the Fe3+ and the h•-Fe3+ chromophores can create dramatic colors, as shown in figure 24.
Beryllium-Diffused Yellow Sapphires. The diffusionof divalent beryllium into natural corundum (Emmett et al., 2003) adds to the concentration of acceptor ions so that equations 3 and 4 become:[acceptors] – [donors] = [Be2+ + Mg2+ + Ni2+] – [Si4+ + Ti4+] > 0 (12)[Be2+ + Mg2+ + Ni2+] – [Si4+ + Ti4+] – [H+ + 2Vo2+ + h+] = 0(13)

The addition of beryllium by diffusion in an oxidizing
atmosphere primarily results in the formation of additional trapped holes, which will form more h•-Fe3+ pairs
with available Fe3+ and thus more yellow coloration.
Because beryllium itself is not the chromophore, thecoloration resulting from trapped holes formed byberyllium excess, by magnesium excess, or by both inexcess is the same, as the chromophore is always h•Fe3+. We have demonstrated this fact by growing two
crystals, the first doped with magnesium and iron andthe second doped with beryllium and iron.
The E⊥c absorption cross sections of the two h•3+
Fe chromophores presented in figure 25 are nearly

Figure 24. This unusual gem from the Bang Kha Cha
deposit is colored by a high concentration of both the
Fe3+ and the h•-Fe3+ chromophores. The color is referred to as “Mekong Whisky.” Photo by Ronnakorn
Manorotkul/Lotus Gemology.

in air, and then by heating again at 1550°C for six
hours in oxygen. Figure 23B illustrates that before
heat treatment there is a minimum at ~495 nm, indicating the absence of the h•-Fe3+ chromophore. Heat
treatment at 900°C substantially increases the depthof the yellow coloration, as shown in figure 23C.

UV-VIS-NIR SPECTRA

ABSORPTION CROSS SECTION IN UNITS OF 10 –17 cm2

3.0

E⊥c
Mg, Fe: Sapphire
Be, Fe: Sapphire

2.5

Figure 25. The UV-VisNIR E⊥c absorption
cross section of theh•-Fe3+ pair in two different synthetic sapphire
crystals. The first isdoped with magnesium
and iron and the second
with beryllium and iron.

2.0

1.5

1.0

0.5

0
200

300

400

500

600

700

800

900

1000

1100

WAVELENGTH (nm)292

CHROMOPHORES IN YELLOW SAPPHIREGEMS & GEMOLOGYFALL 2023

Figure 26. Left: Near-colorless sapphires from the Dry Cottonwood Creek deposit in Montana, with an averagestone size of 4–5 mm. Right: The same group of sapphires after beryllium diffusion. Photos by John L. Emmett.

identical. A small vertical offset (0.127 × 10–17) resulting from internal scatter in the crystal doped withberyllium and iron has been subtracted from its spectrum in figure 25.
Beryllium diffusion to alter the coloration of natural corundum is conducted in an air or pure oxygen
atmosphere at temperatures of 1800°C or above forseveral tens of hours. The diffusion experiments described here were conducted in the Crystal Chemistry laboratory in a pure oxygen atmosphere, as werethe experiments in Emmett et al. (2003). Heating inthis way eliminates all of the H+ and Vo2+ charge compensators, maximizing the degree of acceptor domination and the h•-Fe3+ concentration. This maximizes
the yellow coloration of sapphire. Commercially thisprocess is often conducted in air. If conducting the
diffusion process in air, the water vapor in the air cancontribute hydrogen to the process, contributing tosome reduction of the h•-Fe3+ concentration. In addition, OH bands that would otherwise have been
completely eliminated can appear in the FTIR spectra. This effect in synthetic sapphire is shown inBalmer and Krzemnicki (2015) and Sangsawong(2020).
As previously discussed, sapphires that are colorless or weakly colored are usually neutral or slightly
acceptor dominated. If such stones are beryllium diffused, a strong yellow color results from a substantial
increase in h•-Fe3+ concentration. Figure 26 (left)
shows a group of near-colorless sapphires from the
Dry Cottonwood Creek deposit in Montana. The
great majority of sapphires from this deposit show
little coloration. Figure 26 (right) shows the result ofberyllium-diffusing this group at 1800°C in oxygenfor 33 hours.
The amount of beryllium that can be added tonatural corundum by diffusion often quite exceeds
the acceptor excess that occurs in natural yellow sapphire. Thus, it is possible that even donor-dominatedblue sapphires can be converted to acceptor-dominated yellow sapphires by beryllium diffusion. We
conducted an experiment to demonstrate that possibility using deep blue sapphire mined at Kings Plain,
New South Wales, Australia (figure 27, left).

Figure 27. Left: A group of run-of-mine deep blue sapphires from Kings Plain, New South Wales, Australia, with an average stone size of 4–5 mm. Right: The same group of sapphires after beryllium diffusion. Photos by John L. Emmett.

CHROMOPHORES IN YELLOW SAPPHIREGEMS & GEMOLOGYFALL 2023

293

Figure 28. Synthetic
sapphire colored onlyby the h•-Fe3+ chromophore. The three thickest sections are 12 mm
thick. Photo by John L.
Emmett.

Figure 27 (right) shows these stones after beryllium diffusion treatment. The majority of the blues
have been converted to yellows, which are now acceptor dominated. Although the color has been reduced in degree, stones that were the deepest blue
before diffusion have been lightened but remain
donor dominated.
Synthetic Yellow Sapphires. Synthetic yellow sapphire, which is colored by the yellow h•-Fe3+ chromophore, was grown for the first time as a part of thestudy of the chromophores of natural corundum (Emmett et al., 2017a). The yellow crystal grown by author JS-S at Saint-Gobain Crystal and Detectors wasdoped with magnesium and iron to produce the h•Fe3+ chromophore. Figure 28 shows sections of thisCzochralski-grown boule, of which the three thickest are 12 mm thick. The other three samples are broken, providing some wedge sections. The h•-Fe3+
chromophore concentration is 2.1 ppma ± 9%. Notethe uniformity of color compared to the natural samples colored with the same chromophore, which areoften zoned.
Historically, synthetic sapphires have usually
been doped with nickel or by nickel and chromium
to produce the yellow coloration. The chromophores
used have typically been the transition metals andthe Fe2+-Ti4+ pair. Thus, it is not surprising to see yellow synthetic sapphire colored primarily with Ni3+.

294

CHROMOPHORES IN YELLOW SAPPHIRE

It is also interesting that the nickel concentration
used is quite low compared with the other transition
metals. The yellow coloration from Ni3+ does not result from transitions within the Ni3+ electronic structure, but from a charge transfer transition from theAl2O3 valence band to the Ni3+ ion (Tippins, 1970).
What is most unusual is that this first charge transfer
band of Ni3+ peaks is at 400 nm, providing significant
absorption from 400 to 480 nm and thus strong yellow coloration. All other transition metals studied todate have their first charge transfer band well into
the UV, and thus these bands do not contribute tocolor. For example, the first charge transfer bands forFe3+ and Cr3+ are at 259 and 178 nm, respectively (Tippins, 1970). Charge transfer absorption cross sections
are typically a few hundred times stronger than theinternal electronic transitions in transition metals,
which is why low concentrations of Ni3+ contributesignificant color.
In the course of this study, we examined one crystal grown with the Verneuil process from RusGems
in Bangkok and two crystals grown with the
Czochralski technique by Milan Kokta at Saint-Gobain Crystals and Detectors. The Ni3+ concentrationof the three crystals, analyzed by LA-ICP-MS at GIA
and by SIMS at the California Institute of Technology, ranged from 1.6 to 8.0 ppma. To ensure that all
the nickel was in the trivalent state, the spectroscopic samples cut from the boules were annealed in

GEMS & GEMOLOGYFALL 2023

Figure 29. Sample 1162, a Ni3+-doped sapphire cut
from boule SS9. The c-axis is in the plane of thewafer, which measures 10.0 × 10.0 × 2.14 mm. Photo
by John L. Emmett.

oxygen at 1750°C for 10 hours. The oxygen anneal of
one of the samples cut from the SS9 Czochralskigrown crystal increased the absorption coefficient of

the first charge transfer band by a factor of about
eight. This stems from the fact that the oxygen partial pressure of the growth atmosphere is approximately 10–3 atmospheres, and thus only a portion of
the nickel was in the trivalent state following
growth. The Verneuil-grown crystal from RusGems
was fully oxidized as received. It is unknown
whether this condition resulted from the very different growth conditions of the Verneuil process orfrom an oxygen anneal following growth.
Figure 29 shows sample 1162, cut from Czochralski boule SS9 after the oxygen anneal. Figure 30 shows
UV-Vis-NIR absorption cross sections for both E⊥cand E||c before and after the oxygen anneal. The nickel
concentration in this crystal is 3.58 ppma, as measured by SIMS at the California Institute of Technology, and the thickness is 2.14 mm.
As mentioned earlier, the reason for our interestin determining the Ni3+ absorption cross section is
the fact that nickel in concentrations of up to 15
ppma has been found in natural sapphire. We determined its cross section, as we have done for the othernatural chromophores in corundum. As shown here,
only a few ppma of Ni3+ are necessary to produce sub-UV-VIS-NIR SPECTRA

ABSORPTION CROSS SECTION IN UNITS OF 10 –17 cm2

4.5

E⊥c as grownEǀǀc as grownE⊥c oxygen annealedEǀǀc oxygen annealed

4.0

3.5

3.0

2.5

Figure 30. Sample 1162,
a Ni3+-doped Czochralski-grown sapphire.

2.0

1.5

1.0

0.5

0
200

300

400

500

600

700

800

900

1000

1100

WAVELENGTH (nm)

CHROMOPHORES IN YELLOW SAPPHIREGEMS & GEMOLOGYFALL 2023

295

stantial coloration. Since the absorption cross sectionin the visible region of Ni3+ exceeds even that of theh•-Fe3+ pair, only a few ppma in a neutral or acceptordominated stone would contribute strong absorption
in the 400–480 nm region, adding additional yellow
coloration. It is important to carefully study the absorption spectra of stones containing ≥1 ppma nickel
to determine whether there is a seventh natural chromophore. The first six are discussed in Dubinsky etal. (2020).

CONCLUSIONSThis study of yellow sapphires was motivated by thelack of clarity in the literature as to the origin of theyellow coloration. In retrospect, this uncertainty was
the result of three factors. First, the Fe3+ concentration in natural stones spans a range of approximately
4–4750 ppma, but only concentrations in the
2000–4750 ppma range contribute significantly toyellow coloration. Second, the h•-Fe3+ chromophore

ABOUT THE AUTHORS
Dr. John Emmett is director of Crystal Chemistry in Brush Prairie,
Washington, and a consultant to GIA. Ungkhana Atikarnsakul is a
staff gemologist at GIA’s Bangkok laboratory. Dr. Jennifer StoneSundberg is a technical advisor on GIA education operations anda technical editor of Gems & Gemology located in Portland, Oregon. Dr. Supharart Sangsawong is a former research scientist at
GIA in Bangkok.
ACKNOWLEDGMENTS
The authors would like to thank many people who have contributed to this work over very many years, especially Tom Moses,
Ken Scarratt, Sudarat Saeseaw, Wasura Soonthorntantikul,

is about 1000 times as strong as the Fe3+ chromophore and is often distributed quite inhomogeneously in a given sample. Low concentrations, ≥0.5
ppma, produce substantial coloration, which could
only be determined with the advent of LA-ICP-MSand SIMS in gemology. Third, both of these chromophores contribute to the yellow coloration in
many natural yellow sapphires, making its cause ofcolor difficult to determine.
We hope this paper will form a basis for further
study of the yellow color stability of natural sapphire,
a topic that has become increasingly important in recent years. From our work, it is clear that any studyof color stability must first determine the trace element composition of the sample down to 1 ppma or
better, the UV-Vis-NIR absorption spectra, and the
FTIR absorption spectra of the OH ion. A study ofcolor change by low-temperature heating, visible
light illumination, or UV illumination will beneeded to track the latter two factors.

Tim Thomas, Ziyin Sun, Milan Kokta, George Rossman, Yunbin
Guan, Ren Lu, Zachary Cole and team, Dick Hughes, Wimon
Manorotkul, Emily Dubinsky, and Augustus Pritchett. We would
also like to extend our thanks to all the miners and contacts fortheir efforts in collecting and documenting some of the samples
used for this study, as well as field gemologists Vincent Pardieu
and Wim Vertriest. We want to extend our thanks to Sasithorn
Engniwat, Charuwan Khowpong, and Suwasan Wongchacree
from GIA’s field gemology department for sample fabrication andthe collection of photos and photomicrographs. Anonymous
peer reviewers are also thanked for providing helpful comments
and suggestions.

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297

FEATURE ARTICLESSPECIAL COLORS AND OPTICAL EFFECTS
OF OREGON SUNSTONE: ABSORPTION,SCATTERING, PLEOCHROISM, AND
COLOR ZONING
Shiyun Jin, Aaron C. Palke, Nathan D. Renfro, and Ziyin Sun
Oregon sunstone is a unique feldspar gemstone that displays a wide range of colors and optical effects due to the presence of metallic copper inclusions of various sizes and shapes. Most striking are the distinct green and red colors
caused by copper particles, sometimes with strong red to green pleochroism. Recent computation work suggests that
copper particles can strongly absorb and/or scatter light depending on the relative orientations of the feldspar crystal,the copper particles, and the polarization of the incident light, resulting in dramatically different appearances of thesame crystal when viewed (or cut) along different directions. Common optical properties of Oregon sunstone are described and explained in detail in this paper, based on the optical theories of metallic nanoparticles in anisotropic
media. Notably, it was found that the red to green colors observed can only be explained by considering both selective
absorption and scattering from copper particles. The absorption and extinction spectra of the copper inclusions inOregon sunstone are measured for different crystal orientations and polarizations. The scattering effect of the copper
particles is quantified by subtracting absorption from extinction, the result of which agrees closely with the computed
optical properties of spheroidal copper nanoparticles. The orientation of the anisotropic copper particles relative tothe optical indicatrix of the feldspar is described. The correlation between the copper concentration profile and the“watermelon” color zoning in some Oregon sunstones is examined and discussed, along with a comparison to thecolor zones in treated, copper-diffused feldspar. Further studies on the diffusion and dissolution mechanisms of copperin feldspar crystals are required to fully understand the geologic processes that create these special gemstones innature, which may provide additional methods to differentiate between treated and untreated stones.

O

regon sunstone is a significant feldspar gemstone with growing popularity, and certainly
one of the most famous gemstones producedin the United States. Found in weathered lava flows,these basaltic phenocrysts have poorly developed
cleavages and few inclusions or twins due to the disordered structure resulting from a fast cooling rate (Xuet al., 2017; Jin et al., 2018, 2019), making them moresuitable for faceting than other feldspar gemstones.
Their popularity comes mainly from the wide range
of appealing colors (figure 1), from the more common
yellow, pink, orange, and red to the extremely rare

See end of article for About the Authors and Acknowledgments.
GEMS & GEMOLOGY, Vol. 59, No. 3, pp. 298–322,
http://dx.doi.org/10.5741/GEMS.59.3.298© 2023 Gemological Institute of America

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COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONE

green-blue (www.oregonsunstoneguide.com/typesand-colors.html). Many of these colors have not beenfound in feldspar from anywhere else in the world.
The term sunstone was originally defined (Andersen,
1915) as the gem variety of aventurine feldspar,showing intense play of light and colors caused bystrong reflections from thin oriented metallic inclusions of visible sizes, a phenomenon known as aventurescence. The loosely defined word schiller is morecommonly used to describe the aventurescence ofsunstones in the gem community (the two terms areused interchangeably in this article). Most sunstones,
such as those from India, Norway, and Tanzania, owe
their aventurescence effect to hematite inclusions.
Copper-containing aventurine feldspar was first documented by Andersen (1917), who studied specimens
reportedly from Modoc County, California. Thiscounty borders southern Oregon, an area hosting

GEMS & GEMOLOGYFALL 2023

Figure 1. Faceted Oregon sunstone crystals (2.75–7.71 ct) representing the variety of colors observed. Photo by
Adriana Robinson. Courtesy of the GIA Museum. Gifts of Western Ventures (pear shape), Dust Devil Mining Co.
(oval), and John and Laura Ramsey (emerald cut on the far right).

many deposits known to produce gem-quality copper-included feldspars. Although many of the gem
feldspars from Oregon that display red to bluishgreen colors do not technically fit the definition of“sunstone” due to their lack of reflective visiblesized inclusions, these copper-bearing gem feldspars
from Oregon are all known as “Oregon sunstone” inthe gem trade.

Copper inclusions in non-gem feldspar, thoughextremely rare, have been reported from localities
such as Miyake-Jima in Japan and the Pinacate volcanic field in Mexico (Rossman, 2011). Oregon is the
only verified occurrence of gem-quality copper-bearing sunstone. Similar gem materials have been re-ported from Ethiopia, though the deposit has notbeen independently verified (Kiefert et al., 2019; Sunet al., 2020). Moreover, copper has been shown to easily diffuse into feldspar, which can create rich red colors in originally colorless or light yellow crystals(Emmett and Douthit, 2009; Zhou et al., 2021, 2022).
This discovery was made after a large quantity of red
feldspar purportedly from Asia (Tibet) or Africa(Democratic Republic of the Congo) flooded the gem
market, sparking controversy regarding the origin
and authenticity of these gemstones (Rossman, 2011;
Abduriyim et al., 2011). Major and trace element
compositions can be used to separate copper-bearing
feldspar from different geological origins (Sun et al.,2020), and argon isotope analysis has been used totest high-temperature treatment (Rossman, 2011).
The diffused feldspars have much higher copper concentrations than natural Oregon sunstones of similarcolor intensities (Sun et al., 2020; Jin et al., 2022) andoften show unnatural zoning patterns (McClure,2009). While natural Oregon sunstone is straightforward to identify, there is not yet a simple and definitive way to confirm artificial copper diffusion infeldspar.
The aventurescence effect in Oregon sunstone is
obviously created by the copper flakes oriented along
the cleavage planes of the feldspar crystals, as they
can be observed optically under the microscope (oreven with the unaided eye). The origin of the bodycolor observed in some Oregon sunstone, on the

COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONEGEMS & GEMOLOGYIn Brief• Oregon sunstones owe their colors to both absorption
and scattering of light by copper particles.
• The absorbance of a scattering medium, such as a sunstone, can be measured using an integrating sphere.
• The pleochroism of Oregon sunstone results from theinterplay between the elongated copper particles andthe biaxial feldspar matrix.
• Color zoning in a copper-bearing feldspar is controlled
by diffusion.

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COLOR MAPS FOR SPHEROIDAL COPPER PARTICLES IN FELDSPAR
100

EQUATORIAL RADIUS (nm)80

Disk Shape(oblate)60

Football Shape(prolate)

40

20

0

0.5

1.0

2.0

0.5

1.0

2.0

ASPECT RATIO

Figure 2. Calculated color maps from the computed UV-Vis spectra of spheroidal copper particles with different
sizes (equatorial radius) and shapes (aspect ratio) under plane-polarized light. The particles of different sizes andshapes are schematically positioned on the maps, along with the polarization direction relative to the particles(marked with blue arrow).

other hand, is less obvious. Although the correlation
between the special colors and the copper content is
unmistakable, the exact mechanism by which copper can create these various colors had been elusive.
Hofmeister and Rossman (1985) first studied the origin of color in relationship to the copper content andcopper species in Oregon sunstone. They found thatthe red color in Oregon sunstone is the same as thecolor in red glass colored by copper/gold (“copper
ruby glass”), both of which are caused by the opticalproperties of metallic copper colloids. In fact, copperand gold nanoparticles have been used to produce red
glass for centuries, long before the physics and chemistry behind it was understood (Nakai et al., 1999;
Freestone et al., 2007; Ruivo et al., 2008). The moredesired green-blue colors of Oregon sunstone aremuch more puzzling, because no nanoparticle-colored glass counterpart with similar colors has been
manufactured. Hofmeister and Rossman (1983) first
hypothesized that anisotropic colloids could be causing the strong pleochroism often observed in greenblue Oregon sunstones, later supported by theobserved correlation between the copper particle orientation and the color change (Farfan and Xu, 2008),
as well as the direct transmission electron microscopy (TEM) observation of anisotropic copper
nanoparticles (Wang et al., 2019).

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COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONE

A recent study computed the optical properties ofspheroidal copper particles of various sizes and shapes
embedded in feldspar, using methods based on simplified solutions of Maxwell’s equations (Jin et al., 2022).
The results explained the colors and pleochroism observed in both natural and treated copper-bearing sunstones. Smaller copper particles mainly absorb blueand green light, which can create a vivid red color.
Slightly larger particles strongly scatter red and orange
light to allow more green and blue light to pass directly through the crystal. Intense pleochroism can becreated when elongated particles are aligned in thesame direction, with the absorption and scatteringmuch stronger when the polarization of the incident
light is along the longer direction of the particles. A
color map based on the computational results from Jin
et al. (2022) is provided in figure 2, showing a rather
dramatic change from red to green with increasing
particle size and aspect ratio (the ratio between thelong dimension and the short dimension of the particle). The copper nanoparticles absorb and scatter lightso effectively that only a small amount (~20 ppmw) of
exsolved metallic copper can produce saturated colors
in mostly transparent crystals. Thicker crystals with
denser numbers of copper particles would quickly
turn opaque, allowing only the red backscattered light
to be observed. Optical effects present in copper-bear-

GEMS & GEMOLOGYFALL 2023

TABLE 1. Sunstone samples used in this study.
Sample no.

Weight (ct)Locality

SourceCompositiona

Copper(ppmw)a,b

Color

SCH-001

11.78

Dust Devil mine,
OregonCollected by
Gabriela A. Farfan

An67Ab32Or1

15–60

Strong schiller (figure 5)

SB-001

5.88

Sunstone Butte
mine, Oregon

Collected by
Shane F. McClure

An68Ab31Or1

45–50

Strong red-green pleochroism(figures 6, 8, 9, 12)

A195

4.32

Ponderosa mine,
OregonCollected by
Shane F. McClure

An72Or28Or0

20–130“Watermelon” color with strong
schiller (figure 15)

A278

12.23

Dust Devil mine,
OregonCollected by
Shane F. McClure

An66Ab33Or1

2–100“Watermelon” color with slightschiller (figure 13)

A314

10.96

Dust Devil mine,
OregonMariana Photiou

An62Ab37Or1

5–120“Watermelon” color with moderateschiller (figures 13, 15)

A319

11.80

Dust Devil mine,
OregonMariana Photiou

An67Ab32Or1

5–100“Watermelon” color with slightschiller (figures 13, 15)

A110

0.66

Reportedly from
China

M.P. Gem Corp

An48Ab49Or3

500–700

Red-green pleochroism withzoning (figures B-2, 6)a

The compositions and copper concentrations of the samples were analyzed using LA-ICP-MS.
Detection limit = 0.015 ppmw.

b

ing feldspar can be further complicated by the crystal’s
low triclinic symmetry, resulting in complicated interactions between the light absorption/scattering by
nanoparticles and the optical anisotropy of the matrix,
the combination of which has not been studied in detail before. Given this newly published research about
the optical properties of the copper particles, we revisit
the colors and optical effects of Oregon sunstone inorder to provide a comprehensive explanation of thespecial properties of this unique gemstone.

MATERIALS AND METHODS
The provenance, composition, copper concentration,and color of the seven samples studied in this paper
are listed table 1. The chemical compositions and copper concentrations of the samples were analyzed witha laser ablation–inductively coupled plasma–massspectrometer (LA-ICP-MS), consisting of a Thermo
Scientific iCAP Qc ICP-MS connected to an Elemental Scientific Lasers NWR213 laser ablation system(frequency-quintupled Nd:YAG laser operated in Qswitched mode at a wavelength of 213 nm and pulse
duration of 4 ns). Standard glasses GSD-1G, GSE-1G,
and NIST SRM 610 were used for external calibration.
All samples were analyzed on a traverse from edge toedge through the center of the crystal, with a 55 μm
diameter laser beam size at a 20 Hz repetition rate and

COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONEa fluence (energy density) of ~12 J/cm2. No obvious
zoning of the major elements was observed; therefore,
only the average composition is reported. The copper
concentration showed various zonation and is listed
as a range in table 1.
All the samples were fabricated into wafers (notoptically oriented) with at least two parallel polishedsurfaces for imaging and spectrometry analyses. The
extinction spectra were collected on a PerkinElmer
Lambda 950 ultraviolet/visible (UV-Vis) spectrometer, whereas the absorption spectra were collectedwith a GIA UV-Vis spectrometer, which is constructed from an AvaSphere-50 integrating sphere, an
AvaLight-DS deuterium light source, and a QE Pro
high-performance spectrometer. Custom-blended
Laser Liquid from Cargille with a refractive index of
1.540 was used as an index-matching (immersion)liquid for orienting the feldspar crystals and measuring extinction spectra in different directions.

ABSORPTION AND SCATTERING BY
COPPER PARTICLESThe feldspars containing copper inclusions are either
colloids or suspensions, in which the copper particles
are dispersed inside the feldspar host as a separate
phase (see box A). The copper particles larger than a
few micrometers in size (copper suspension) are al-

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BOX A: DISPERSION SYSTEMS AND THEIR OPTICAL PROPERTIES
Due to the second law of thermodynamics, everythingin nature is a mixture of multiple components, resulting
from the tendency to maximize entropy. Even the purest
material created by humans, single-crystal silicon formaking computer chips, contains a few impure atoms
for every trillion silicon atoms. It is simply impossible
to make anything 100% pure. Most of the mixtures are
systems known as dispersions, in which distributed particles of one material are dispersed in a continuous phaseof another material. The two materials mixed in the dispersion system can be in identical or different states ofmatter (gas, liquid, or solid). The dispersions can be divided into three different categories (solution, colloid,and suspension) with very distinct optical properties,
separated by the sizes of the dispersed particles.
In a solution, the dispersed particles are at the atomic
scale, resulting in a thermodynamically stable homogenous system with one single phase. Common examples
of solutions are air, an alcoholic beverage, seawater, andmetal alloys. Most allochromatic gemstones (sapphire,
ruby, emerald, and alexandrite) are solid solutions withtrace amounts of color-absorbing ions (e.g., chromium,iron, or titanium) or point defects (vacancies) evenly dissolved in the crystal lattice of the gem mineral. Solutions
almost exclusively absorb light through electron transitions (excitation) involving the dissolved ions and molecules, which can result in a color for the transmitted
light (with back lighting or internal reflection in the caseof faceted stones). When light is shining from the side
(perpendicular to the viewing direction), the light beam
is invisible inside the solution (figure A-1) because nolight is deflected to the observer by the solution. Although Rayleigh and Raman scattering is theoreticallypossible in a solution, these effects are so weak that theycannot be observed unless on an extremely large scale(for example, Rayleigh scattering by the atmospherecauses the sky to appear blue).
In a suspension, the dispersed particles are at the micrometer scale or larger and optically visible (either underan optical microscope or without magnification). Suspensions are unstable heterogeneous mixtures, from which
the suspended particles will settle out of the (fluid) mixture
if left undisturbed for a prolonged period. Common examples of suspensions include muddy water, sandstorms, andlava flow. Minerals and gemstones with eye-visible inclusions are also suspensions, such as aventurine feldspar, rutilated quartz, “horsetail” demantoid, and star sapphire.
The larger particles in suspension behave similarly to bulk
macroscopic objects, which reflect and absorb light rays
mostly on their surfaces. Depending on the particle density, a light beam shining from the side can be outlined by
the illumined particles in the suspension, if not completely
blocked by the dense particles (figure A-1).
A colloid is a dispersion with particle sizes in between
those of solutions and suspensions. The particles in a colloid
are smaller than a couple hundred nanometers, which is
below the resolution of optical microscopy and cannot bedirectly observed using any optical device. However, thecolloidal particles (also called nanoparticles) are much larger
than atoms and small molecules, and they can strongly scatter light in all directions (Mie scattering). Therefore, thelight path in a colloid is illuminated by the uniform scat-

Figure A-1. Diagram showing light interaction with dispersion systems of different particle sizes. Only the particles in asuspension are large enough to be visually observed. When a beam of light is shining from the side, no visible effect canbe observed in a solution, but an illuminated path of light can be seen in a colloid and a suspension due to the scattering effect by the dispersed particles.
Solution

Colloid

Suspension

Invisible Particles
Invisible Light Beam

Invisible Particles
Visible Light Beam

Visible Particles
Visible Light Beam

Fiber-Optic Light302

COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONEGEMS & GEMOLOGYFALL 2023

Figure A-2. Sample A110, a copper-diffused feldspar from China, under darkfield (left) and brightfield (right) illumination.
The deep red color on the left is the color of the light scattered by the copper nanoparticles (Tyndall effect). The colors on
the right are from the light transmitted through the crystal (neither absorbed nor scattered). The red zone on the left perfectly matches the green zone on the right, whereas the red rim on the right is almost invisible on the left. This means thegreen color is mainly produced by the scattering of red light, while the red color is mostly from the absorption of the copper particles. Photos by Shiyun Jin; field of view 7.1 mm.

tering from the invisible particles, known as the Tyndall effect (figures A-1 and A-2, left). A colloid is a metastable heterogeneous system, meaning the particles will not settle out
when left undisturbed, but it can easily be destabilized if
the particles’ surface properties are altered (e.g., by adding
an electrolyte in the host phase). Common examples of colloids are clouds, smoke, milk, ink, and cranberry glass (redglass colored by copper/gold). Colloid gemstones (those containing colloid particles, such as cloudy sapphire, fancy
white diamond, and opal) often appear translucent due tothe scattering particles. The metallic copper colloid in red
copper-bearing feldspar has stronger absorption and weaker
scattering than dielectric colloid particles, thus showing asolution-like appearance similar to other colored stones(note that the red rim in figure A-2, right, is almost invisible
in figure A-2, left). The color of a colloid is also more complicated because the color from the scattered light and thetransmitted light are often complementary to each other,making the observed color highly dependent on the type of
illumination (back lighting, darkfield illumination, fiberoptic lighting) and the viewing direction.
When light travels through an absorbing solution, its
intensity is absorbed following the Beer-Lambert law,with the absorbance (Abs) proportional to the path
length in the medium (l), the molar absorption coefficient (ε), and the concentration (c) of the absorber:
Abs = εlc

(A-1)

in which the absorbance is defined as the negative logarithm of transmittance (T), where T is the fraction of unabsorbed light intensity, I0 is the incident light intensity,and IA is the absorbed light intensity:
I –I
Abs = – log T = – log 0 A
I0

COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONE(A-2)

In a scattering (non-absorbing) medium (colloid or suspension), the scatterance1 (Sca) can be defined in the same way
using scattered intensity (Is):I –I
Sca = – log 0 S
I0

(A-3)

For a medium that both absorbs and scatters light, thetotal effect of absorption and scattering is called extinction (Ext, or attenuation), for which the quantity of extinctance can be defined following the Beer-Lambert law:
Ext = –logT = Abs + Sca(A-4)

Note that transmittance (T) is the quantity that canbe easily measured, which means extinctance is the mostdirectly analyzed. In non-scattering solutions, which isthe case for most colored gemstones, extinctance is thesame as absorbance. The scattering effect, on the other
hand, is much harder to measure directly, which requires
specially designed instruments (e.g., Sullivan and Twardowski, 2009) and has only been successfully done on liquid suspensions (e.g., seawater) and jet streams of small
particles. Fortunately, the absorbance of a scattering
medium, though not as easy to measure as extinctance,
can be measured using an integrating sphere (Elterman,
1970; Fry et al., 1992; Nelson and Prézelin, 1993; Mignani
et al., 2009), which blends the scattered light together
with the transmitted light. Therefore, scatterance is best
quantified by subtracting absorbance from extinctance.

1

This term (along with “extinctance” in the next paragraph) is uncommon because there are very few studies on the scattering properties ofnanoparticles.

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303

A

+
e–

e–

+

+

e–

B

most all aligned along the cleavage planes of thefeldspar host, which means their shiny surfaces would
reflect light simultaneously to create the schiller effect. However, if the copper particles are smaller thana couple hundred nanometers in size (copper colloid),
they cannot be directly resolved optically. The copper
nanoparticles, unlike the larger inclusions with flaky
shapes, are mostly spherical or spheroidal, as directly
observed by TEM (Wang et al., 2019). This is because
the system always tries to minimize the excessive interfacial energy by reducing the surface area of thenanoparticles. The smaller the particle size, the larger
the relative surface area (surface area per unit
mass/volume) and the closer the particle shape is to asphere. The anisotropy of the interfacial energy (withthe anisotropic feldspar host) may distort the nanoparticles to spheroid or ellipsoid shapes.
The interaction between light and copper
nanoparticles is quite complicated. The free electrons in metallic copper—what make copper a very
good conductor of heat and electricity—are pushed
back and forth from one end of the particle to theother by the incident light (oscillation of the electricand magnetic field) at extremely high frequencies(the frequency of visible light is ~400–800 terahertz,
or 4–8 × 1014 cycles per second) (figure 3A). Therefore,
the free electrons in the copper particles can absorb

304

COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONE

Figure 3. Schematic diagramof light (electromagneticwave) interacting with copper (or other metal) nanoparticles. A: Free electrons in
the nanoparticles oscillate
under the force from an oscillating electric field, which
absorb the energy of the incident light and transform it
into heat. B: The bouncing
electrons confined by theparticle boundary create hot
spots of electric field on thesurface of the particles,
which enhances their scattering power. Particles muchsmaller than the wavelengthof the incident light behave
as oscillating dipole moments (left), whereas larger
particles that extend morethan one spatial period of
the incident light show
more complicated multipole
behavior (right).

the energy from incident light and transform it into
heat (kinetic energy of the electrons). This absorption
is the strongest when the frequency of the light coincides with the intrinsic frequency of the free electrons—an effect known as localized surface plasmon
resonance (LSPR) (Petryayeva and Krull, 2011)—creating a peak in the UV-Vis absorption spectrum thatis strongly dependent on the size and shape of thecopper particle. The oscillating free electrons constrained by the particle boundary create hot spots ofelectric field on the surface of the nanoparticle,
which would enhance the scattering power of thecopper particles. Particles much smaller than the
wavelength of the incident light behave as oscillating
electric dipoles, whereas larger particles that extend
more than one spatial period of the incident lightshow more complicated multipole behavior (figure3). The oscillating electrons also emit light (at thesame frequency as the incident light) that contributes
to the light scattered by the copper particle. The
metal nanoparticles are sometimes referred to as “optical nano-antennas” due to their electromagnetic
resonating properties (He et al., 2009), and thestrongly concentrated electromagnetic field (at theparticle surface) is often used to enhance signals froma small region in spectroscopic analyses such as
Raman spectroscopy (Hossain et al., 2009) or laser-

GEMS & GEMOLOGYFALL 2023

induced breakdown spectroscopy (LIBS) (Palásti et al.,2020).
The extinctance of the copper particles in Oregon
sunstone can be measured easily using a regular UVVis spectrometer by comparing the intensity of the attenuated light (transmitted through the medium) to
the unattenuated intensity (directly from light source
to detector) (figure 4A). However, the extinctancemeasured this way is not the same as absorbance, but
the sum of absorbance and scatterance. The true absorbance needs to be measured inside an integrating
sphere, with the detector positioned away from the direct path of the incident light (figure 4B). An integrat-

ing sphere is an optical device consisting of a hollow
spherical cavity with its interior covered with a diffusely reflective coating. Light entering the integrating
sphere is reflected and scattered multiple times before
leaving through another port not in line with the incident light, which preserves the power of the lightbut destroys any directional or spatial information.
When a scattering material is placed inside an integrating sphere, both the transmitted light and scattered light are blended together before entering thedetector (figure 4B), which means the measured extinction (by comparing the detected intensity withand without the scattering crystal) is only due to the

Figure 4. Schematic configuration for measuring extinction (A) and absorption (B) spectra of a scattering crystal.
The to-scale configuration used in this study is shown in C.

ACScattered Light
Light
SourceIncident
Light

TransmittedLight
Absorption
Detector

Scattered Light
Film polarizer

B

Fiber-Optic Light

Scattered Light
Light
SourceIncident
Light

CrystalTransmittedLight

Absorption

Scattered Light

COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONEIn

Detector

te

gr

at
in
g

Sp

he

re

BaffleGEMS & GEMOLOGY

Optical Fiber toSpectrometer

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absorption of the crystal. It should be noted that, unlike the direct path configuration (figure 4A), light in
the integrating sphere can be absorbed multiple times
by the crystal itself, which may result in a higher absorbance depending on the relative size of the crystal
to the integrating sphere. To minimize these errors,
the integrating sphere used for optical measurement
should be much larger than the measured sample. The
to-scale illustration of the setup used in this study isshown in figure 4C. Once the total extinctance andabsorbance are measured, scatterance of the copper
particles in Oregon sunstone can be calculated by subtracting absorbance from extinctance.
The extinction spectrum (Ext, measured using theconfiguration in figure 4A) and absorption spectrum(Abs, see figures 4B and 4C) of sample SCH-001 areshown in figure 5. Because the micron-scaled copper
inclusions in the schiller sunstone are large enough toblock visible light of all wavelengths evenly, the extinction spectrum has a shape similar to a copper-freefeldspar but elevated along the vertical axis. Because
almost all the flaky copper inclusions are oriented in
the same direction, the reflectance (backscattering)
spectrum can also be directly measured. The measured reflectance spectrum (Ref) is the same as bulk
copper metal (as expected), which accounts for mostof the scattering by the copper inclusions. Therefore,
the scattering spectrum (Sca) obtained from subtracting the absorbance from extinctance has a shape similar to the reflectance spectrum of copper, with red andorange light (>570 nm) preferably reflected (scattered)and blue/green light (<570 nm) absorbed. Note that reflectance is the intensity ratio (in percentage) between
the reflected light and the incident light, whereas scatterance (by subtracting absorption from extinction) is
the logarithm of the intensity ratio, which is why theycannot be plotted on the same scale and have slightly
different shapes.
The absorption and scattering spectra of colloidal
copper particles in feldspar can also be measured thisway. As shown by the computational results from Jin
et al. (2022), strong pleochroism can be created byelongated copper nanoparticles aligned in the same
direction. To measure the scattering power with different polarization, a film polarizer is placed between
the fiber-optic light and the feldspar crystal (again,
see figure 4C). The experimentally measured spectra(figure 6) of two pleochroic copper-sunstone samples—sample A110 (treated) and sample SB-001 (natural)—match the computational result well. The redcolor is almost exclusively from absorption, withminimum scattering (extinction is almost the same

Figure 5. Left: The extinction (Ext), absorption (Abs), scattering (Sca), and reflection (Ref) spectra of sample SCH-001.
Right: The schiller effect of the sample from micron-scaled copper inclusions under reflective light. The scatterance iscalculated by subtracting absorbance from extinctance. The reflectance (in percentage) is the fraction of light beingreflected by the copper particles and thus cannot be plotted on the same scale as extinctance, absorbance, and scatterance, which are logarithms of intensity ratios. Photomicrograph by Shiyun Jin; field of view 13.5 mm.

UV-VIS-NIR SPECTRA
2.0

80
Ext
Abs
Sca

60

1.0

40

0.5

20

0

400

500

600

700

800

REFLECTANCE (%)

ATTENUANCE

1.5

Ref

0

WAVELENGTH (nm)306

COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONEGEMS & GEMOLOGYFALL 2023

UV-VIS-NIR SPECTRA
2.0

2.0

B

ATTENUANCE

A
1.5

1.5

1.0

1.0

0.5

0.5

0

0

2.0

2.0

D

ATTENUANCE

C
1.5

1.5

1.0

1.0

0.5

0.5

0

400

500

600

700

800

0

400

WAVELENGTH (nm)

500

600

700

800

WAVELENGTH (nm)
Ext

Abs

Sca

Figure 6. Polarized extinction (Ext), absorption (Abs), and scattering (Sca) spectra of sample A110 (A and B) andsample SB-001 (C and D). The stone color (with polarization direction marked by arrows) corresponding to each
spectrum is shown alongside the plotted spectra. A red color in sunstone is mostly created by absorption, and agreen color gets significant contribution (if not all) from scattering, as predicted by the computational results fromJin et al. (2022). Photomicrographs by Shiyun Jin; fields of view 5 mm (A and B) and 20 mm (C and D).

as absorption in figure 6, A and C), whereas the green
color gets significant contribution from the scattering of red light (λ >600 nm) (figure 6, B and D). Notethat the absorption spectrum for the polarization direction in which the sample appears green (figure 6B)
is very similar to that corresponding to the polarization direction in which the sample appears red (figure
6A), with an LSPR peak at ~570 nm followed by adrop-off (less steep in figure 6B than in 6A), which ischaracteristic of particles with a radius of ~35 nm
and an aspect ratio of ~1.3 (Jin et al., 2022). Sample
SB-001, on the other hand, shows an absorption peakat 600 nm that is enhanced by the scattering peak at
625 nm, adding up to an exceptionally strong peakin the extinction spectra at 613 nm (figure 6D). This
indicates a smaller particle radius (~20 nm) with ahigher aspect ratio (~1.7) (Jin et al., 2022).

COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONEGEMS & GEMOLOGYPLEOCHROISM OF OREGON SUNSTONE2
As shown by the computational results from Jin etal. (2022), a higher aspect ratio of the copper or othermetal nanoparticles produces stronger pleochroism.
In an optically isotropic medium such as glass, the2

The scattering effect is not considered in this section. Therefore, only
terms related to absorption are used for simplicity, even though it isextinction being measured in the experiments.

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transmittance (T) of light polarized at an angle of θ
relative to the longer direction of the copper particles(assuming they are all oriented vertically) is an average of vertical (long direction of the particle, l) andhorizontal (short direction of the particle, s) transmittance (figure 7A):
T(θ) = Ts sin2 θ +Tl cos2 θ = 10–As sin2 θ + 10–Al cos2 θ (1)This is because the vertical and horizontal components of the incident light are absorbed independently by the particle. Therefore, the absorbance (A)as a function of angle θ (figure 7, A and B) is:–As

A(θ) = –log(10sin θ + 10
2–Al

cos θ)
2(2)

But in an optically anisotropic medium, thepleochroism (maximum and minimum absorption)
for a certain orientation is determined by the opticalA

orientation of the host crystal (feldspar in the case ofOregon sunstone) instead of the orientation of thelight absorber (copper particles) (Libowitzky andRossman, 1996). Light inside an anisotropic crystal
is split into two perpendicularly polarized light rays
following the directions of maximum/minimum refractive index (n1/n2) (the dashed arrows in figure 7C),
which are absorbed (attenuated) independently. The
absorbance is thus dependent on the angle α (between polarization direction and n2) instead of θ (figure 7, C and D):
A(α)= –log(10–A1 sin2 α +10–A2 cos2 α)

The absorbance A1 and A2 (along n1 and n2), on theother hand, are determined by the angle β between
the long dimension of copper particles and n2 (figure
7C):

0°

B

θ
270°

90°

180°

n

C

D

2

0°

27
0°

β

α

n

1

90
18
0

308

COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONE(3)°

GEMS & GEMOLOGY°

Figure 7. A: Plane-polarized incident light (at anangle of θ with the long
direction of the copper
particle) in an isotropic
medium can be decomposed into vertical andhorizontal components(dashed arrows) that interact with the particle
independently. B: The
angular absorption figure (the polar plot of absorbance against θ at a
certain wavelength) corresponding to the interaction shown in A. C:
Plane-polarized incident
light in an anisotropic
medium is split into twopolarized rays following
the elliptical section of
the optical indicatrix.
The copper particles act
on each ray independently. D: The orientationof the angular absorption figure of ananisotropic medium isdetermined by the optical orientation of thematrix instead of theorientation of the copper
particles.

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The orientation of the copper particles cannot bedirectly extracted from the angular absorption figures, as it is masked by the optical anisotropy of thefeldspar. It also means the pleochroism of ananisotropic crystal is generally smaller than theanisotropy of the absorber (figure 7, B and D). The
angle β determines the shape of the angular absorption figure but does not change its orientation. (Toview how the angular absorption figure changes withβ, go to www.gia.edu/gems-gemology/fall-2023oregon-sunstone-effects and see videos 1 and 2.)
The “absorption ellipsoid” (Dowty, 1978; Sambridge et al., 2008) of a triclinic crystal is completely
independent from its optical indicatrix, because there
is no symmetrical constraint on either of them by the
crystal structure. Although the optical indicatrix canbe easily determined, the orientation of the absorber(absorption ellipsoid) cannot be independently measured due to the splitting and polarization of light rays
inside the crystal (figure 7, C and D). In theory, theorientation of an anisotropic absorber in a triclinic
medium can be determined by reconstructing the absorbance ellipsoid from the absorption coefficient of
six independent directions (Dowty, 1978). In practice,
however, such reconstruction is very difficult (if not
impossible), as it requires complete deconvolution of
the absorption spectra into independent absorption
bands with accurate baseline corrections. Therefore,
the absorber orientation (absorption ellipsoid) has
never been quantified for any triclinic crystals. Moreover, the absorption spectra of copper particles cannot
be simplified as combinations of several independent
bands with fixed peak widths and positions, because
the absorption/scattering peak continuously changes
with different particle sizes and shapes. Nonetheless,
it is still possible to determine the orientation of thecopper particles in a strongly pleochroic Oregon sunstone, as the intense absorption peak at ~600–630 nm
only occurs when light is polarized along (or at aslight angle with) the long direction of these particles.
The optical orientation (principal axes X, Y, and Z
of the optical indicatrix) of the feldspar crystal needs
to be determined first, because the polarized absorption spectrum is only meaningful when measured
along special optical directions. This can be achieved
by locating the two optic axes using an optical orienting device (Thomas et al., 2014). The Y-axis is perpendicular to the optical plane (a plane containing two
optic axes), whereas the X- and Z-axes are the bisectorsof the two optic axes. Sample SB-001 mounted on a
dop along the Z-axis of the optical indicatrix (with theX-axis facing downward) is shown in figure 8A, alongwith the relative orientation between the optical indicatrix (white ellipsoid) and the crystallographic axesof the feldspar (figure 8, C and D). The orientation of
the optical indicatrix agrees with the composition following the stereographic plot by Smith and Brown(1988, p. 200). Similar to most Oregon sunstone crystals, sample SB-001 is tabular shaped, with the table
face parallel to the (010) cleavage/twin plane (Stewart
et al., 1966). The (001) cleavage is relatively poorly developed but still recognizable on one side of the crystal(figure 8A). Note that even with the two cleavage
planes (010) and (001) correctly identified, though they
are not always obvious, the direction of the a-axis stillcannot be unambiguously determined (there are two
possibilities for positive and negative direction) without any developed crystal faces (other than the cleavages). Without using X-ray diffraction, measuring theoptical orientation of the crystal is necessary to determine its crystallographic axes.
When the polarization plane of the incident light(red plane in figure 9A) is perpendicular to the X-axis(vertical blue line), the polarization will remain unchanged inside the crystal (immersed in an indexmatching liquid to avoid irregular surface refraction).
This property allows the absorbance to be measured
along any direction perpendicular to the X-axis. Here
we define the longest axis of the copper particles as
P (black line in figure 9A), the angle between P and
the X-axis as γ, and the angle between the P-X plane(yellow plane) and the plane normal to the incident
light (green plane) as ϕ. As the crystal is rotatedaround the X-axis (perpendicular to the polarization
plane), the projection of the copper particle along thehorizontal direction (polarization direction marked as
red arrow in figure 9B) is dependent on the angle ϕ,which is maximized at ϕ = 0° and minimized at ϕ =COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONEGEMS & GEMOLOGY

A1 = Al sin2 β + As cos2 β(4)

A2 = Al cos2 β + As sin2 β(5)Note that absorbance is used in equations 4 and 5
rather than transmittance, as seen in equation 1. Thisis because the two polarized rays inside the crystal(corresponding to n1/n2) are constrained by the opticalanisotropy of the medium and cannot be further decomposed. (The assertion by Sambridge et al. [2008]
that absorbance can be simply replaced by transmittance in all equations is incorrect, as transmittance isused if and only if there are multiple rays or components being attenuated independently.) Combining
equations 3, 4, and 5 results in:
A(α)= –log(10 –Al sin β–As cos β sin2 α + 10 –Al cos β–As sin β cos2 α) (6)
2

2

2

2

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A
c

)
01
(0

b

(0
10
)

a

C
Z

(0
10
)

)
01
(0

B

(001)
Y
X

Z
Y

Figure 8. A: Sample SB001 mounted on a dop(behind the sample)
along the Z-axis of the
optical indicatrix (withthe X-axis facing downward). The crystallographic axes of thecrystal and the twocleavage planes are labeled. Photo by Shiyun
Jin. B: Diagram of thefeldspar crystal (withideal crystal habit) inthe same orientation as
in A, showing the orientation relation between
the crystallographic
axes and the optical indicatrix of sample SB001. C: The sameorientation relation asin B but viewed from amore general direction,
similar to figure 8.8 in
Smith and Brown(1988).

c

c

a

b

a

90°. (To view the full rotation around the X-axis,
along with the corresponding particle projection, seevideo 3.) The color change during the rotation is more

310

(01
0)

X

COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONEb

dramatic when P is closer to the horizontal plane (γangle closer to 90°). The images in figures 9C and 9D
show the two extreme colors (green at ϕ ≈ 0° and red

GEMS & GEMOLOGYFALL 2023

A

P

B

X

nα

ϕ

γ

n2

C

ϕ ≈ 0°

Dϕ ≈ 90°

X
X

Y

Z

Y

Z

Figure 9. A: When the feldspar crystal is rotated around the X-axis, the horizontally polarized incident light (redplane perpendicular to the X-axis) does not change polarization inside the crystal, while the longest axis of the copper
particle (P-axis) would revolve around the X-axis. The angle between P and the X-axis is defined as γ, and the angle
between the P-X plane (yellow plane) and the plane normal to the incident light (green plane) is defined as j. B: Diagram showing the projection of a copper particle along the incident light direction as it revolves around the X-axis(vertical) during the rotation. The absorbance for the horizontally polarized light (red arrow) is determined by the
projection of the copper particle along the horizontal direction, which is maximized at j = 0° and minimized at j =90°. C: Photo of sample SB-001 with maximum absorption at j ≈ 0° showing deep bluish green color under horizontally polarized light. D: Sample SB-001 rotated 90° and showing red color with minimum absorption. Photos by
Shiyun Jin.

at ϕ ≈ 90°) during the rotation around the X-axis
under horizontally polarized light (to see the color
change during the full rotation, see video 4). The P-

X plane can thus be determined by the position of
maximum (ϕ = 0°) and minimum (ϕ = 90°) absorption. The same measurement can be performed by

COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONEGEMS & GEMOLOGYFALL 2023

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UV-VIS-NIR SPECTRA
2.0

A

6°
51°
96°

ATTENUANCE

1.5

1.0

0.5
Horizontal polarization
0
2.0

B

6°
51°
96°

ATTENUANCE

2.0

2.0

1.5
Vertical polarization
0
400

500

600

700

800

WAVELENGTH (nm)Figure 10. A: UV-Vis-NIR absorption spectra of sample SB-001 under horizontally polarized light corresponding to the positions in figures 9C (maximum
absorption, 6°), 9D (minimum absorption, 96°), andhalfway in between (intermediate absorption, 51°).
The angles 6°, 51°, and 96° indicate the angle between
the incident light and the Z-axis for each spectrum. B:
Absorption spectra of sample SB-001 under verticalpolarization at the same positions as in A. Note thatthe intermediate absorption spectrum (51°) is a direct
average of the maximum (6°) and minimum (96°) absorption spectra, instead of the negative logarithm of
averaged transmittance.

rotation around the Z-axis to determine the P-Z
plane, and the orientation of the copper particles P
can be determined as the intersection of the P-X and
P-Z planes. It should be noted that measurementwith rotation around the Y-axis should be avoided
because incident light at a slight angle with the optic
axis may result in incompletely polarized rays andirregular refraction (Sambridge et al., 2008).

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COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONE

The absorption spectra of sample SB-001 with polarization perpendicular and parallel to the X-axis (atthree different angles between the incident direction
and the Z-axis) are shown in figure 10, measured
using the same configuration in figures 9C and 9D.
The maximum and minimum absorption occur
when the incident light is at an angle of 6(±3)° and96(±3)° with the Z-axis, indicating the P-X plane (redplane in figure 11) is at an angle of ~6° with the X-Yplane (~96° with the X-Z optical plane). Note that the
spectra with polarization parallel to the X-axis barelychange with rotation of the crystal; this is expected
because the projection of the copper particles along
the vertical direction does not change during rotationof the crystal (figure 9B and video 3). It should also
be noted that the minimum absorption spectrum infigure 10A (96°) does not show any shoulders at ~610
nm, agreeing with the computational results by Jin
et al. (2022). This indicates that the shoulder of theabsorption spectrum in figure 6C is contributed by
the long direction of the copper particles (Al) resulting from a small β angle in equation 6. The spectrum
under vertical polarization is also very similar to the
6° spectrum under horizontal polarization (figure
10A), indicating the γ angle (figure 9A) between P
and X is close to 45° (β = γ when ϕ = 0°). Performing
the same measurements around the Z-axis results inan angle of ~135° between the P-Z (blue plane in figure 11) and X-Z planes. The intersection of the P-X
and P-Z planes aligns well with the a-axis of thefeldspar crystal (figure 11). This makes sense because
the (010) and (001) cleavage planes have the weakest
connections (chemical bonds) in the feldspar structure, which means the copper particles encounter the
least resistance when growing along the intersectionof these two planes (a-axis). It is also possible that afaster copper diffusion rate along the a-axis contributes to the particle elongation. As the particles
get larger, one of the cleavage planes dominates thegrowth, resulting in flaky inclusions that create theschiller effect (figure 5).
If the crystal is viewed along P (see figure 9A),
which has the smallest projection of the copper particles, only red color would be observed (nopleochroism) because the absorption is always minimal no matter the polarization of the incident light.
On the other hand, viewing perpendicular to P does
not guarantee maximum absorption or pleochroism
because the polarization of light inside the crystal iscontrolled by the optical anisotropy of the feldspar
matrix (figure 7). The maximum pleochroism occurs
only when the polarization of light inside the

GEMS & GEMOLOGYFALL 2023

B

A

Z

Z

X

X

Y

Y

c

a

c

b

a

b

Figure 11. The orientations of the P-X (red) and P-Z (blue) planes of sample SB-001 relative to its crystallographic
and optic axes, viewed down the crystal’s a-axis (A) and b-axis (B). The intersection of the P-X and P-Z planes (P,thick black arrow in B) aligns perfectly with the a-axis, which is also the intersection of the two cleavage planes.
The cyan plane in A marks the direction of maximum pleochroism, with the elliptical section perfectly alignedwith the copper particles.

feldspar crystal is perfectly parallel or perpendicular
to P (β = 0°/90° in equation 6 and figure 7C). This
special optical direction can be found easily by total
optical extinction between crossed polarizers, because the incident light (horizontal plane in figure12A) polarized perpendicular to P would be completely extinguished by the analyzer (gray plane infigure 12A, with vertical polarization parallel to P)
after the crystal only when the elliptical optical section is aligned with P. Images of sample SB-001
mounted on a dop along the a-axis (particle orientation P) between cross polarizers at two different positions are shown in figure 12, B and C, along withthe schematic relations between the particle orientation and the optical sections. (To view the crystal
going in and out of complete extinction during thefull rotation, see video 5.) The orientation of the optical section that creates complete extinction in figure 12D is marked as the cyan plane in figure 11A,
which is at an angle of ~30° with the (010) cleavage.
This is the orientation with the strongest pleochroism, as shown by the polarized extinction spectra in
figure 12D. This method can be used to maximize(or minimize) the pleochroism while faceting Oregon sunstone crystals, as the copper particles should

mostly elongate along the a-axis (the easiest direction to identify in a rough feldspar crystal), eventhough the exact orientation of the section withmaximum pleochroism may vary depending on thecomposition of the feldspar.

COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONEGEMS & GEMOLOGYCOLOR ZONING IN OREGON SUNSTONE
Another special feature commonly observed in Oregon sunstone is color zoning. Unlike most coloredstones, in which the color intensity is directly proportional to chromophore concentration, the colors
and color intensities of copper feldspars are strongly
dependent on the density, size, and shape of the copper particles (Jin et al., 2022), which are not simply
correlated with the concentration of copper in the
crystal. The treated (copper-diffused) feldspars from
China (e.g., sample A110) contain much more copper than natural Oregon sunstone of similar (if notstronger) colors (table 1) (Jin et al., 2022). Thismeans the color zoning in Oregon sunstone, controlled by multiple factors (copper concentration,
defects, temperature/cooling rate, and oxygen fugacity), is much more complicated than the zoning inother gemstones.

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A
P

Po
lar
ize
r
Cr
ys

tal

UV-VIS-NIR SPECTRA
2.0

D

B

ATTENUANCE

1.5

Verticalpolarization
Horizontal
polarization

1.0

C
0.5

0

400

500

600

700

800

WAVELENGTH (nm)Figure 12. In A, an arbitrarily oriented feldspar crystal splits the horizontally polarized incident light into two perpendicularly polarized beams of light (traveling at different speeds inside the crystal); the phase shift due to the
speed difference allows the light to partially pass through the vertical polarizer positioned after the crystal (grayplane). However, if the optical section of the feldspar is aligned with the horizontal and/or vertical direction, the incident light remains horizontally polarized inside the crystal (as in figure 9), which would be completely extinguished by the vertical polarizer. Images of sample SB-001 between cross polarizers (horizontal and vertical),rotated around P (a-axis) at two different positions (non-extinction and extinction), are shown in B and C, respectively, along with the schematic relations between the copper particles and the optical sections. Polarized (verticaland horizontal) UV-Vis-NIR absorption spectra corresponding to the extinction position in C are shown in D,
which represents the maximum possible pleochroism that can be observed in sample SB-001.

The most recognizable zoning pattern of Oregon
sunstone is the “watermelon” color, with a red-colored core followed by a thin green-colored (pleochroic)

314

COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONE

rim, before turning near-colorless close to the surfaceof the crystal. Images of Oregon sunstone crystals
with typical “watermelon” colors under transmitted

GEMS & GEMOLOGYFALL 2023

light (brightfield illumination) are shown in figure 13.
The color zones in Oregon sunstone have smoothtransitions and curved boundaries (e.g., Rossman,
2011; Xu et al., 2017), with the shape roughly following the irregular shape of the feldspar crystal (or cracks
inside the crystal; e.g., sample A319 in figure 13). The
colorless zone near the surface of the crystal is often
wider than the colored zone.
The color zoning in almost all other gemstones,
such as ametrine or “watermelon” tourmaline, is adirect reflection of the changes in chemical composition (chromophore concentration) of the crystal
across the different zones, which are controlled byeither the conditions (chemistry, temperature, pressure) during crystal growth (i.e., growth zoning), or
the crystal chemistry of the zoned mineral itself (i.e.,
sector zoning). The light-absorbing elements (e.g.,iron, titanium, chromium, vanadium, or manganese)

typically diffuse very slowly in crystals, and thus any
abrupt changes in their concentrations (sharp color
zone boundaries) can only be created during crystal
growth. Gradual changes in the chemistry of thefluid that feeds the crystal growth would create thesmooth color changes that are often observed insome elongated crystals (e.g., bicolor tourmaline).
Slow diffusion of the chromophores after crystallization can gradually smooth out some of the initiallysharp boundaries over thousands or millions of years
in certain geological settings, which is often used toestimate the cooling rate or annealing time of chemically zoned minerals. Whether sharp or smooth, thecolor zoning in gemstones other than Oregon sunstone almost always follows the crystal habit of themineral (i.e., concentric/radial hexagonal zones withsharp corners or along the c-axis in elongated crystal
form).

Figure 13. Three Oregon sunstone crystals show typical “watermelon” colors from two different directions. The
third direction of each sample only displays a light red color with no obvious zoning and is therefore not shownin the figure. The cracks in sample A314 were accidentally created during sample preparation (which is why it
did not affect the color zoning as in sample A319). The dashed line on sample A314 indicates the position of the
LA-ICP-MS analysis traverse. Photos by Shiyun Jin.

A319
A278
A314

COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONEGEMS & GEMOLOGYFALL 2023

315

COPPER CONCENTRATION PROFILES
300

A314

Cu CONCENTRATION (ppmw)

Schiller
200

100

0
2

4

6

8

2

3

4

900

Cu CONCENTRATION (ppmw)

A110

Figure 14. Copper concentration profiles
across the different colorzones in natural Oregon
sunstone (top, sample
A314) and diffused sunstone (bottom, sampleA110) from LA-ICP-MS
analyses. The corresponding color changes along
the profiles are drawn inthe background. The position of the line analysis on sample A314 ismarked in figure 13 withthe dotted line.

600

300

0
0

1

DISTANCE (mm)The unique zoning characteristics of Oregon sunstone are clearly not growth controlled, with no correlation to the common crystal habit of feldspar.
Copper has been shown to diffuse in feldspar extraordinarily rapidly (Emmett and Douthit, 2009; Audétat

316

COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONE

et al., 2018; Zhou et al., 2021, 2022), which means theconcentration profile of copper in feldspar may change
noticeably within weeks or even days at magmatic
temperatures. The “watermelon”-colored Oregon
sunstones have most of their copper near the core of

GEMS & GEMOLOGYFALL 2023

the crystal and almost no copper at the surface, as
shown by the LA-ICP-MS analysis results for sample
A314 in figure 14. However, the color zoning appears
to contradict the copper concentration profile, as the
red color corresponds to a smaller copper particle size
than the green color, as shown in figure 2. This apparent anomaly can be resolved with a closer look at the“watermelon”-colored Oregon sunstone, revealing thelarger micron-scaled particles with schiller effect in
the red-colored cores (figure 15). The copper would

precipitate first at the core of the crystal with higher
copper concentrations, which had longer growthtime to form larger particles than the zones closer tothe surface with lower copper concentrations. Mostof the copper particles in the green zone stopped
growing at intermediate sizes (~100 nm in diameter)
before they became large enough to produce schiller.
Smaller particles that produce a red color (<80 nm indiameter) would precipitate at a later stage, presumably due to further cooling. The larger particles with

Figure 15. “Watermelon”-colored Oregon sunstone crystals all show larger flakes of copper inclusions at the core,creating the schiller effect at certain orientations that is most obvious under an optical microscope. A: Sample
A314 in transmitted light (brightfield illumination). B: The same area of sample A314 under reflected light; thered-colored core shows much stronger reflection (schiller) than the green-colored rim. C: The copper flakes insample A195 noticeably decrease in size from the red core to the green rim before disappearing in the clear edge.
D: The flaky copper inclusions align along the cleavage planes in sample A319. Photomicrographs by Shiyun Jin;
fields of view 8.76 mm (A–C) and 4.38 mm (D).

A

B

C

D

COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONEGEMS & GEMOLOGYFALL 2023

317

schiller effect have a flat extinction spectrum (figure5) and therefore cannot mask the red color from thesmall particles in transmitted light. The intermediate particles that generate a green color, on the other
hand, have the strongest extinction power (Jin et al.,2022) and would overshadow the red color from any
small particles (figures 6, 11, and 13). The copper profile (figure 14) in the artificially diffused sample A110
(figure A-2), in contrast, is mostly flat with dips near
the very edge, showing no obvious correlation between the color zones and copper concentration.
Copper could partition into the feldspar phenocrysts during their growth from the basaltic
magma at depth. It has been hypothesized that copper in natural sunstone may have been introduced
through diffusion after crystallization (Rossman,2011). Because of the extraordinarily rapid diffusion
rate of copper in feldspar crystals, the original copper
distribution during crystal growth would be erased
by diffusion during cooling. Some theoretical inward
and outward diffusion profiles are illustrated in figure
16. With a long enough diffusion time, the two initially independent diffusion profiles from opposite
surfaces (figure 16, A and D) would meet at the center of the crystal (figure 16, B and E), after which theprofile would slowly approach a perfect sine function
with decreasing amplitude (half a wavelength; figure
16C). The concentration profile of sample A314 (figure 14) and other Oregon sunstones with “watermelon” color is most certainly created by prolonged
outward diffusion (figure 16E), starting from a moreor less homogeneous copper concentration throughout the crystal. It should be noted that once the copper in the feldspar starts to precipitate as
nanoparticles, they cannot move anymore, but can
only grow by absorbing the copper dissolved in thefeldspar lattice around it. This is why the copper profile of sample A314 in figure 14 follows a sine function (cyan curve) in the clear zone but deviates fromit in the colored zones.
Using the diffusion coefficients of copper in plagioclase feldspar measured by Audétat et al. (2018),
it can be roughly estimated that at least 100 days ofoutward diffusion at 1000°C are required to create
the profile in sample A314. The real time to create
the color zoning in sample A314 would be significantly longer, considering the time needed for copper
to diffuse (or partition) into the feldspar crystal, as
well as the significantly lower average temperature(and slower diffusion) during cooling. This is why thetypical “watermelon” colors seen in figure 13, showing a wide clear zone near the entire surface of the318

COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONE

stone and a red-colored core with schiller, have only
been found in natural feldspar. It is likely that the extraordinarily strong pleochroism of sample SB-001(figures 6, 9, and 12), indicating a higher aspect ratioof the copper particles, is also caused by slow coolingin natural basaltic lava flow.
In contrast, the copper profile of sample A110 indicates a complete inward diffusion of copper at hightemperature (homogeneous copper concentration
throughout the crystal) followed by a rapid cooling
period, resulting in the precipitation of a small number of copper particles (color zones) and a very shallow outward diffusion profile near the surface (clearrim). This is the case for most artificially copper-diffused feldspars, with a concentration profile similar
to the one in figure 16D. (The diffusion depth ofsample A110 is comparable to the size of the laser
beam and therefore not resolved in figure 14.) The
copper particles would precipitate slightly later near
the edge, which explains the “reverse-watermelon”
color commonly observed in copper-treated feldspars
with red rims outside green cores (McClure, 2009;
Rossman, 2011). Rossman (2011) reported unusual“watermelon” color (red core and green rim; figure15) in some artificially diffused feldspars, characterized by thin clear edges at the surface, often followedby thin red rims outside the green rings (similar tosample A110 in figure A-2, right), which are distinctively different from the natural “watermelon” Oregon sunstone. Incomplete inward diffusion (figure16C) followed by rapid cooling (figure 16D), which
would create a ring of peak copper concentration
with less copper at both the core and the edge (figure
16F), may explain the color profile of the treated“watermelon” feldspars. The red core in sample
A110 might be caused by the temperature or theoxygen fugacity gradient inside the crystal during
copper diffusion and precipitation, since no obvious
difference in copper concentration is detected relative to the green zone. Further study is needed to understand the detailed chemical reactions involved in
the copper diffusion and exsolution processes.
Schiller at the center of feldspar crystals (e.g., figures
5 and 15) has never been observed in artificially diffused feldspar, likely due to the much longer time it
would take for the larger copper particles to form.
Even if it were possible, producing these less desirable colors rather than vivid red and green colors
would not be economically sound. Some coppertreated feldspars showing a pinkish sheen at the surface are probably the result of failed diffusion
experiments.

GEMS & GEMOLOGYFALL 2023

CONCENTRATIONCONCENTRATIONCONCENTRATIONDIFFUSER ELEMENT CONCENTRATION PROFILES
L2
4D

D

tout = 0.01

tin = 0.1

L2
4D

E

tout = 0.1

tin = 0.5

L2
4D

F

A

tin = 0.01

B

C

L2
4D

L2
4D

L
Edge

Center

Edge

Edge

CenterEdge

Figure 16. Schematic concentration profiles of a diffuser element (with a diffusion coefficient of D) in a crystal(with a thickness of L) after various diffusion processes with different directions and times. A, B, and C show typical inward diffusion profiles (initial concentration in the crystal is zero) with increasing times (at t = 0.01L2/4D,
t = 0.1L2/4D and t = 0.5L2/4D, respectively). D and E show outward diffusion profiles starting from a homogeneous concentration (at t = 0.01L2/4D and t = 0.1L2/4D, respectively). Note that inward diffusion and outward diffusion are exactly inverse processes. The profile in F can result from an incomplete inward diffusion followed by ashort period of outward diffusion (C + D).

CONCLUSIONSThis paper has described and discussed in detail the
special color effects observed in Oregon sunstone (figure 17), including aventurescence (schiller), pleochroism, and color zoning. All the special optical effects ofOregon sunstone can be explained as combinations of
the properties described here. The scattering power ofthe colloidal copper particles in Oregon sunstone isquantified for the first time. The strong scatteringpeak at ~625 nm agrees with the computed opticalproperties of copper nanoparticles in feldspar (Jin etal., 2022). The long axis of the particle is measured to

be aligned along the a-axis in strongly pleochroic Oregon sunstones, which is expected from the weak connections along the (010) and (001) cleavage planes in
the feldspar structure. To our knowledge, this is also
the first time the anisotropic absorption (extinction)
property has been quantified (oriented) in any triclinic
mineral. The accurate orientation of the copper particles along with the optical indicatrix of the hostfeldspar would allow gem cutters to optimize the colorand optical effects while faceting Oregon sunstones.
The pleochroism in natural Oregon sunstone appears
much stronger than in copper-treated feldspars, indi-

COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONEGEMS & GEMOLOGYFALL 2023

319

Figure 17. A 31.12 ct
Oregon sunstone carving by Larry Woods.
Photo by Robert Weldon; courtesy of John D.
Woodmark, Desert Sun
Mining & Gems.

cating higher aspect ratios of the copper particles,
likely due to a slower cooling rate. More specimens
need to be tested to confirm this as a universal characteristic of natural pleochroic Oregon sunstone,
which could be used to identify natural stones without schiller. The concentration of copper in natural

320

COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONE

Oregon sunstones with “watermelon” color shows an
outward diffusion profile that takes at least a few
months (most likely years) to form, distinctly separating them from the artificially copper-diffused feldspars
with no obvious correlation between color and copper
content. The schiller at the core of the feldspar crys-

GEMS & GEMOLOGYFALL 2023

tals, created by copper flakes a few micrometers in sizeor larger, has been found exclusively in natural stones.
Further study on the dissolution and diffusion mech-

ABOUT THE AUTHORS
Dr. Shiyun Jin is a research scientist, Dr. Aaron Palke is senior
manager of research, Nathan Renfro is senior manager of coloredstone identification, and Ziyin Sun is a senior research associate,at GIA in Carlsbad, California.

anisms of copper in feldspar is necessary to create acomprehensive method of separating treated coppercolored feldspars from natural ones.

ACKNOWLEDGMENTS
The authors would like to thank Shane F. McClure, Mariana
Photiou, Gabriela A. Farfan, Huifang Xu, and M.P. Gem Corp forproviding the samples used in this study, along with their locality information. We also thank Christopher M. Breeding for his
assistance with the UV-Vis spectrometer.

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Dowty E. (1978) Absorption optics of low-symmetry crystals—Application to titanian clinopyroxene spectra. Physics and Chemistry of Minerals, Vol. 3, No. 2, pp. 173–181,
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Elterman P. (1970) Integrating cavity spectroscopy. Applied Optics, Vol. 9, No. 9, pp. 2140–2142, http://dx.doi.org/
10.1364/AO.9.002140
Emmett J.L., Douthit T.R. (2009) Copper diffusion in plagioclase.
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and X-ray diffraction study. Acta Crystallographica Section B:
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8, article no. 920, http://dx.doi.org/10.3390/min12080920

2023

CONGRATULATIONSThis year, readers from all over the world took the 2023 Gems & Gemology Challenge. Participants tested theirknowledge by answering questions listed in the Spring 2023 issue. Those who earned a score of 75% or higher
received a GIA Certificate of Completion. Participants who earned a perfect score are listed below.

G&G Challenge Winners
Australia

United States

Barbara Wodecki

Lo Combs
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Ana Lucia Tres

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Answers
See pages 92–93 of the Spring 2023 issue forthe questions.
1 (a), 2 (b), 3 (d), 4 (a), 5 (d), 6 (b), 7 (d), 8 (d), 9(b), 10 (c), 11 (b), 12 (a), 13 (b), 14 (c), 15 (a), 16(d), 17 (a), 18 (c), 19 (a), 20 (a), 21 (c), 22 (c), 23(b), 24 (d), 25 (b)GEMS & GEMOLOGYFALL 2023

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FEATURE ARTICLESETCH PITS IN HELIODOR AND GREEN BERYLFROM THE VOLYN PEGMATITES, NORTHWEST
UKRAINE: A DIAGNOSTIC FEATURE
Gerhard Franz, Oleksii A. Vyshnevskyi, Volodymyr M. Khomenko, Peter Lyckberg, and Ulrich Gernert
Green beryl and the yellow beryl variety heliodor are well known from the Volyn pegmatite field in Ukraine,and this study presents details of their morphological characteristics. Visible etch pits are characteristic of berylfrom this locality. In addition, they may contain an organic matter called kerite. Formation of the etch pits is
associated with a fluorine-rich, late-stage fluid phase. Etch pits on the pinacoidal face have a hexagonal outline
and a pointed bottom (originating at linear defects) transitioning to etch pits with very steep walls, and theyoccur in three different orders of magnitude: ≤500 μm, ≤50 μm, and 1–3 μm. On the first-order prismatic faces,
etch pits with a flat bottom (originating from point defects) or pointed bottom are square to rectangular, thelatter oriented parallel or perpendicular to the beryl’s c-axis. Flat bottom etch pits are more abundant thanpointed bottom and also occur in three different orders of magnitude. In addition, small etch pits with a canoe
shape and porosity on the nanometer scale were observed. Scanning electron microscopy of these etch pits wasused to distinguish uncut stones from other pegmatitic beryls, but these phenomena are also visible with an optical microscope or even with a loupe.

P

aleoproterozoic pegmatites near Khoroshiv(formerly Volodarsk-Volynskyi) in northwest
Ukraine are known as a source of high-quality
green beryl and the yellow beryl variety heliodor (e.g.,
Koshil et al., 1991; Simmons, 2014, and references
therein). The green beryl’s color can be very intense
but never reaches the typical emerald color. Aquamarine is also claimed from the Volyn pegmatitefield, but very rarely, and emerald from southeast
Ukraine (Franz et al., 2020) is the country’s only
other important gem beryl occurrence. The finest
specimens of Volyn beryl with color between green
and yellow (figure 1) and size of up to 140 cm inlength are displayed in the Museum of Precious andDecorative Stones in Khoroshiv (Vasylyshyn et al.,2001), while gem-quality crystals up to 55 cm havebeen sold abroad.
Since 1931, the pegmatites have been mined extensively for piezo quartz, which occurs in exceptionallylarge crystals from one to two meters (Lyckberg, 2005;

See end of article for About the Authors and Acknowledgments.
GEMS & GEMOLOGY, Vol. 59, No. 3, pp. 324–339,
http://dx.doi.org/10.5741/GEMS.59.3.324© 2023 Gemological Institute of America

324

ETCH PITS IN UKRAINIAN BERYL

Lyckberg et al., 2009, 2019). In addition to beryl, which
is present in ~2% of the pegmatites, approximately
10% of these pegmatites contain gem-quality topaz.
Among the first gem findings was topaz in 1931–1933,
and later gem beryl was recovered as a byproduct.

In Brief• Gem-quality crystals of green beryl and heliodor fromthe Volyn pegmatite field in Ukraine show spectaculardissolution features.
• Etch pits resulting from the dissolution are rectangularon prismatic faces and hexagonal on pinacoidal
faces.
• The etch pits occur in different sizes and overlap eachother, indicating different stages of dissolution that are
diagnostic for uncut stones.
• Fossil organic matter known as kerite can be attachedto the beryl crystals.

Mining for heliodor continued after World War II, but
this material was only available in Eastern Europe.
The first crystals reached the Western European market in 1980, and in 1987 Volyn beryl finally emerged

GEMS & GEMOLOGYFALL 2023

A

B

C

D

E

F

at the gem shows in Tucson. Many of these were used
for cutting (figure 2), but a large number of fine mineral specimens were also preserved.
Since the early 1990s, the Volyn Quartz Samotsvety Company has conducted mining for beryl andtopaz. Lyckberg et al. (2009) reviewed the mining andcollecting history, the geology, and the formation ofchambers within the pegmatites. That article also
contains a list of accompanying minerals in the pegmatite and several photos showing the etch pits.
A characteristic feature of Volyn beryl crystals istheir morphology, with exceptionally large dissolution
features (etch pits), described on the basis of light microscopy by Bartoshinskiy et al. (1969) and Lazarenko

Figure 1. Yellowish green heliodor and green beryl fromVolyn, oriented with the caxis vertical (top row) anddetail views of the prismatic
faces (bottom row). A and D:
An 8.5 cm crystal with welldeveloped prismatic faces
and strongly sculptured end
faces; the detail view shows
F-type etch pits elongatedparallel to the c-axis. B and
E: Transition from prismatic
face (1.8 cm wide) to sculptured pyramidal face withtriangular hillocks (comparewith the SEM image in figure10); the detail view shows
dominantly asymmetrical Ptype etch pits. C and F: A
2.683 kg deep green crystal
measuring 18 × 10 × 9 cm
with rectangular F-type etchpits, mostly oriented perpendicular to the c-axis. The
samples in A and B are fromthe collection of Peter Lyckberg. The sample in C,
named “Professor Pavlyshyn,” is from the Museumof Precious and Decorative
Stones in Khoroshiv. Photos
by Peter Lyckberg (A and B)
and Gerhard Franz (C).

et al. (1973). The prism faces show rectangular cavities
with negative crystal faces, while the basal planes
show negative, hexagonally shaped crystal faces. Al-

Figure 2. Rectangular step-cut green beryl from theVolyn pegmatite, 190.00 ct and 28.55 × 39.02 × 21.71
mm, with a rough unpolished table containing etchpits. Faceting and photo by Konstantyn Zalizko.

ETCH PITS IN UKRAINIAN BERYLGEMS & GEMOLOGYFALL 2023

325

BELARUS

RUSSIAN FEDERATION
Korosten Pluton
Volyn heliodor

POLAND

Kyiv

Lviv
Dnipro-Donets Rift
Volyno-Podolian Platform

UKRAINIAN SHIELD

Carpathians

DonetsFold Belt

DniproMOLDOVA
Kruta Balka emerald
ROMANIA

OdessaBlack Sea Depression

S E A

B L AC K

S E A

O F

A Z OV

Skyth

m
Cr i

e an

t
Bel
100 kmFigure 3. Map of the major tectonic units of Ukraine (based on the geological map in Shestopalov et al., 2020, andfrom Wikimedia Commons) with the Ukrainian Precambrian Shield, part of the East European Platform.
Ukraine’s two gem beryl occurrences: heliodor in the Volyn pegmatite field in the southwest of the Korosten Pluton and emerald at Kruta Balka, are each indicated with a star.

though dissolution of early precipitated minerals during a later stage is frequently observed in pegmatites(Černý, 2002; London, 2008), these features together
with the yellow-green color and their size are diagnostic for Volyn beryl. In large crystals, they are visible
without magnification, and in smaller crystals they
can be seen with the aid of a loupe or optical microscope.
This study presents an investigation of the morphology of the etch pits on prismatic and basal faces
in a number of beryl specimens from Volyn by meansof scanning electron microscopy (SEM), which
helped to identify the origin of uncut stones, extending the work of Kurumathoor and Franz (2018) on
etch pits as a provenance indicator. The nomenclature for etch pits follows from that work: F-type = flatbottom, which originate at point defects; P-type =pointed bottom, which originate at linear defects (i.e.,edge and screw dislocations); H-type = hollow, withwalls almost perpendicular to a face and bottom thatis not visible; and C-type = canoe-shaped.

326

ETCH PITS IN UKRAINIAN BERYLGEOLOGICAL SETTING
The Volyn pegmatites are genetically and spatiallyconnected with granites of the Korosten plutonic
complex in the northwest Ukrainian shield. This pluton crops out over an area of approximately 15,000
km2 and has intrusion ages of 1800–1760 Ma (figure 3;
Shumlyanskyy et al., 2017, 2021). Its country rocks
are high-grade migmatic gneisses. In the Khoroshiv region at its western margin, pegmatites are found in acontact zone between granite and gabbroic rocks overa distance of 22 km and a width of 300 to 1500 m (e.g.,Lazarenko et al., 1973; Koshil et al., 1991; Ivanovichand Alekseevich, 2007). Miarolitic cavities, produced
during the late-stage crystallization by fluids, occur allalong the edges of the pluton. The southern and eastern sides have been test-mined for quartz.
The pegmatites belong to the category of shallow
pegmatites, fully differentiated, and are irregularly distributed in the granites (Ivanovich and Alekseevich,2007). According to the nomenclature of Černý and
Ercit (2005) and Linnen et al. (2012), they belong to the

GEMS & GEMOLOGYFALL 2023

miarolitic interior-type of the niobium-, yttrium-, andfluorine-enriched family. They reach a size of several
tens of meters; individual pegmatite bodies are typically about 30 m by 40 to 45 m wide (Kalyuzhnyi etal., 1971; Lyckberg, 2005; Ivanovich and Alekseevich,2007). The pegmatites contain large miarolitic pockets
and are referred to in the Ukrainian literature as“chamber pegmatites.” They are zoned around an
open cavity, which can reach a volume of approximately 40 m3 and in extreme cases up to 4000 m3(Lyckberg et al., 2009).
A cross section of a pegmatite is shown in figure 4.
Quartz crystals one or two meters in size, most often
as smoky quartz, grew from the hanging wall into thechamber. The floor is covered with quartz that has
fallen down, as well as smaller crystals of mica, topaz,beryl, and albite. Chambers in which beryl and topaz

occur together as large crystals are the exception inthese deposits, because during the multistage crystallization conditions they can replace each other. The
footwall of the chambers consists of a strongly leachedfeldspar-rich horizon up to 36 m deep (Lyckberg et al.,2009) where quartz was dissolved, and further downlies a zone of broken-up pegmatite. New mineral formations in this zone include albite, siderite, topaz, fluorite, beryl (rarely), and phenakite. The zones aroundthe chamber are a quartz core above the chamber, thena more or less concentric feldspar zone, a blocky
quartz-feldspar zone with large pegmatitic crystals,
and next to the granite a zone with graphic-textured
quartz-feldspar intergrowths.
Some of the chambers were destroyed forming a
breccia, which consists of broken crystals and rockfragments, cemented by quartz, opal, chalcedony,

Figure 4. Cross section of pegmatite number 487 from the Volyn pegmatite field, showing the different zones. The
strongly leached feldspar-rich horizon below the chamber is typical, as is a quartz core above the chamber. The pegmatite measures approximately 12 m × 7.5 m, and the chamber is approximately 5 m × 2 m wide. The vertical scaleon the far left represents depth in meters. Modified by D. Lyckberg after © Volyn Quartz Samotsvety Company.
Pegmatite 487
59m level, shaft 2, Volyn, Ukraine

Graphic pegmatite

es
ur

fis
s

Rapakivi granite

Coo
ling
/co
ntr
ac
tio
n

Pegmatoid
Blocky pegmatite

Massive grey quartz core

Crystal chamber
MorionTopaz

Leached zone

Small pegmatoid graphic

Rapakivi granite
Mafic zone (granite enriched with amphibole)
5m

ETCH PITS IN UKRAINIAN BERYLGEMS & GEMOLOGYFALL 2023

327

and clay minerals. Pseudomorph replacements ofberyl, which now consist of opal, muscovite, bertrandite, buddingtonite, and minor amounts of euclase,
albite, potassium feldspar, columbite, pyrite, barite,rare earth element minerals, and organic matter,occur in the breccia (Bartoshinskiy et al., 1969; Franzet al., 2017). The authors recently obtained anothersample of a beryl pseudomorph, which contains
siderite next to the beryllium minerals bertrandite
and euclase, pointing to a variable fluid composition
during the latest alteration stages. We also obtained
a sample with a pseudomorph of white mica replacing topaz, which again shows the importance of alate alteration stage for the gemstones.
Based on fluid inclusion studies, the pegmatites
crystallized at a high fluid pressure, first in a closed
system (from 600° to 390°C) and then during coolingin an open system (at 365° ± 15°C) at a depth of 2.0
to 2.5 km (Lukashev, 1976; Kalyuzhnyi et al., 1971;
Voznyak, 2007; Voznyak et al., 2012).
Another very special feature of these pegmatites
is the occurrence of an organic matter called kerite.
It is found within the cavities, attached to feldspar
and topaz. It was interpreted as an example of abiogenic (of inorganic origin) formation (Ginzburg et al.,
1987; Luk’yanova et al., 1992)—in other words, inorganic synthesis of fibrous carbon hydrates in thecooling stage of the pegmatites. They were later

reinterpreted as fossil cyanobacteria (Gorlenko et al.,
2000; Zhmur, 2003) from a geyser-type deposit. Recent investigations (Franz et al., 2022, 2023) havealso identified a number of different fossils, some ofthem fungi-like. Filamentous fossils and remnantsof former biofilms were also identified in etch pits
of beryl.

PREVIOUS WORK ON VOLYN BERYL
Beryl is an accessory mineral found in 2% of the approximately 1500 pegmatite bodies at Volyn. It ispresent in small amounts in cavity areas, found inclay primarily in the lower sections of the pocket
where they once grew on albite (Lyckberg et al., 2009),
but their initial location is mostly lost. Beryl crystals
are usually sitting in a clay-mica-quartz rock matrix
or in a soft clay mass of dominantly kaolinite, as variably oriented individual crystals. Anhedral crystals,
measuring no more than 0.5 cm in cross section, also
occur in the leached zone under the chamber, where
beryl fills cracks and leaching voids in albite.
The Vickers hardness of the Volyn beryl as measured on prismatic faces ranges from 1380 to 1395
kg/mm2 (Lazarenko et al., 1973), corresponding to 7–8
on the Mohs scale. Volyn beryl is characterized by arelatively low density of 2.65 to 2.75 g/cm3 (Gurskyi etal., 2006) and low refractive indices of no = 1.568–1.570
and nε = 1.566–1.567 (Lazarenko et al., 1973).

TABLE 1. Morphological characteristics of Volyn beryl (Lazarenko et al., 1973).
Morphological
types

Type 1

Type 2Type 3

Light yellow,
Greenish yellow,
yellowish green yellowish green

Type 4

Type 5Light green tolight blue

Brownish green
to yellow

Color

Brownish green

Shape

Prismatic

Long-prismatic,columnar

Needle-shaped,
prismatic

Prismatic

Irregular

Other faces,
etch pits

Rarely developed{0001} and−{11 21} faces

Deep etch pits;
only remants ofedges

Curved−{11 21}

Microporous
dissolution

Only relics oforiginal faces

External features(rough crystals)–0} prismatic faces, hexagonal etch
All types: Rectangular and nearly square etch pits on {101pits on {0001} basal planes, and triangular hillocks between prismatic faces and on
transition from prismatic faces to pyramidal faces

328

ETCH PITS IN UKRAINIAN BERYLGEMS & GEMOLOGYFALL 2023

Figure 5. Crystal drawings, reproduced from
Bartoshinskiy et al.
(1969), showing the typical crystal habit with
Miller indices of thecommon faces (top left),spectacular long funnelshaped etch pits (ending
in thin tubes) on thepinacoidal face (topright) and the typical Ptype and F-type etch pits
on the first-order prismatic face. The outline
can be symmetrical or
asymmetrical, and Ftypes can overlap eachother. No scale wasgiven in the original
publication.

Only a few chemical analyses of Volyn beryl areavailable in the literature (Lazarenko et al., 1973;
Khomenko et al., 2007; Voznyak et al., 2012). The
low contents of sodium (≤0.10 wt.%), lithium (≤0.02
wt.%), cesium (≤0.24 wt.%), iron(II) (≤0.29 wt.%), andmagnesium (≤0.03 wt.%), as well as the relatively
high Fe3+/Fe2+ ratios (on average near 7:1), are characteristic for Volyn beryl. Titanium, manganese,
chromium, vanadium, potassium, calcium, gallium,
niobium, molybdenum, and barium were reported asminor elements, mostly below or near 0.01 wt.%.
Lazarenko et al. (1973) described different stagesof the dissolution of beryl, which produced five mor-

phological types of columnar-prismatic, occasionally
spicular beryl crystals (table 1), with characteristicallysculptured crystal faces as well as systems of dissolution cones (so-called etch channels). Basal faces are
rare. Bartoshinskiy et al. (1969) showed rectangular Fand P-type etch pits with a partly curved outline on
the first-order prismatic face and spectacular long, tubular etch pits on the pinacoidal face (figure 5).
The nature of the typical color for Volyn heliodor
was studied by Khomenko et al. (2007, 2010) using
optical spectroscopy. Variations in the color of Volynberyl are due to the intensity of the absorption seenin the slope from 28000 to 22000 cm–1 of the strong

ETCH PITS IN UKRAINIAN BERYLGEMS & GEMOLOGYFALL 2023

329

A BSORPTION SPECTRA

10000(rare greenish blue crystals are displayed in the Museum of Precious and Decorative Stones; Vasylyshyn
et al., 2001). Although some reproductions of crystals
from Volyn show an intense green color resemblingthat of emerald, this is purely a matter of color reproduction: Typical emerald color is not reported fromVolyn crystals, in line with the absence of chromium
and vanadium. Ukraine’s only emerald occurrence is
Kruta Balka (Franz et al., 2020), and the attributionof emerald to Volyn in Grundmann and Giuliani(2002) is probably erroneous. Morganite has not beenreported from Ukraine, either.

Eǀǀc
E ⊥c

MATERIALS AND METHODS

Eǀǀc
E ⊥c

LINEAR EXTINCTION COEFFICIENT k (cm –1)

6.0

4.0

O 2–-Fe 3+ CT
Fe 2+ (IV)Fe 2+ (IV)

H 2O

2.0

0
25000

20000

15000

O 2–-Fe 3+ CT

2.5

2.0
Fe 3+
1.5
Fe 3+
1.0
0
28000

26000

24000

22000

20000

WAVENUMBER (cm–1)

Figure 6. A: Representative polarized optical absorption spectra of a weakly colored yellow-green berylfrom Volyn. Linear extinction coefficient k (cm–1) isthe absorption coefficient of a sample 1 cm thick. Arrows point to a strong long-wave tail of the O2–-Fe3+
charge transfer (CT) in the UV region and the weakFe2+ and H2O bands in the NIR region. B: Absorption
along the long-wavelength slope of a strong O2–-Fe3+
charge transfer band that causes yellow color in thepolarized spectra of heliodor. Spin-forbidden absorption bands of Fe3+ ions are labeled with arrows; modified from Khomenko et al. (2007, 2010).

UV absorption, caused by the O2–-Fe3+ charge transfer
in structural or interstitial positions near the cationicvacancies (figure 6). This slope covers the blue absorption at approximately 450–470 nm (22222–21276
cm–1). Fe2+ ions do not affect the color of Volyn heliodors, confirming results of previous spectroscopic
investigations by Wood and Nassau (1968), Schmetzer et al. (1975), and Platonov et al. (1984, 2016).
Volyn beryl’s color varies from saturated brownish green to light yellow-green (figures 1 and 2). Mostof the gem-quality crystals can be regarded as typicalheliodor with a yellowish shade of variable intensity

330

ETCH PITS IN UKRAINIAN BERYLFor SEM investigation of the etch pits, nine light yellowish green beryl crystals were selected from thecollection of the Institute of Geochemistry, Mineralogy and Ore Formation of the National Academyof Sciences of Ukraine (IGMOF NASU) in Kyiv, alongwith three nearly colorless prismatic crystals (collected by author GF from the mine tailings in 2008)
with a partly developed pinacoidal face.
SEM and energy-dispersive X-ray spectroscopy(EDS) were performed using a JEOL JSM-6700F fieldemission scanning electron microscope equippedwith a JED-2300 EDS spectrometer at IGMOF
NASU. The spectra were collected with an accelerating voltage of 15 kV and a beam current of 0.65 nA.
Operating conditions for each analysis were identical. The samples were cleaned in an ultrasonic bathand then sputtered with a platinum layer of 15 nm
thickness. The three nearly colorless crystals werecarbon coated and investigated at the Central Facility
for Electron Microscopy (ZELMI), Technical University Berlin, using a Hitachi SU8030 cold field emission SEM equipped with an EDAX Octane-A Plus 30
mm2 silicon drift detector for qualitative EDS analysis. Some images were obtained in combination witha backscattered electron (BSE) and SE detector.
Standard gemological properties were investigated on another 16 rough beryl crystals (mineralogical collection of IGMOF NASU) and 20 faceted
beryls (private collection of O. Vyshnevskyi) from theVolyn pegmatite field. For refractive index and birefringence measurements, we used an Eickhorst GemLED refractometer. Sartorius CP 224 S analytical lab
balances equipped with a hydrostatic attachment
were used for specific gravity (YDK01MS Density
Determination Kit), using distilled water as fluid.
The color of the samples was graded using a GIA
GemSet in strong, diffuse daylight.

GEMS & GEMOLOGYFALL 2023

TABLE 2. Gemological characteristics of Volyn beryl.
Color

Refractive index

Birefringence

Very light to medium
yellowish green

Very light to strongly
yellowish green

Very light to medium greenyellow

nw = 1.569–1.570

nw = 1.568–1.571

nw = 1.568–1.571

nε = 1.562–1.564

nε = 1.562–1.566

nε = 1.562–1.566

0.006–0.008

0.005–0.006

0.005–0.006

Specific gravity
Rough crystals (16)

2.62–2.70

Faceted stones (20)

2.66–2.70

Crystals, liquid (fluid) inclusions, fingerprints, etch tubes, fracturesRESULTS
Gemological Characteristics. The standard gemological properties of the 16 rough and 20 faceted samples
are summarized in table 2. All were transparent totranslucent, and none showed fluorescence undershort- and long-wave UV light. Their refractive indices of 1.568–1.571 (nw) and 1.562–1.566 (nε) and birefringence values of 0.005–0.008 were typical fornatural beryl. Samples with stronger yellowish hue
showed slightly lower birefringence. Eye-clean roughcrystals, as well as all faceted stones, had a specificgravity in the range of 2.66–2.70. Translucent roughberyl had a lower specific gravity range of 2.62–2.65,
which may be due to numerous fluid inclusions. Solid
inclusions with a size of tens to hundreds of micrometers, brought to the surface in polished sections,were identified by EDS analysis as albite, microcline,

muscovite, columbite, iron sulfide, titanium oxide,and iron oxide/hydroxide (possibly goethite).
Morphology of Etch Pits. In general, the “negative
faces” of an etch pit follow the symmetry patternof the crystal. The nomenclature for the description
is outlined in figure 7. Faces of the etch pits can beoutlinerectangularInternal features{1010}
sector[0001]
ct

or

bo

un
d

ar

y

asymmetric

line

ETCH PITS IN UKRAINIAN BERYL

se

out

Figure 7. Nomenclature for the description of etch pits,
shown for F-type (flat bottom, originating from point defects) on the first-order prism. P-type etch pits show thesame features, except that the bottom is pointed. The outline of the etch pit indicates the relative dissolution vectors in the directions parallel and perpendicular to thec-axis. If the vectors do not change with increasing depth,
sectors with straight boundaries are formed. If the dissolution vectors change relative to each other, sector boundaries become curved toward an hourglass shape. Stepped
sectors indicate discontinuities in the dissolution velocity.
An asymmetric outline indicates heterogeneous dissolution within a growing etch pit. Etch pits of one kind or one
generation can be overprinted by another type.

overprinted by C-type[1010][1120]
dissolution vectors

GEMS & GEMOLOGYFALL 2023

331

A

B

C

2nd
1st

500 μm

D

200 μm

E

}
20
{11

{1010}

20 μm

F{1011}

}
12
{10

20 μm

20 μm

Figure 8. SEM images of Volyn beryl crystals with etch pits on {0001}. A: Deep first-order P-type etch pits (transitional to
H-type) with no steps, straight, regular in outline, and up to 500 μm in diameter together with irregular second-order Ptype etch pits approximately 50 μm in diameter. B: Second-order P-type etch pits overlapping each other. C: Second- andthird-order P-type etch pits. D: Star-like bottom in a second-order P-type etch pit, produced by dissolution following first––the {1010} first-order prismatic face, then in the deep part the {1120} second-order prismatic face. E: Diagram of a P-type––etch pit; the negative crystal produced by dissolution shows the {1010} first-order prismatic face with the {1011} in the–upper part and at the bottom the {1012} first-order hexagonal dipyramidal faces. This indicates a change in the relative
dissolution velocity with increasing depth. F: Third-order P-type etch pits, with about 1 to 3 μm diameter.

even or stepped; outlines of an etch pit on the crystal face can be straight or curved, symmetrical or
asymmetrical.
Rectangular etch pits on first-order prismatic faces–{1010} and hexagonal etch pits on the basal planes{0001} were formed by dissolution. Pyramidal faces are
rarely developed on the investigated crystals. Etch pits
on {0001} are P-type; however, some with very steep
faces are transitional to H-type (figure 8). We observe
three orders of magnitude: First-order etch pits are up
to 500 μm in diameter and have a mostly straighthexagonal outline (figure 8A). Second-order etch pits
are on the order of 50 μm wide, with a less regular outline, partly overlapping each other (figure 8, B and C).
Many of these show at their bottom a star-like pattern(figure 8D), produced by a change from dissolution par–allel to the first-order prismatic face {1010} to dissolu–
tion parallel to the second-order prismatic face {1121}(figure 8E). The negative crystal produced by dissolu–
tion shows the {1010} first-order prismatic face with––the {1012} and {1011} first-order hexagonal dipyramidal
faces. Third-order etch pits are 1 to 3 μm wide, irreg-

332

ETCH PITS IN UKRAINIAN BERYL

ular in outline and very abundant, covering all of{0001} (figure 8, C and F).
Figure 9 shows etch pits on the first-order prism–{1010} of the beryl crystals. These etch pits are rectangular, with their long side parallel to the c-axis (figure
9A), perpendicular (figure 9B), or nearly square (figure
9C). Similar to {0001}, F-type etch pits were observedin three orders of magnitude; however, P-types aremuch rarer and only observed in first-order, whereas
F-types dominate and there is a more or less continuous spectrum for second- and third-order types. In addition, there are small C-type etch pits, which can beoriented perpendicular to the c-axis (figure 9, D and E)
or parallel to the c-axis (figure 9, F and G). First-orderP-types with stepped sides can be strongly elongatedparallel to the c-axis (figure 9, D and E). F-type etchpits show a smooth curvature of the edges (figure 9G),
which was also observed in P-types (figure 9B).
Edges between two first-order prisms are characterized by the presence of triangular hillocks, formedby dissolution of the prismatic faces (figure 1B and figure 10). Pyramidal faces were only observed in one

GEMS & GEMOLOGYFALL 2023

B

A

C

2nd F-type

1st F-type

1st P-type

1st F-type

B

500 μm

D

120 μm

1st P-type

1st P-type

500 μmE

1st P-type

E

500 μmF

3rd C-type

50 μm

G

3rd C-type

2nd F-type

500 μm

100 μm–Figure 9. SEM images of Volyn beryl crystals with etch pits on first-order prismatic faces {101 0}, with theorientation of the c-axis nearly vertical. Some etch pits are outlined (black dotted lines). A: Rectangular
first-order P-type etch pit and asymmetric first-order (1st) F-type etch pit (approximately 500 μm × 1000μm) with long edge parallel to the c-axis. Note the second-order (2nd) approximately 50 μm × 100 μm wide
F-type etch pits within first-order F-type etch pit. B: Rectangular symmetric first-order P-type etch pit withsmoothly curved, hourglass-shaped sector boundaries. C: Two large first-order F-type etch pits, and an
asymmetrical first-order P-type etch pit, overprinted by very flat second- to third-order F-type etch pits. D:
Stepped first-order P-type etch pit with bottom elongated into a line. Note the small triangular etch pits inthe left and right sector walls (arrows). E: Lower sector wall with third-order C-type etch pit, oriented perpendicular to the c-axis. F: Second-order F-type etch pits with large, asymmetric third-order C-type etch pitsoriented parallel to the c-axis (magnified in G).

ETCH PITS IN UKRAINIAN BERYLGEMS & GEMOLOGYFALL 2023

333

300 μm

30 μm

Figure 10. SEM image (left) of the edge between two prism faces, forming triangular hillocks, magnified on the right.

crystal and contained triangular steps (figure 11). On
the steps, small third-order F-type etch pits developed.
Analysis of the crystals’ surfaces yielded more information about the dissolution process and thecomponents in the dissolving fluid. The faces show
a porosity, both outside and inside the etch pits (figure 12) and also within the small C-type etch pits.
Individual pores show no crystallographic outline
and are on the order of several hundred nanometers,
mostly elongated parallel to the c-axis. Very small
grains of a titanium dioxide polymorph and gold(with minor contents of silver, copper, and iron as
determined by EDS) were observed (figure 12D), as
well as particles consisting of a mixture of chloride
and sulfate crystals and a particle with light rareearth elements and phosphorous.

DISCUSSION
On a crystal face, the distinction between growthfeatures and dissolution features is based mainly on
the observation that growth features, such as
growth hillocks, are positive (on the basal planewith a hexagonal shape), rising up on top of eachother in stepwise fashion (Sahama, 1966), whereas
pits that extend down into the crystal are negativeand indicate dissolution. Flamini et al. (1983)
showed SEM images of growth features on red(manganese-bearing) beryl from Utah. The featuresdescribed above on the pinacoid and the prism ofVolyn beryl are all interpreted as dissolution features. Phenomena observed at crystal edges and thetransition from the prism to the pinacoid are lessclear, and we propose that for a provenance study

Figure 11. Left: SEM image of dissolution features on the transition from prismatic face (lower part, with rectangular F-type etch pits) to pyramidal face (upper right), with the formation of triangular hillocks. Right: Small etchpits (indicated by arrows) formed parallel to the larger F-type etch pits on the prismatic face.

140 μm334

ETCH PITS IN UKRAINIAN BERYL

25 μm

GEMS & GEMOLOGYFALL 2023

A

10 μm

B

C
C

2.5 μm

750 nmD
Au (Ag, Cu, Fe)

Figure 12. Combined
SEM-BSE images of enlarged areas of a prism
face, showing smallscale porosity. A: Theedge of an etch pit withelongated C-type etchpits. The smooth surface
is a prism face near theetch pit. B and C: Enlarged part of the prism
face, showing anhedral
pores measuring several
hundred nanometers. D:
A surface with verysmall grains of goldwith silver, copper, andiron impurities, as wellas titanium dioxide(chemical composition
determined by EDS).

TiO2

6 μm

ETCH PITS IN UKRAINIAN BERYLGEMS & GEMOLOGYFALL 2023

335

only the pinacoid and the first-order prism should
be used.
Dissolution of a mineral can be considered as“negative growth.” As growth is generally describedby growth laws (e.g., Philpotts and Ague, 2022), similarly we can describe dissolution by “negative
growth laws.” On a crystal face, dissolution is first
dominated by the number of defects (i.e., dislocationcontrolled). Removal of atoms is energetically favored at defects compared to an ideal surface. At
point defects, the defect disappears and removal ofatoms produces steps on the crystal’s face in two directions. On a linear defect, the defect extends in thethird dimension into the crystal and removal ofatoms continues in three dimensions. Once a deep
etch pit has formed, dissolution can be rate-determined by diffusion of the components in solution
out of the etch pit, and it is then diffusion-controlled.
This is what is seen on the pinacoidal face. There area few first-order etch pits, due to a small number oflarge screw dislocation bundles, which were formed
parallel to the c-axis during growth of the crystal.
They can probably extend deep into the crystal as
etch channels (Bartoshinskiy et al., 1969).
We interpret the second-order etch pits as due tosmaller screw dislocations, which are more frequent(figure 8A) and formed a negative first-order prism.
When these etch pits became deep, the dislocationcontrolled dissolution was followed by a diffusioncontrolled dissolution, and the negative crystal ends––
with the {1012} and {1011} first-order hexagonal
dipyramidal faces. Relative dissolution velocity is—
as estimated from the shape and depth of the etchpits—in the sequence of the directions [0001] >––[1120] > [1010]. The deep channels (figure 5) shownby Bartoshinskiy et al. (1969) demonstrate that
[0001] is a preferred direction for dissolution. Verydeep P-type etch pits are also shown in figure 8A,
where the bottom is no longer visible. In the diagramin figure 8E, the negative crystal produced by dissolution is shown in the upper part the outline of the––{1010} first-order prismatic face with the {1011}
hexagonal dipyramidal faces. This indicates faster––dissolution in [1120] compared to [1010], leaving
traces of the first-order prismatic faces. At the bottom (i.e., with increasing depth), a change is seen–from the {1011} first-order hexagonal dipyramidal–
faces to the {1012} hexagonal dipyramidal faces (figure 8, C and D). This indicates a change in the relative dissolution velocity with increasing depth.
Therefore, at the end of the etch pit, the sequence of––relative velocities is [0001] > [1010] > [1120], which

336

ETCH PITS IN UKRAINIAN BERYL–
also implies that dissolution in [1120] is not very dif–ferent from that in [1010].
Third-order etch pits are very abundant, covering
the basal plane completely and also occurring within
second-order etch pits (figure 8, C and F). Such a highnumber of growth dislocations seems unlikely, andwe propose that dissolution on these faces is controlled by the crystal structure of beryl with its pronounced channels parallel to the c-axis. Demianets
et al. (2006) determined by atomic force microscopythat experimentally produced hydrothermal berylshows growth hills on the pinacoidal face, with a separation distance of approximately 0.2 μm. The cavities around such growth hills might be correlatedwith the sites where dissolution is effective in theabsence of dislocations, producing the third-orderetch pits (figure 8F). The centers of the Si6O18 rings
within the beryl structure aligned with the basal
planes are sites of preferential removal of atoms, supported by the fact that first-order etch pits are very
deep and can extend into the long, funnel-shaped
tubes (figure 5; Bartoshinskiy et al., 1969; Lazarenko
et al., 1973) perpendicular to the Si6O18 rings. This isalso in line with the sodium-poor composition ofVolyn beryl. Sodium would be situated within thechannels, generally bound to a tetrahedral site witha stronger bond than water and could thus preventstrong dissolution in [0001].
On the first-order prismatic faces, P-type etchpits are rare and most are large, first-order P-types(figure 9), indicating relatively few screw dislocations perpendicular to the c-axis. Prismatic faces are
dominated by F-type etch pits, associated with point
defects (substituting atoms, vacancies, and interstitial atoms). These etch pits are very abundant, often
covering the whole face and overlapping each other(figure 1). They range in size from a centimeter down
to several micrometers (figure 9), and though weclassified them as first- and second-order etch pits,
there is more or less a continuum in size. The outline is rectangular, elongated perpendicular or parallel to the c-axis, or nearly square, indicating
essentially equal dissolution velocities in both directions. The sector boundaries are mostly straight,
with equal dissolution velocities in both directions,but some show curved boundaries, indicating continuously changing dissolution velocities, also observed in P-type etch pits (figure 9B). First-order etchpits can also be asymmetrical.
Our investigations also yielded some information
about the fluid phase, which caused the dissolution.
This fluid must have been able to transport elements

GEMS & GEMOLOGYFALL 2023

c-axis{1010}~ 500 μm F

Volyn

~ 200 μm P

fluorite as an accessory mineral, and by the occurrence of fluorine-rich muscovite (Franz et al., 2017).
Beryl and topaz can be related to each other by thechemical equilibriumPP~ 5 μmSichuan~ 100 μm
Luumäki–Figure 13. Diagrams of etch pits on {101 0} of heliodor
from Volyn compared to heliodor from Luumäki (Finland) and aquamarine from Sichuan (China), Minas
Gerais (Brazil), Dassu, Shigar, and Yuno (Himalayas),from Kurumathoor and Franz (2018). F- and P-typeetch pits are larger on crystals from Volyn and overlap
each other, but note that they can also be asymmetrical. Within F-type etch pits, C-type etch pits oriented
perpendicular to the c-axis are larger and more irregular in outline than on crystals from Minas Gerais. In
P-type etch pits on crystals from Luumäki, abundant
steps were identified, whereas the steps in crystals
from Volyn are less frequent or missing, and sector
boundaries can be curved.

Be3Al2Si6O18 + 28 HF ⇌ Al2F2SiO4 + 3 BeF2fluid + 5 SiF4fluid 14 H2OP

such as titanium, rare earth elements, and even gold
(figure 12D). Furthermore, we know from the localitythat in these niobium-, yttrium-, and fluorine-richpegmatites, fluorine is a major component, as shownby the abundance of topaz as a typical gemstone, by

where beryllium fluoride and silicon fluoride are fluid
species. This chemical equilibrium might explain
why either beryl or topaz crystallized in the chambers, but rarely both, and it also hints toward a fluorine-rich fluid phase for the late-stage dissolution of
the beryl. Lyckberg et al. (2009) noted that when topaz
and beryl occur together, one grows at the expense of
the other, which supports the interpretation inferredfrom the chemical equilibrium above.
The large gem-quality crystals with their dissolution phenomena on the surfaces show that possible
products of newly formed minerals during incongruent dissolution have been transported away. In otherwords, we are dealing with an open system. Possible
remnants of beryl dissolution as clay-like material
are observed in some etch pits, but it is not yet clear
whether this material is a product of the dissolutionof the beryl or contamination from a late-stage hydrothermal process of other minerals such as
feldspar.
The shape and size of the etch pits on Volyn beryl
are different from those of other pegmatitic beryls(figure 13; Kurumathoor and Franz, 2018) and may
well help to identify uncut crystals. A characteristicfeature in particular is that F-type etch pits are muchlarger (figures 8 and 9) compared to beryl from mostother localities. They frequently overlap each other(figures 1, 8, and 9), which is rarely observed in berylfrom other localities. The different dissolution stagesof the Volyn beryl with varying acidity-alkalinity described in the literature (Barthoshinskiy et al., 1969;

ETCH PITS IN UKRAINIAN BERYLGEMS & GEMOLOGY

F~ 200 μm
C
~ 200 μm

Minas Gerais~ 10 μm

C~ 50 μm
Dassu

F~ 100 μm

P

Shigar~ 100 μm

P

F

~ 100 μm

Yuno

~ 100 μm

FALL 2023

337

Lazarenko et al., 1973) might be correlated with ourobservation of first-, second-, and third-order etchpits. The degree of etching is quite variable on different crystals, and it even varies from pegmatite to pegmatite (Lyckberg et al., 2009).
The crystal from Minas Gerais in figure 13 shows
similarly large F- and P-type etch pits and also small
third-order C-type etch pits within F-type etch pits. At
Volyn, however, the C-type etch pits are much larger,
more abundant, and less regular in outline (figure
13A). C- and P-type etch pits measuring from 5 up to
50 μm, as observed in crystals from Dassu (Himalayas)
and China, were not found in Volyn beryl, nor was theorthorhombic outline of F- and P-type etch pits incrystals from Yuno (Himalayas). Aquamarine from
Shigar (Himalayas) shows P-type etch pits with acurved outline, which is not observed in the Volyncrystals, but its flat F-type etch pits are similar. Although the geological background of Volyn beryl ismost similar to that from Luumäki, Finland (Lahti andKinnunen, 1993; Lyckberg, 2004), with a similar heliodor color and a similar relation to a Precambriananorthosite pluton, the etch pits on the prism faces
are quite different (figure 13). On the crystals from Luumäki, only P-type etch pits were observed in Kurumathoor and Franz (2018), with an oblique outline andfrequent steps. It must be noted, however, that for Kurumathoor and Franz (2018) only a small fragment

ABOUT THE AUTHORSDr. Gerhard Franz is a retired professor of mineralogy and petrology at the Institute for Applied Geosciences, Technical University
Berlin. Dr. Oleksii Vyshnevskyi and Dr. Volodymyr Khomenko aresenior researchers at the Institute of Geochemistry, Mineralogy
and Ore Formation of the National Academy of Sciences of
Ukraine. Dr. Peter Lyckberg is a researcher and scientific collaborator with the National Museum of Natural History in Luxembourg.
Ulrich Gernert is a physics engineer at the Central Facility for Electron Microscopy (ZELMI), Technical University Berlin.

from one crystal was available, which might not be
representative for the occurrence. Furthermore, it
must be kept in mind that up to now only a few crystals from the different localities could be investigatedusing SEM. Future research should be concentrated on
more crystals from the same localities and on otherberyl varieties with different composition (e.g., rich insodium-magnesium, lithium-cesium, or manganese)that are also from non-pegmatitic environments.

CONCLUSIONS
Beryl crystals from Volyn are associated with the Korosten pluton in the Precambrian Ukrainian Shield.
These crystals are of gem quality, exceptional in size,and have excellent color—both the yellow heliodor
and the green beryl. They are found in chambers of
pegmatites that formed in the cooling and crystallization stages of the pegmatite melt. A special feature ofthese beryl crystals is the appearance of dissolution
features known as etch pits. On the prismatic face,these etch pits are very pronounced and often cover
the whole crystal. On the basal face, they form sixsided channels, ending with pyramidal faces or penetrating deep into the crystal. The shape and size of theetch pits are characteristic for the Volyn occurrence
and easily distinguish them from beryl crystals fromother localities.

ACKNOWLEDGMENTS
The Alexander von Humboldt Foundation financially supported
author V. Khomenko’s stay in Berlin in 2022–2023. The Museum for Precious and Decorative Stones in Khoroshiv isthanked for the permission to use the photograph of a beryl
crystal in figure 1. We also thank the peer reviewers and editors
for handling the manuscript, as well V. Chournousenko for stimulating discussions and for sharing his knowledge about theVolyn pegmatite field.

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FEATURE ARTICLESIOLITE FROM THE THOR-ODIN DOME,BRITISH COLUMBIA, CANADA: GEOLOGY,CHEMICAL COMPOSITION, INCLUSIONS,AND CAUSE OF CHATOYANCY
Philippe M. Belley
The Blue Arrow and Blue Bear iolite (gem cordierite) claims located in the Thor-Odin dome (near Revelstoke in
British Columbia, Canada) are two new examples of iolite occurrences hosted in hydrothermally altered mafic
volcanic rocks metamorphosed at upper amphibolite to granulite facies, in which cordierite (XMg = 0.81–0.90)
occurs in association with gedrite, iron-rich phlogopite, quartz, almandine-pyrope garnet, and various traceminerals. Strongly pleochroic, gem-quality cordierite rough from these claims produced dark violet-blue facetedstones generally smaller than 2 ct (both localities) and light grayish blue to dark slightly grayish violet-blue chatoyant cabochons up to approximately 12 ct (Blue Bear only). Coarse-grained cordierite-rich rocks are widespread in the Thor-Odin dome, which indicates good potential for future iolite discoveries in the region. Primarymineral inclusions in Blue Bear iolite include iron-rich phlogopite, muscovite, zircon, xenotime-(Y), hematite,
and apatite. Where cordierite is traversed by rehealed fractures, mica inclusions are altered to unidentified hydrous aluminosilicates rarely containing trace pyrrhotite or barite. Three types of fluid inclusions occur in ThorOdin iolite: (1) primary blocky fluid inclusions; (2) primary acicular fluid inclusions oriented parallel to thea-axis of cordierite, which cause chatoyancy when present in high concentrations (at Blue Bear only); and (3)secondary subequant fluid inclusions occurring along rehealed subconchoidal fractures.

T

wo gem-quality cordierite (iolite) localities, theBlue Arrow and Blue Bear occurrences, were recently discovered (in 2016 and 2021, respectively) in the mountainous Thor-Odin dome,
southwest of Revelstoke in British Columbia,
Canada (figure 1). They have so far produced small
quantities of faceted stones (generally <2 ct, though
the author has seen faceted windowed stones up to
4.76 ct from Blue Bear) and attractive cat’s-eye cabochons (up to 12.02 ct, Blue Bear only). The present
study seeks to gain new insights into: (1) the geological origin of the occurrences in a regional context
and how they compare to the geology of other iolite
deposits, (2) the nature of iolite mineralization at theBlue Arrow and Blue Bear localities, (3) the chemical

See end of article for About the Author and Acknowledgments.
GEMS & GEMOLOGY, Vol. 59, No. 3, pp. 340–355,
http://dx.doi.org/10.5741/GEMS.59.3.340© 2023 Gemological Institute of America

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composition and inclusions of Thor-Odin dome iolite, and (4) the nature of the inclusions responsible
for chatoyancy.

GEOLOGY OF IOLITE DEPOSITS
Deposits of iolite (gem-quality cordierite) occur inmagnesium- and aluminum-rich metamorphic rocks
formed at amphibolite to granulite facies conditions.
Many iolite occurrences are found around the world,
but relatively few have been comprehensively characterized from a geological (i.e., petrological, geochemical, and geochronological) perspective.
Significant deposits occur in the Bandarguha andOrabahala areas of Kalahandi District, India, where
gem cordierite (with XMg = [Mg]/[Mg + Fe] = 0.82–0.88)
is hosted in granulite facies cordierite-biotite schists
and gneisses, which can contain the following accessory minerals: quartz, potassium feldspar, plagioclase, sillimanite, corundum, and orthopyroxene(Das and Mohanty, 2017).

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PAC I F

I C

O C E A N

N

BritishColumbia

Figure 1. Location mapof the Thor-Odin dome
near Revelstoke in
British Columbia,
Canada.

CANADA

Thor-Odin dome

UNITED STATESAt the Geco copper-zinc mine—a volcanogenic
massive sulfide deposit in Manitouwadge, Ontario,
Canada—cordierite (XMg ≈ 0.80; Fox and Yakymchuk,2017) occurs in amphibolite facies (650° ± 30°C, 6 ±1 kbar; Petersen, 1984) rocks consisting of a meta-morphosed hydrothermally altered (calcium- andsodium-depleted) basalt and a cordierite-rich dikepossibly representing an altered synvolcanic dike

(Pan and Fleet, 1995). The typical cordierite host rockat the Geco mine is a cordierite-gedrite gneiss often
containing quartz, sulfides (generally chalcopyrite),and garnet; but it is also found in quartz-rich segregations (Pan and Fleet, 1995; P. Belley, hand sample
observations). The Geco mine is a significant iolite
occurrence that did not produce much gem roughdue to the exclusive focus on copper and zinc mining(B. Wilson, pers. comm., 2018). Wilson (2014) noted
beautiful violet-blue faceted stones of 2.00 and 2.20
ct from the Geco deposit, as well as an overly dark
6.10 ct stone.
Gem cordierite occurs in magnesium- and aluminum-rich, orthoamphibole-bearing rocks (similar
to those at the Geco mine but not associated withsulfide deposits) at Palmer Canyon in the Laramie
Range of Wyoming, United States (Patel et al., 1999;
Hausel, 2002), and in the Passmore area, Shuswap
metamorphic complex, southern British Columbia(Simandl et al., 2000). In the Passmore area, the gem
cordierite mineralization occurs in contact metasomatic deposits that are in contact with anthophyllite-rich granulite facies metamorphic rocks (1)between more siliceous intrusive or crosscutting
rocks such as quartz veins and pegmatites and (2) inbiotite-rich rocks (Simandl et al., 2000).

IOLITE FROM BRITISH COLUMBIA, CANADAGEMS & GEMOLOGYIn Brief• Violet-blue faceted iolite gems (usually <2 ct) andchatoyant cabochons (up to 12 ct) are describedfrom occurrences in the Thor-Odin dome in British
Columbia, Canada.
• The occurrences are hosted in gedrite-cordierite
rocks consisting of altered mafic volcanic rocks
metamorphosed under granulite facies conditions.
• Chatoyancy is caused by an abundance of acicular
fluid inclusions parallel to the a-axis of cordierite.
• Mineral inclusions include biotite, muscovite, zircon,
xenotime-(Y), hematite, and apatite.

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’

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Three Valley Gap
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˚4Figure 2. Simplified geological map of the ThorOdin dome showing thelocation of known
cordierite-orthoamphibole rock occurrences,including the Blue Bearand Blue Arrow gem
cordierite occurrences.
Modified after Goergen
and Whitney (2012), reproduced with permission from SNCSC (©
2011 Springer-Verlag.
Contrib. Mineral.
Petrol., Volume 163).

THOR-ODIN DOME
B.C., CANADA

30

11
8˚

11
8˚

10
’

60
50

˚3

0’

F1 / F2 Axial Plane

Fault

GraniteF3 Axial Plane

Form Surface

Middle Unit

F4 Axial Plane

Dip of Transposition
Foliation

Lower Unit

Amp.-Crd Rock(blue = gem Crd)

Scale

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1

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1 2 3 4 5
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Figure 3. View of Arrow Lake from the talus at the Blue Arrow iolite occurrence. Photo by D.J. Lake.

HISTORY AND ACCESSIBILITY
While gem-quality cordierite was not discovered in
the Thor-Odin dome until 2016, it has been known
to occur in a nearby geological terrane, the Valhallametamorphic core complex, since the 1990s. In 1998,
exploration near Passmore in British Columbia(within the Valhalla complex) by Anglo Swiss Resources Inc. generated about 50 kg of high-grade
rough iolite, which yielded less than 100 carats of finished gems (Wight, 1999; Laird, 2002). Passmore produced both “bright and lively [faceted stones] with ablue-violet color and excellent clarity” up to 0.6 ct(Simandl et al., 2000; Wilson, 2014) and cabochons
up to 1.5 ct (Simandl et al., 2000), some of which display asterism (Wight, 1999).
Cordierite-orthoamphibole rocks have been documented from 20 localities within the Thor-Odin dome
(Duncan, 1984; Norlander et al., 2002; Hinchey andCarr, 2007; Goergen and Whitney, 2012; see figure 2of the present study), although as many as 30
cordierite localities are known to local prospectors (H.
Hyder, pers. comm., 2022). In 2016, prospector Herb
Hyder discovered the Blue Arrow iolite occurrence intalus overlooking Arrow Lake (figures 3 and 4), and in
2021 he discovered the Blue Bear iolite occurrence

IOLITE FROM BRITISH COLUMBIA, CANADA(currently under claim) in outcrop at high elevation(approximately 2000–2400 m above mean sea level) 1–
2 km south of Dickinson Lake and west of Horsefly
Figure 4. Aerial image of the Blue Arrow iolite occurrence, which consists of cordierite-bearing boulders
near the forest’s edge. Photo by Philippe Belley.

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Figure 5. Cordieritegedrite-biotite-garnet
boulder at the Blue
Arrow occurrence.
Photo by D.J. Lake.

Peak. The iolite occurrences are located on relatively
steep slopes (see figures 4 and 5) and very difficult terrain. Both localities can be accessed by hiking fromthe nearest logging road, but the Blue Bear claim ismost easily accessed via helicopter since it is a substantially difficult hike with considerable elevation
gain. Due to its high elevation, the Blue Bear locality
is covered by snow except for one or two summer
months (generally in August). At the time of this writing, collecting is not permitted at either claim and themineral exploration rights are privately owned.

GEOLOGICAL SETTING
Regional Geology. The study area is located in theThor-Odin dome, which forms the southern portionof the Monashee complex—the deepest structurallevel of the Shuswap metamorphic core complex in
the Omineca Belt of southern British Columbia. The
Thor-Odin dome consists of Paleoproterozoic basement rocks (primarily migmatitic paragneiss and orthogneiss) and Paleoproterozoic to possibly Paleozoic
supracrustal metasedimentary cover rocks, both ofwhich experienced high-grade (granulite facies; 750°–
800°C) metamorphism concomitant with decompression from above 8–10 kbar to below 5 kbar during
exhumation, resulting in anatexis.1 Partial melting
occurred approximately 60 to 54 Ma, possibly continuing until 51 Ma (Vanderhaeghe et al., 1999; Norlander et al., 2002; Hinchey et al., 2006).

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Cordierite-bearing gedrite-rich gneisses occur atregional scale and exclusively within the paragneissin the basement sequence of the Thor-Odin dome(the “Lower Unit,” also containing orthogneiss),
which consists mostly of cordierite-biotite-quartzfeldspar gneiss, migmatitic garnet-sillimanite-quartzfeldspar gneiss, with local calc-silicate rock, marble,
and quartzite (Norlander et al., 2002; Hinchey andCarr, 2007). The only previously reported gem materials found within the basement sequence of theThor-Odin dome are large crystals, averaging 5 cm
across and up to 10 cm, of opaque to rarely translucent, internally fractured, zoned dark blue and darkgrayish brown corundum (the latter being star sapphire). These corundum are reported from a single locality, where they occur within restite boudins2 inleucosome3 of metapelitic4 origin, having formed asa result of muscovite breakdown and partial melting
along the prograde P-T path at or before 60 Ma, nearpeak granulite facies metamorphic conditions (Abdale et al., 2023).

1

Anatexis: The partial melting of crustal rocks.
Restite: Residual material left at the site of partial melting, such as ina migmatite. Boudin: A sausage-like pinch-and-swell structure formedby extension of originally continuous rock layers.
3
Leucosome: The light-colored portion of migmatite, which is a product of partial melting.
4
Metapelitic: Sourced from or related to metapelites (metamorphosed
pelitic rock such as shale or mudstone).
2

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Figure 6. Cordierite-bearing quartz vein (approximately 10 cm wide) cutting gedrite-cordierite rocks at the Blue
Bear occurrence. Photos by Herb Hyder.

Gedrite-Cordierite Rocks. Gedrite-cordierite-richrocks are widespread in the Thor-Odin dome (Goergen and Whitney, 2012; figure 2), where they occur aselongated boudins up to 500 m in length (Norlander
et al., 2002). Two variants of this rock are known: one
containing almandine-pyrope garnet porphyroblasts5
averaging 5 cm across and another containing sapphirine (Norlander et al., 2002). The mineral assemblage is variable but always includes gedrite (2–25 cm
long and the most abundant mineral, comprising>30–40% of the rock), cordierite (usually as interstitial masses between gedrite blades), and biotite (phlogopite, with XMg ≈ 0.8). Other minerals observed inrock samples include garnet (up to 15 cm in diameter,where present), quartz, sillimanite, kyanite, sapphirine, corundum, and/or staurolite. Minerals occurring in trace amounts include ilmenite, rutile, apatite,magnesium-rich hercynite, plagioclase, and monazite(Norlander et al., 2002; Hinchey and Carr, 2007).
Gedrite-cordierite rocks in the Thor-Odin dome
have a distinctive bulk chemical composition that ischaracterized by a relative depletion in alkali elements, calcium, and low field strength minor andtrace elements, along with enrichment in aluminum,magnesium, iron, and high field strength trace elements. The rocks are interpreted to consist of Paleoproterozoic mafic volcanic rocks that underwent
hydrothermal alteration, likely by seawater, andwere subsequently metamorphosed in the
Cordilleran orogeny (Hinchey and Carr, 2007).
Hinchey and Carr’s genetic model is based on a combination of whole rock major and trace element
chemical composition (including rare earth element
patterns) and neodymium isotopic systematics.

DESCRIPTION OF THE IOLITE OCCURRENCES

Porphyroblast: In metamorphic rocks, a larger mineral grain that
occurs in a groundmass composed of relatively smaller grains.

Blue Arrow Occurrence. Violet-blue, strongly
pleochroic cordierite occurs in approximately half adozen large boulders (up to 2.5 m across; figure 5) on
the northern side of an east-facing talus overlooking
Arrow Lake. The talus measures approximately 1.0 by
0.7 km. The cordierite host rock consists of very
coarse-grained (average 1–4 cm grain size) gedrite-richgneiss consisting of the following minerals in rockforming amounts (highest to lowest abundance):gedrite, anhedral cordierite (often occurring in the in-

IOLITE FROM BRITISH COLUMBIA, CANADAGEMS & GEMOLOGY

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Figure 7. Large, euhedral
cordierite crystal containing facetable andcabochon-quality sections. Note the deformation-induced fractures
visible on prism faces in
the photo on the right.
The crystal measures
11.3 cm wide. Photos by
Philippe Belley.

terstices of gedrite crystals), biotite, subhedral garnet
porphyroblasts, and quartz. The cordierite occurs astranslucent to transparent, dark blue masses up to approximately 8 cm. Cordierite porphyroblasts arehighly fractured, with facetable gem rough that could
produce finished stones 0.5 ct or larger coming only
from small zones in a minority of the porphyroblasts.

Figure 8. Freshly recovered rough chatoyant cordierite
from the Thor-Odin dome. Note the lighter color andabundant acicular inclusions in the smaller fragments. Photo by Herb Hyder.

Blue Bear Occurrence. Violet-blue, strongly pleochroic cordierite occurs as subhedral to euhedral porphyroblasts in gedrite-cordierite-biotite-(±quartz)
gneiss, and in quartz-cordierite veins (<10 cm thick)cutting these rocks (figure 6). The geographic extent
of cordierite mineralization is currently unknown,and exploration in this steep, high-elevation terrain
is ongoing. Cordierite crystals shown to the author
averaged 5 cm across, and the largest crystal measured 11.3 cm long (figure 7). Cordierite crystals varyfrom anhedral to euhedral. Most large euhedral crystals are enveloped by biotite. The cordierite is highly
fractured, translucent to transparent, and locally contains zones suitable for production of polished gemstones. Higher-quality material is shown in figures 7
and 8, and representative cordierite, quartz, andgedrite are shown in figure 9. In some crystals (in partor in whole), the cordierite is moderately to heavily
included by parallel acicular inclusions. Cordierite
Figure 9. Representative cordierite (top left), quartz
with cordierite (top right), and gedrite (bottom) fromBlue Bear. As is common in metamorphic gem deposits, only a small fraction of the mineral occurs in
gem quality. The gedrite fragment measures 65 mm
across. Photo by Philippe Belley.

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Figure 10. Cordieritebearing biotite-quartzofeldspathic gneiss near
the Blue Bear occurrence. This lithology is
not known to containgem-quality cordierite.
Photo by Herb Hyder.

that is not of gem quality occurs in biotite-rich quartzofeldspathic gneiss in the area (figure 10).

Samples. The cordierite samples examined consist of~4 kg of unprocessed rough from the Blue Bear claim
and ~100 g from the Blue Arrow claim, both provided
by Herb Hyder, an independent prospector. A 1.15 ct
faceted stone from Blue Arrow was acquired fromlocal gem cutter Russell Crick, who visited the locality. Additional samples were recovered by D.J.
Lake and the author on a site visit to the Blue Arrowlocality; however, none of this material was of sufficient quality to facet into stones larger than 0.3 ct.
Rough samples provided by Herb Hyder were
processed by the author, and the highest-quality material was sent for cutting and polishing, resulting in
the production of faceted stones. The largest stones
described in this study are a 1.15 ct faceted stone
from Blue Arrow (figure 11), a 1.24 ct faceted stone
from Blue Bear (figure 12), and chatoyant cabochons
up to 12.02 ct from Blue Bear (figure 13). The largest
faceted gemstones reported to the author from each
locality consist of a very dark, included 2.0 ct stone

IOLITE FROM BRITISH COLUMBIA, CANADAGEMS & GEMOLOGY

MATERIALS AND METHODS

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Figure 11. Rough and cut iolite from the Blue Arrow occurrence. The faceted stones weigh up to 1.15 ct, withthe two matching triangular stones weighing approximately 0.8 ct in total. Photo by Michael Bainbridge.

Figure 12. Rough and cut iolite from Blue Bear. Fromleft to right, the faceted stones weigh 1.24, 0.48, 0.48,
and 0.88 ct. Photo by Philippe Belley.

from Blue Arrow and a dark 4.46 ct windowed stone
from Blue Bear. The specific gravity of the largest
stone of each variety and locality was obtained hydrostatically, and the refractive indices of Blue Arrow
and Blue Bear faceted iolites were measured with a
Kruss professional refractometer with a monochromatic sodium light source.

Sample Examination and Preparation. Rock samples
were examined to characterize the mineralogy andmineral habits. Selected samples of individual minerals and rock fragments were polished into 25 mm sections, carbon coated (coating thickness 5–10 nm), andexamined using Memorial University of Newfoundland’s FEI Quanta 650 SEG scanning electron microscope (SEM). The SEM was equipped with two Bruker
XFlash 5030 energy-dispersive X-ray spectrometers(EDS) for mineral identification, with a 30 s X-ray acquisition time, and a secondary electron detector (SE)for acicular inclusion examination, using an operating
voltage of 25 kV. Gem-quality rough was cut and polished (faceted stones and cabochons) and examinedusing a binocular gemological microscope.

Figure 13. Chatoyant iolite cabochons from Blue Bear.
The three largest stones weigh 12.02, 8.52, and 7.24
ct. The remainder weigh between 0.96 to 3.71 ct. Notethe variation in color, where lighter-colored cabochons have a higher concentration of acicular inclusions. Photo by Philippe Belley.

Chemical Analysis. Five spots, each spread out over a~1 cm2 area, were analyzed each on two representative
gem-quality pieces from each locality. Chemical compositions of cordierite were obtained with a JEOL JXA8230 electron microprobe at Memorial University of
Newfoundland. Operating voltage was 15 kV with 20
nA beam current and 3 μm beam diameter. Count
times were 40 s (except 20 s for sodium, potassium,and silicon, and 30 s for calcium) with background collection times equal to half of the count times. The following standards were used: orthoclase (potassium),
albite (sodium), diopside (calcium, magnesium, andsilicon), almandine (aluminum and iron), rhodonite(manganese), and sphalerite (zinc). Matrix correction
calculations were done using the eponymous “PAP”
method (Pouchou and Pichoir, 1985).

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RESULTS
Properties of Rough. More than 90% of the cordierite
at both Thor-Odin localities is not of gem quality,
owing mostly to extensive post-formation fracturing,a common feature in metamorphic gem deposits, andless commonly owing to an abundance of primarymineral or secondary fluid inclusions. Gem-quality
cordierite is present only in portions of single crystals, with a fairly random distribution, and the best
gem rough generally comes from the largest crystals.
The cordierite crystal in figure 7 illustrates deformation-controlled fracturing where the dominant fractures show some degree of preferential orientation.
This crystal also contains facetable rough in an area
where fracturing is less pervasive.
At the Blue Bear locality, cordierite contains avariable degree of parallel acicular inclusions ranging
from nearly absent (not visible in a hand sample) toextremely abundant, often varying from both extremes within a single crystal. The portions richestin these inclusions are light slightly grayish blue andshow strong chatoyancy (see the smaller rough fragments in figure 8).
Standard Gemological Properties. Cordierite fromboth localities is very strongly pleochroic, showing
three different colors in different orientations: dark
saturated violet-blue, medium-light blue-gray, andlight gray-yellow with low color saturation. Specificgravity values for the three largest iolite gemstones
from each locality and variety were 2.61 (Blue Arrow,faceted), 2.60 (Blue Bear, faceted), and 2.59 (Blue Bear,
chatoyant cabochon). Refractive indices were measured on one faceted stone from each locality. These
refractive index values were 1.610–1.620 for Blue
Arrow and 1.538–1.548 for Blue Bear.

the cordierite. The stones are therefore grayish yellow if backlit through the base. They display a distinct chatoyant band perpendicular to the long axisof the acicular inclusions (described below) that isvisible even in diffuse indoor and outdoor lighting
conditions. The cabochons occur in two color varieties: dark grayish blue-violet with a light gray cat’seye, and light gray-blue with a very light graycat’s-eye (see figure 13). Darker cabochons contain alesser amount of acicular inclusions and are moretransparent than the lighter, more heavily included
cabochons. The strength and sharpness of the cat’seye is identical in the dark and light varieties.
Chemical Composition. Gem cordierite from theThor-Odin dome, which was analyzed using electronprobe microanalysis (EPMA), is strongly cordieritedominant, with only a minor component (<20mol.%) of sekaninaite, the Fe2+ end member of thecordierite-sekaninaite solid solution series. The Blue
Bear cordierite sample analyzed has a higher XMgvalue than the sample from Blue Arrow, with valuesof 0.90 and 0.81, respectively (table 1). The higher
iron concentration in Blue Arrow cordierite correlates with a darker color in larger stones (compare figures 11 and 12). The analyzed sample of Blue Bearcordierite is richer in manganese, zinc, and sodium
than the Blue Arrow cordierite sample.

Cabochons. Cat’s-eye cabochon samples were fromthe Blue Bear locality only, and all of this material
consists of translucent cordierite containing abundant parallel acicular inclusions. They were cut withthe base perpendicular to the grayish yellow axis ofMineral Associations and Inclusions. At both localities, inclusions observed in rough crystals include
millimeter- to centimeter-scale grains and clusters of
biotite (likely iron-rich phlogopite), gedrite, andquartz. These minerals form a stable assemblage
with cordierite in the gneiss. Muscovite occurs locally in Blue Bear gneiss and, based on grain boundary relationships, appears to form a stable assemblage
with gedrite and cordierite. A 1.5 mm yellow hexagonal prismatic apatite crystal was observed incordierite from Blue Bear.
Rehealed fractures, indicated by the presence of
secondary fluid inclusions along subconchoidal fractures, are common in iolite from the two localities.
Relatively few gem-quality samples were available
from Blue Arrow, so the following inclusion descriptions focus on Blue Bear iolite, where a combinationof optical microscope and SEM-EDS was used to examine faceted stones and flat-polished samples. Some
subconchoidal fractures in iolite are filled with atranslucent white unidentified hydrous aluminosilicate phase containing equal molar proportions of alu-

IOLITE FROM BRITISH COLUMBIA, CANADAGEMS & GEMOLOGY

Properties of Cut Stones. Faceted Stones. When correctly oriented to maximize the violet-blue color,
faceted iolite stones show a beautiful, saturated violet-blue color that is of medium lightness in stonesin the half-carat range, while dark in stones of approximately 1 ct (figures 11 and 12). In similarly sized
examples, faceted gems from Blue Arrow are darker
than those from Blue Bear.

FALL 2023

349

TABLE 1. Average chemical composition (in wt.%), measured with EPMA, of two
representative samples of gem cordierite (iolite) from the Thor-Odin dome, Canada.
Blue Arrow(5 analyses)

Standard
deviation

Blue Bear(5 analyses)

Standard
deviation

Detection limit

SiO2

49.39

0.13

49.55

0.18

0.01

TiO2

bdla

—

bdl

—

0.01

Al2O3

33.79

0.08

33.93

0.06

0.01

FeO

4.60

0.12

2.35

0.06

0.01

MnO

0.03

0.01

0.07

0.01

0.01

ZnO

0.02

0.02

0.03

0.01

0.01

MgO

10.96

0.04

12.17

0.13

0.01

CaO

0.02

<0.01

0.01

<0.01

0.01

Na2O

0.26

0.02

0.47

0.01

0.01

K2O

0.01

<0.01

0.01

<0.01

0.01

Totalb

98.55

0.41

98.13

0.18

XMg

0.81

0.90

a

bdl = below detection limit
The slightly low total weight % indicates the presence of volatiles in structural channels (i.e., H2O and CO2 ; Vry et al.,1990) in the cordierite.
b

minum and silicon, and 0.5–2.2 mol.% of iron, magnesium, calcium, and potassium (figure 14). This mineral may be impure halloysite, a common alteration
mineral of cordierite (Layman, 1963). The most common inclusions observed in Blue Bear iolite are fluidinclusions (both primary blocky fluid inclusions andsecondary subequant rounded inclusions along rehealed subconchoidal fractures), acicular inclusions(causing chatoyancy when abundant; described below),euhedral platy brown iron-rich phlogopite (figure 15),
tabular to equant white muscovite (commonly altered
to hydrous aluminosilicates containing aluminum andsilicon in similar molar abundances, and possibly consisting of clay minerals; figures 16 and 17), zircon (figure 17), anhedral uranium-bearing xenotime-(Y) (figure18), and platy hematite (≤20 μm crystals). Zircon isfairly common in cordierite as groupings of closely

Figure 15. Light brown iron-rich phlogopite, a white
unidentified mineral (possibly muscovite), and acicular
fluid inclusions in a faceted iolite from Blue Bear. Photomicrograph by Philippe Belley; field of view 2.4 mm.

Figure 14. Unidentified aluminosilicate (possibly halloysite or another clay mineral) filling a fracture incordierite from Blue Bear. Backscattered electron
SEM image; field of view 650 μm.

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IOLITE FROM BRITISH COLUMBIA, CANADAGEMS & GEMOLOGYFALL 2023

Alt.

Fl. incl.

Alt.

Alt.
Alt.

Ms

Crd

Figure 16. Inclusion of primary muscovite (Ms), whichis partly altered to unidentified aluminosilicates(Alt.), and blocky fluid inclusions (Fl. incl.) in
cordierite (Crd) from Blue Bear. Note the fractures
leading to the partly altered muscovite. Backscattered electron SEM image; field of view 165 μm.

Figure 17. Densely included cordierite from Blue Bearcontaining a grouping of zircon crystals with yellowish radiohalos, white muscovite, and small colorless
fluid inclusions. The fluid inclusions proximal to thezircon group appear primary, while those in the lower
right corner occur on a rehealed fracture and are thus
secondary. Photomicrograph by Philippe Belley; fieldof view 1.88 mm.

spaced, euhedral to subhedral, light-colored or colorless prismatic crystals associated with biotite (figures
17 and 19). The zircons appear yellowish due to thepresence of radiation damage in cordierite proximal tothese grains, making their true color difficult to assess.
Partly to wholly altered muscovite and less commonly
biotite, which are replaced by hydrous aluminosilicates, occur in close proximity to fluid inclusions thatindicate the presence of rehealed fractures. Unalteredmica-species grains occur in areas not traversed bysuch features. This hydrous aluminosilicate alteration
rarely contains very small (approximately 5–10 μm) inclusions of pyrrhotite or barite.

Chatoyancy-Causing Inclusions. Acicular inclusions
are common in cordierite from the Blue Bear claim.
They are less abundant in facetable material (figure15) and more abundant (to the point of decreasingtransparency) in chatoyant material. The inclusions
are straight and oriented parallel to the direction ofgreatest light absorption in cordierite (the dark violetblue axis). Even in single cabochons, these inclusions
vary in concentration, forming bands of slightly
higher or lower inclusion density (figure 20, left). The
inclusions vary in length and, to a lesser degree, diameter (figures 15 and 20, right). A polished sectionof chatoyant cordierite, cut at an oblique angle to the

Figure 18. Zircon (Zrn) and xenotime-(Y) (Xtm-Y) inclusions in gem cordierite (Crd) from Blue Bear. Backscattered electron SEM image; field of view 150 μm.

Figure 19. Zircon (white) associated with biotite (Bt;iron-rich phlogopite) in gem-quality cordierite fromBlue Bear. Backscattered electron SEM image; field ofview 120 μm.

Xtm-Y

Zrn

Bt
Crd

IOLITE FROM BRITISH COLUMBIA, CANADAGEMS & GEMOLOGYFALL 2023

351

Figure 20. Magnified view of the 12.02 ct cat’s-eye iolite cabochon from Blue Bear. Note the variable inclusion density within the stone and the variable length of the acicular inclusions. Photomicrographs by Philippe Belley;
fields of view 5.57 mm (left) and 2.81 mm (right).

acicular inclusions, was examined using SEM in secondary electron mode. This revealed that the exposed
inclusions (≤2 μm in diameter) are hollow and thus
do not consist of solid inclusions (figure 21).

DISCUSSION
Deposit Geology. The iolite occurrences in the ThorOdin dome of British Columbia bear many resemblances to other deposits and occurrences worldwide:
They all occur within, or contiguous to, upper amphibolite to granulite facies magnesium- and aluminum-rich metamorphic rocks. Cordierite-bearing
rocks in the Thor-Odin dome, like those in mostother localities, are associated with rock-forming
amounts of orthoamphibole (gedrite, though somelocalities contain anthophyllite). These rocks differ
from those reported in the Bandarguha and Oraba-

hala areas of India, which are principally composedof cordierite and biotite (Das and Mohanty, 2017). Iolite localities that have been subject to more extensive petrological and geochemical research, such as
Thor-Odin (Hinchey and Carr, 2007) and the Gecomine (Pan and Fleet, 1995) consist of alkali- and calcium-depleted, magnesium- and aluminum-enriched
metamorphic rocks with hydrothermally alteredmafic volcanic (i.e., basaltic) protoliths.
Gem cordierite mineralization at Blue Arrow issimilar in habit to another locality in the Thor-Odin
dome, where cordierite occurs as anhedral crystals in
the interstices of large gedrite crystals (Hinchey andCarr, 2007). While the Blue Bear locality occurs in acomparable geological setting—cordierite-gedrite
rocks—the nature of the mineralization differs fromthat at Blue Arrow: Cordierite crystals are anhedral

Figure 21. Secondary electron images of a chatoyant Blue Bear iolite that was polished to crosscut the acicular inclusions. The images clearly show that the elongated cylindrical inclusions consist of fluid inclusions and are notfilled with any solid material. Fields of view 200 μm (left) and 15 μm (right).

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IOLITE FROM BRITISH COLUMBIA, CANADAGEMS & GEMOLOGYFALL 2023

TABLE 2. Summary of inclusions occurring in gem-quality cordierite.
Thor-Odin, Canada(present study)

India (Kammerling and Koivula, 1991;
Madagascar
Das and Mohanty, 2017; Muyal and(Graziani and Guidi, 1978)
Mattlin, 2019)

Biotitexx

Hematitexx

RutilexApatitex

QuartzMuscovite

x

x
x
x

x

Orthopyroxene
Xenotime-(Y)xx

Zirconxx

Monazitex

Dolomitex

PyriteWhite acicularinclusions

Wyoming,
United States (Hausel,
2003)

x
x

x

to euhedral and occur both within the gedrite-richrock (where they are sometimes enveloped by a layerof biotite) and in quartz veins cutting this rock. The
occurrence of cordierite within crosscutting quartz
segregations is reported from the Thor-Odin dome forthe first time in the present study, but similar mineralization is known at the nearby localities in Passmore, British Columbia (Simandl et al., 2000), and at
the Geco mine in Ontario (Pan and Fleet, 1995).

x

factors would also present challenges to gem mining,should a significant deposit be discovered.

Potential for Future Production. Beautiful facetedstones, generally smaller than 2 ct, have been produced in very small quantities from the Blue Arrow
and Blue Bear localities, and attractive cat’s-eye cabochons to ~12 ct have come from Blue Bear. The original bedrock source for the iolite-bearing boulders at
Blue Arrow has yet to be discovered. Iolite was only
discovered recently at Blue Bear, and the area has beenminimally explored. Further exploration at both sites
has the potential to yield new iolite-producing zones.
Extremely coarse-grained cordierite-bearing metamorphic rocks are widespread throughout the ThorOdin dome (Duncan, 1984; Norlander et al., 2002;
Hinchey and Carr, 2007; Goergen and Whitney, 2012;
present study). Potential targets for gem cordierite
exploration are therefore numerous, and prospectors
may discover new iolite deposits in the future. Exploration in this region is not without its challenges.
The rugged terrain and remoteness of most of themetamorphic dome make exploration difficult, timeconsuming, costly, and sometimes dangerous. These

Inclusions. Inclusions reported in gem-quality
cordierite include a variety of silicates, oxides, sulfides, and phosphates (table 2), with the most common being hematite and biotite. Hematite occurs in
the “bloodshot” iolite variety (India and Sri Lanka;
Kammerling and Koivula, 1991; Muyal and Mattlin,2019) as oriented crystals causing asterism (Bui et al.,2022) or as small isolated inclusions (Thor-Odin
dome; present study). Iron-rich phlogopite (biotite)
inclusions are reported from Wyoming (Hausel,
2003), India (Das and Mohanty, 2017), and Thor-Odin(present study). Rutile is reported in iolite from
Madagascar (Graziani and Guidi, 2018) and India(Das and Mohanty, 2017) but was not observed inThor-Odin samples. Quartz was observed as inclusions in iolite from India (Das and Mohanty, 2017);
at the Thor-Odin dome, quartz was found as large inclusions on the edge of large fractured crystals but
was absent in rough and faceted stones. Zircon inclusions were noted from both India (Das and Mohanty,2017) and Thor-Odin. In the present study, zircon
generally occurred in small groupings of numerous
crystals associated with iron-rich phlogopite inclusions. Apatite inclusions have only been reported
from Madagascar (hydroxyl end member; Graziani
and Guidi, 1978) and from Thor-Odin (present study).
Mineral inclusions reported only from a single locality are the following: dolomite (Madagascar; Graziani

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353

and Guidi, 1978); monazite, orthopyroxene, quartz,
and pyrite (India; Das and Mohanty, 2017); and muscovite, xenotime-(Y), and secondary alteration products from mica inclusion breakdown, which include
an unidentified hydrous aluminosilicate (possibly
clay minerals) and rare pyrrhotite and barite (ThorOdin; present study). Muscovite is commonly associated with cordierite, where it is a retrograde
metamorphic alteration product of the latter (e.g.,
Layman, 1963). Muscovite inclusions—which, basedon grain boundary relationships, form a stable assemblage with gem cordierite at the Blue Bear occurrence—are to the author’s knowledge, reported here
as primary inclusions for the first time.
Primary blocky fluid inclusions and secondary
fluid inclusions, the latter occurring on rehealed subconchoidal fractures, are common in Thor-Odin iolite. While they have not been reported from other
iolite localities worldwide, such inclusions are likely
to occur at most if not all of these localities due tothe prevalence of similar fluid inclusions in mostmetamorphic gem minerals.
White acicular inclusions in cordierite, describedas possibly being sillimanite, are reported by Hausel(2003) in gemstones from Wyoming. Parallel acicularinclusions are also reported from an iolite cabochonof Indian origin, where these inclusions are the causeof chatoyancy (Kammerling and Koivula, 1991). Acicular inclusions resembling those from the Indian iolite cabochon occur abundantly in Blue Bearcordierite. These are discussed next.

Cause of Chatoyancy. Chatoyancy in iolite is rare,caused by fine acicular inclusions (Kammerling andKoivula, 1991). In the cat’s-eye iolites from Blue Bear,the orientation of the fine acicular inclusions is parallel to the direction of strongest visible light absorption (dark violet-blue) and perpendicular to the otherpleochroic colors (light grayish yellow and light bluegray). In iron-bearing cordierite, the strongest absorption occurs parallel to the direction of highest refractive index (γ), which is the Z vibration axis, wherelight is absorbed by intervalence charge-transfer(IVCT) between Fe2+ in the octahedral site and Fe3+ at
the T1 tetrahedral site of the cordierite structure—and to a much greater extent than absorption by
IVCT occurring along the Y axis (Goldman et al.,1977). The γ-axis, along which visible light is moststrongly absorbed, is oriented parallel to the a-axis ofcordierite (see figure 4 of Fritsch and Rossman, 1988).
The Blue Bear chatoyancy-causing inclusions occur

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IOLITE FROM BRITISH COLUMBIA, CANADA

in the same orientation as similar colorless acicularinclusions previously reported in chatoyant iolitefrom South India (Kammerling and Koivula, 1991),
as well as colored acicular hematite inclusions causing chatoyancy in iolite cabochons from an unknown
country of origin (Bui et al., 2022). The nature andcomposition of the colorless acicular inclusions reported by Kammerling and Koivula (1991) were not
identified. In the present study, the secondary electron images of a chatoyant cordierite polished at an
oblique angle to the long axis of the acicular inclusions reveal that they consist of “empty” depressions. Thus, the inclusions in Blue Bear chatoyant
iolite, and likely that in iolite reported by Kammerling and Koivula (1991), consist of fluid inclusions.

CONCLUSIONSThe Blue Arrow and Blue Bear gem cordierite claims
at the Thor-Odin dome near Revelstoke in British Columbia, Canada, are two new examples of iolite occurrences hosted in hydrothermally altered mafic
rocks metamorphosed at upper amphibolite to granulite facies. At these occurrences, cordierite (XMg =0.81–0.90) occurs in association with gedrite, iron-rich
phlogopite (biotite), quartz, almandine-pyrope garnet,
and various accessory and trace minerals. Test sampling on the claims yielded strongly pleochroic, gemquality rough that produced dark violet-blue facetedstones generally smaller than 2 ct (both localities), as
well as light grayish blue to dark slightly grayish violet-blue chatoyant cabochons up to 12.02 ct (Blue Bear
only). Coarse-grained cordierite-rich rocks are widespread in the Thor-Odin dome, which indicates goodpotential for future iolite discoveries in the region.
Primary mineral inclusions in gem-quality Blue
Bear cordierite include iron-rich phlogopite, muscovite, zircon, xenotime-(Y), hematite, and apatite.
Muscovite and xenotime-(Y) are, to the author’s
knowledge, the first reported examples of these minerals as primary inclusions in iolite. Where cordierite
is traversed by rehealed fractures, mica inclusions arealtered to unidentified hydrous aluminosilicatesrarely containing trace pyrrhotite or barite.
Three types of fluid inclusions occur in ThorOdin iolite: (1) primary blocky fluid inclusions; (2)primary acicular fluid inclusions oriented parallel tothe a-axis of cordierite, which cause chatoyancy
when present in high concentrations (at Blue Bear
only); and (3) secondary, generally subequant, fluidinclusions occurring along rehealed subconchoidal
fractures.

GEMS & GEMOLOGYFALL 2023

ABOUT THE AUTHOR
Dr. Philippe Belley manages the Gem Science Research Groupin his role as assistant professor in the Department of Earth Sciences, Memorial University of Newfoundland in St. John’s,
Canada.

ACKNOWLEDGMENTS
The author thanks Herb Hyder for providing samples, information, and photographs; D.J. Lake for help in the field and forproviding additional field photographs; Dylan Goudie for support with SEM-EDS and sample preparation; and Dr. Wanda
Aylward for support with EPMA. The manuscript was improved
thanks to detailed comments from three anonymous reviewers.

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Lab Notes
Editors
Thomas M. Moses | Shane F. McClure | Sally Eaton-Magaña

DIAMOND
Etch Features Reveal theMorphology of Diamond
Natural monocrystalline diamonds
formed in the earth’s mantle have
two major growth forms: octahedronand cube. It has been suggested that
diamond growth habit depends on
multiple factors, including crystallization temperature, growth rate,
and the degree of carbon supersaturation in the diamond-forming
fluids (J.W. Harris et al., “Morphology of monocrystalline diamond andits inclusions,” Reviews in Mineralogy and Geochemistry, Vol. 88, No.
1, 2022, pp. 119–166). If a diamond
forms under changing conditions, it
might not have a regular octahedral
or cubic morphology but a combination of both. In some cases, diamonds
grow without any well-defined crystal faces and are referred to as “irregular diamonds” (see examples in R.
Tappert and M.C. Tappert, Diamonds in Nature: A Guide to RoughDiamonds, Springer-Verlag, Berlin,
2011, p. 28). While octahedral or
cubic faces cannot always be identified visually in irregular diamonds,
the morphology can be inferred byetch features left on the diamond by
fluids in the mantle or during kimberlite eruption. Trigons and tetragons are the most common of these

Editors’ note: All items were written by staff
members of GIA laboratories.
GEMS & GEMOLOGY, Vol. 59, No. 3, pp. 356–369.
© 2023 Gemological Institute of America

356

LAB NOTESTrigons

Tetragons

Figure 1. Table (left) and pavilion (right) views of a chameleon diamondwith visible trigons and tetragons. The trigons and tetragons are shown inmore detail in figures 2 and 3. The diamond measures 1.28 cm in length.

etch features—restricted to octahedral
and cubic crystal faces, respectively.
The Carlsbad laboratory recently
examined a 2.00 ct Fancy browngreenish yellow diamond with both
trigons and tetragons on its surface(figure 1). The trigons were locatedwithin a small indented natural near
the table facet, while the tetragons
were preserved within an indented
natural at the culet (figures 2 and 3).
Co-occurring trigons and tetragons
are very uncommon and indicate anirregular morphology, characterized
by both octahedral and cubic growth.
This stone was identified as achameleon diamond. For such a classification, the diamond must include agreen color component, show phosphorescence to short-wave UV, andchange from a greenish color to a yellow or orange color with gentle heatingor when kept in the dark for an extended period (C.M. Breeding et al.,“Natural-color green diamonds: A
beautiful conundrum,” Spring 2018
G&G, pp. 2–27). These diamonds typically have a broad visible absorption
band centered at approximately 480
nm and are referred to as “480 nm
band diamonds.” A study on roughyellow diamonds from Canada showedthat all of those colored by the 480 nm
absorption band have irregular morphology (M.Y. Lai et al., “Yellow diamonds with colourless cores –evidence for episodic diamond growth
beneath Chidliak and the Ekati mine,
Canada,” Mineralogy and Petrology,
Vol. 114, 2020, pp. 91–103), suggesting

GEMS & GEMOLOGYFALL 2023

Pointed-bottom trigon
with circular etch pit

Flat-bottom trigon

Figure 2. Flat-bottom and pointed-bottom trigons (surface features restricted to an octahedral crystal face) preserved on an unpolished portionof the diamond. A circular etch pit overprinting a pointed-bottom trigon
indicates a late-stage diamond dissolution event that postdated the formation of the trigon. Field of view 1.26 mm.

that diamonds with the 480 nm absorption band may form under changing conditions in the mantle.
Trigons on diamond have two
major morphologies: either a flat bottom or a pointed bottom (pyramidal).
The type of trigon is primarily dependent on the temperature and pressure atthe position where diamond dissolu-

tion occurs, as well as on the carbon dioxide and water contents in the fluidthat etches diamonds. Experimental
work indicates that fluids with higher
carbon dioxide content tend to produce
pointed-bottom rather than flat-bottom trigons (Y. Fedortchouk, “A new
approach to understanding diamondsurface features based on a review of

Figure 3. Tetragons (surface features restricted to the cubic crystal face)preserved at the culet of the diamond. Field of view 1.26 mm.

LAB NOTESGEMS & GEMOLOGY

experimental and natural diamondstudies,” Earth-Science Reviews, Vol.
193, 2019, pp. 45–65). Both forms of trigons were observed on the chameleon
diamond we examined, as well as a circular etch pit overprinting a pointedbottom trigon (figure 2). Unlike
pointed-bottom trigons, circular pits
are typically created by fluids with low
carbon dioxide contents, indicating alate-stage etching event after the formation of the trigon.
The combination of surface features provides clues for the morphology of diamonds in rough form, as
well as the interactions between diamonds and fluids throughout theirjourney from mantle residence to theearth’s surface.
Mei Yan Lai and Matthew Hardman

Natural Diamond with Unusual
Phosphorescence
Phosphorescence spectroscopy showsthat natural type IIb diamonds canhave two broad phosphorescencebands: one centered at 500 nm that appears greenish blue and one centered at
660 nm that appears red. Generally,
one of these phosphorescence bands isdominant. In the Hope diamond, forexample, the phosphorescence initiallyappears orangy red and transitions to
red as the subordinate greenish blue
phosphorescence fades away (S. EatonMagaña et al., “Using phosphorescence as a fingerprint for the Hope andother blue diamonds,” Geology, Vol.
36, No. 1, 2008, pp. 83–86). Occasionally, the faster-decaying greenish
blue phosphorescence band will transition to the slower-decaying red band
(Spring 2007 Lab Notes, pp. 47–48).
Recently, a 0.90 ct diamond with
D color and I1 clarity was submitted
to GIA’s laboratory in Ramat Gan, Israel, for a diamond dossier report. Infrared (IR) absorption spectroscopy
indicated that the natural diamond
was nominally type IIa. However,photoluminescence (PL) spectroscopy
revealed that the diamond containedboron-related features, including the648.2 nm peak attributed to a boron
interstitial (B. Green, “Optical and

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357

Figure 4. These deep-UV (<225 nm) phosphorescence images of the table (left) and pavilion (right) in a 0.90 ct
D-color natural diamond with very low boron showed localized areas of the 500 nm (greenish blue) and 660
nm (red) bands.
magnetic resonance studies of point
defects in single crystal diamond,”
PhD thesis, University of Warwick,2013), along with the observed phosphorescence. These features indicatedthat the diamond did contain boron,
but below the detection level of IR absorption spectroscopy. The distinctive
aspect was that both the red andgreenish blue phosphorescence werelocalized and visible within the sameimage (figure 4).
The phosphorescence is attributed
to donor-acceptor pair recombination.
For the 500 nm band, this recombination has been identified as interactions
between boron acceptors and isolated
nitrogen donors (J. Zhao, “Fluorescence, phosphorescence, thermoluminescence and charge transfer insynthetic diamond,” PhD thesis, University of Warwick, 2022), while the660 nm band is attributed to boron acceptors and unknown donors that arelikely related to plastic deformation(S. Eaton-Magaña and R. Lu, “Phosphorescence in type IIb diamonds,”
Diamond and Related Materials, Vol.
20, No. 7, 2011, pp. 983–989). PL maps
were collected on the table and pavilion with 455, 532, and 785 nm excitation; only an undefined PL feature at
813 nm showed an apparent increaseon the pavilion within the red-phosphorescing region (figure 4, right).
Type IIb diamonds grown by highpressure, high-temperature (HPHT)
methods almost always show the 500358

LAB NOTES

nm phosphorescence and occasionally a separate orange phosphorescence band centered at 580 nm. In a
few instances, HPHT-grown diamonds have shown the 500 and 580
nm bands simultaneously in different
portions of the diamond (Eaton-Magaña and Lu, 2011; Spring 2023 Gem
News International, pp. 155–156).
This is the first time the authors
have seen spatially distinct observations of red and greenish blue phosphorescence in a natural diamond, an
unusual find.
Sally Eaton-Magaña,
Evelina Goldort, and Dani BinyaminType IIa diamonds typically display a blue fluorescence pattern underdeep UV as a result of “band A” fluorescence or the N3 defect in the stone(U.F.S. D’Haenens-Johansson et al.,“Synthesis of diamonds and theiridentification,” Reviews in Mineralogy and Geochemistry, Vol. 88, No. 1,
2022, pp. 689–753). High amounts ofnitrogen vacancy (NV) centers cancause red fluorescence in diamonds;

Figure 5. This 3.58 ct natural typeIIa diamond with H color and SI2
clarity displayed a unique fluorescence pattern when exposed todeep-UV light.

Natural Type IIa Diamond with
Unusual Red Fluorescence
Distribution
The Carlsbad laboratory received anatural type IIa diamond with some
unusual characteristics. As part of thestandard data collection, the diamondtype was determined by Fouriertransform infrared (FTIR) spectroscopy and deep-UV fluorescence
images collected using the DiamondView instrument. The DiamondView
images revealed predominantly red
fluorescence on one side of the diamond and blue fluorescence on theother. This 3.58 ct pear modified brilliant diamond had H color and received an SI2 clarity grade due tocavities, chips, and a feather (figure 5).

GEMS & GEMOLOGYFALL 2023

PL SPECTRAINTENSITY

Red-fluorescing region
Blue-fluorescing region

NV0

NV–
Raman line

560

580

600

620

640

WAVELENGTH (nm)Figure 6. 514 nm PL spectra collected on the pavilion of opposite sides of the stone. The spectrum plotted in red
corresponds with the red-fluorescing region, and the blue spectrum corresponds with the blue-fluorescing region,
with Raman lines at 552 nm scaled as equal. Spectra are offset vertically for clarity. PL analysis spots are circled in
the deep-UV fluorescence images.
however, this is rare in type IIa stones,
as they are defined by nitrogen concentrations low enough to be undetectable by FTIR (Summer 2016 Lab
Notes, pp. 189–190). The NV centers
can be detected in a negative charge
state (NV–, zero-phonon line at 637nm) or a neutral state (NV0, zerophonon line at 575 nm). Photoluminescence (PL) testing revealed muchhigher concentrations of both NV
centers in the red-fluorescing region of
the stone (figure 6). This distribution
could have occurred if traces of isolated nitrogen were concentrated in
one growth area of the stone.
PL mapping using 532 nm laser
excitation (figure 7) displayed the relative intensities of these defects, demonstrating the wide distribution of theNV centers in opposite sides of thetable and crown facets of this diamond. Analysis of this interesting fluorescence feature demonstrated that
nitrogen could have been distributed
unevenly in a relatively pure type IIa
diamond when it initially formed.
Taryn Linzmeyer andBarbara Whalen

151

NV0 intensity

NV– intensity

Figure 7. False-color 532 nm PL map showing the Raman-normalized peak area of NV0 (left) and NV– (right). The
maps were compiled from 47,040 spectra. The pronounced gradient in NV centers across the diamond resulted
in the different fluorescence color observations (see figure 6 insets).

00

LAB NOTESGEMS & GEMOLOGYFALL 2023

359

Figure 8. The etch channel in this CVD laboratory-grown diamond extends across the table facet into the crown. Field of view 7.19 mm.

LABORATORY-GROWN DIAMOND
CVD “Etch” Channel
GIA’s New York laboratory recently
examined a diamond that had beengrown via chemical vapor deposition(CVD) and processed using a high-pressure, high-temperature (HPHT) treatment. Standard grading procedures
identified it as a 4.00 ct H-color cushion modified brilliant with SI1 clarity.
The diamond was identified as HPHT
processed primarily due to the green
fluorescence observed in the stone (seeFigure 9. Dark residue is visible insome parts of the channel, as seen
through the pavilion. The appearance of the residue is similar tothat of graphitized carbon but
could also be attributed to nondiamond carbon. Field of view
1.99 mm.

360

LAB NOTES

figure 10), while distinct fluorescent
growth layers across the table and pavilion confirmed its CVD origin.
While GIA regularly receives
CVD-grown diamonds submitted forlaboratory-grown diamond reports,
this particular example exhibited aunique characteristic. An etch channel–like structure was plainly visible
across the table of the stone, a feature
not observed in CVD laboratorygrown diamonds at GIA so far. The
channel broke the surface of the tableand extended across the stone, ending
near a pavilion facet on the other side
(figure 8). When viewed under highmagnification, a dark residue (figure9) was observed around parts of thechannel. This residue was not present
throughout the channel and appeared
to be opaque. Close to the surfacereaching channel on the table facet
were multiple smaller channels contained within the diamond. The surface-reaching opening of the channel
was imaged with a scanning electron
microscope (SEM), which revealed ajagged, slit-shaped entrance, suggesting that the shape of the channel extending throughout the diamondstructure was similar in nature (figure11). Some fragmentary material
trapped just inside the entrance of the
opening appeared to extend into thechannel itself. This fragmentary ma-

GEMS & GEMOLOGY

Figure 10. DiamondView imaging
revealed that the channel was
surrounded by blue-fluorescing
dislocation bundles extending across the growth layers of the CVD
diamond.

terial could have been the same typeof opaque residue observed in someareas of the channel.
Fluorescence imaging with the
DiamondView instrument also revealed the presence of this channel,
which could be discerned by the bluefluorescing line of dislocation bundles
following the channel’s path (figure10). These dislocation bundles appeared almost perpendicular to thegreen-fluorescing growth layer boundaries, suggesting the channel was

Figure 11. A secondary electron
SEM image showed the finer
structure of the channel opening,
revealing a jagged, slit-like feature. Field of view 16 μm.

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PL SPECTRA
NV0

Typical CVD
This CVD

INTENSITY

NV–

SiV–

600

650

700

750

466

WAVELENGTH (nm)

467

468

469

470

WAVELENGTH (nm)Figure 12. The PL spectra for a 1.40 ct CVD-grown diamond are shown in red. Left: The 514 nm PL spectrum reveals relatively weak NV centers and no detectable silicon. For comparison, the 514 nm PL spectrum for a morecommonly observed as-grown CVD (purple) shows much stronger NV centers and detectable silicon. The Ramanlines (not shown) are scaled as equal. Spectra are offset vertically for clarity. Right: Many locations on the CVDgrown diamond were analyzed with 457 and 633 nm excitation to verify whether the 468 nm center or the SiV–
center could be detected; this very weak peak of the 468 nm center was only detected at the culet.

created by exploiting weaker dislocation bundles that propagated throughthe diamond during CVD growth.
The presence of this channel in a
CVD laboratory-grown diamond andthe shape of the channel opening wereunlike any natural etch channel opening or laser drill hole opening previously seen. It is unclear whether thischannel was created during initial
CVD growth or during HPHT processing. The opaque residue visible insome parts of the channel as well as
the material visible at the channel surface could point to the presence ofgraphitized carbon, suggesting thechannel was a byproduct of HPHT
treatment, or it might indicate thepresence of non-diamond carbon,
which is a common feature found in
untreated CVD laboratory-grown diamonds. In summary, the residue mayarise from either the CVD growth process or post-growth treatment. Because this feature has not previously
been documented at GIA, futurestudies may help determine the mechanism of action that could createchannels in CVD-grown diamonds.
Elina Myagkaya

LAB NOTESCVD-Grown Diamond with Few
Diagnostic Features
Diamonds grown by chemical vapor
deposition (CVD) have evolved rapidly in the last two decades. However, the vast majority submitted
to GIA for reports still show several
diagnostic features such as the silicon vacancy or SiV– defect (a doublet
at 736.6 and 736.9 nm) and/or the
468 nm peak when analyzed by photoluminescence (PL) spectroscopy.
Additionally, most CVD diamonds
have distinctive fluorescence images
when analyzed by deep UV with the
DiamondView instrument.
Therefore, it was interesting to analyze a 1.40 ct E-color, VVS2 type IIa
diamond submitted for a laboratorygrown diamond report. Initial PL datacollection did not show either the SiV–
center or the 468 nm peak. The features observed were a 3H peak at 503.5
nm using 455 nm excitation and relatively weak nitrogen vacancy (NV)centers at 575 (NV0) and 637 (NV–) nm
using 514 nm excitation (figure 12,left). The stone showed weak anomalous birefringence indicating strain;
while this observation is helpful in

GEMS & GEMOLOGY

confirming whether a diamond isgrown by high-temperature, high-pressure (HPHT) methods, such birefringence cannot distinguish between
natural and CVD origins. The deep-UV
fluorescence image showed only deep
blue coloration, which appeared generally comparable with the vast majority of natural type IIa diamonds (figure
13, left). Therefore, many of the standard markers of CVD origin were notpresent even when examined using advanced methods. Careful analysis of
the fluorescence image revealed somedistinct differences from the majorityof natural diamonds, namely thepatchiness of the fluorescence withsome inert regions.
Many models of the DiamondView
instrument include a series of filtersthat permit fluorescence imaging inwhich portions of the visible spectrum
are blocked out. Using these filters, theblue portion of the visible spectrum
was blocked in order to observe
weaker underlying fluorescence features. With the orange long-pass filter
blocking wavelengths less than 550
nm, NV-related fluorescence clearly
showed the striations indicative of

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361

Figure 13. Left: The deep-UV (<230 nm) fluorescence image showed blue coloration (due to the presence of dislocation bundles) that appeared nominally similar to DiamondView images of natural type IIa diamonds. Right:
When the fluorescence was filtered by an orange long-pass filter that blocked wavelengths below 550 nm, theNV-related fluorescence and the striations indicative of CVD growth became apparent.

CVD growth (figure 13, right). Also,
several locations of the diamond wereanalyzed with PL spectroscopy to determine if either the 468 nm center or
the SiV– could be detected. Only at theculet was a small 468 nm feature observed (figure 12, right).
The author concluded that thiswas a CVD-grown diamond with noevidence of post-growth treatment.
This example is a noteworthy indicator that as CVD growth methods
improve, telltale defects are being
minimized or eliminated entirely.
Careful examination of all data, both
spectroscopic and fluorescence imaging, is needed to provide accurate
origins for diamond.

and an outer diameter of 20.32–20.40
mm. The ring was produced by Dutch
Diamond Technologies in collaboration with the Belgian jewelry store
Heursel, established in 1745.
This laboratory-grown diamondring was cut from an 8.54 ct plategrown by chemical vapor deposition(CVD). Laser cutting produced a nearperfect circular ring, as demonstratedusing digital imaging analysis to measure its inner and outer circumferences(figure 15).
The manufacture of a solid singlecrystal diamond ring is a complex andchallenging process. Once material isremoved from the crystal, very fine
fractures and imbalanced strain canhave severe consequences, and the ringcan shatter. Processing can take more

Figure 14. The 4.04 ct ring fashioned from a single-crystal CVDgrown diamond.

Figure 15. OpenCV software wasused to draw perfect circles fromthe center of the ring, marked bya green dot.

Sally Eaton-Magaña

Solid Laboratory-Grown SingleCrystal Diamond Ring
Carved single-crystal diamond rings
are rare, with few examples to reference (see Spring 2020 Lab Notes, pp.
132–133). But with advancing technology in the laboratory-grown diamond industry, the creation of solid
faceted diamond rings is now a reality.
Recently submitted to the New Yorklaboratory was a 4.04 ct single-crystal
laboratory-grown diamond ring (figure14). The 3.03 mm thick band had an
inner diameter of 16.35–16.40 mm

362

LAB NOTESGEMS & GEMOLOGYFALL 2023

PEARLS
A Traditional Bombay Pearl Bunch

Figure 16. Strong birefringence observed under cross-polarized light (left)
and blue fluorescence observed in the DiamondView (right).

than six months. Creating the ringfrom a single-crystal laboratory-growndiamond involved about 1,131 hours ofprocessing (including pre-grinding) in
the Dutch town of Cuijk, followed by
751 hours of polishing in Belgium.
Infrared absorption analysis revealed it was a type IIa diamond (nodetectable nitrogen), and no hydrogenrelated peaks were detected at 3107 or
3123 cm–1. Photoluminescence spectroscopy showed interesting vacancyrelated defects indicative of amulti-step growth process. A verystrong emission from the silicon vacancy (SiV–) defect was observed with
633 nm laser excitation; this is a common feature used to identify CVDgrown diamond. The conclusion wasthat this ring had not undergone postgrowth treatment; nevertheless, it wasnotable that the ring showed relatively
low nitrogen-related defects such as
NV centers, along with no detectable
468 nm peak or the 596/597 nm doublet. P. Martineau et al. (“Identification of synthetic diamond grownusing chemical vapor deposition(CVD),” Spring 2004 G&G, pp. 2–25)
observed the 596/597 doublet with a
514 nm laser. Also of note, there were
no H3 or 468 nm centers detectedwith 457 nm laser excitation.
Strong birefringence was observed
using cross-polarized light under microscopic analysis, with an unusual radial pattern (figure 16, left) signifying

LAB NOTES

unique growth conditions compared
to typical CVD diamonds available inthe market today. The deep ultraviolet
illumination of the DiamondView (figure 16, right) confirmed these unusual
growth features with a lack of postgrowth fluorescence colors such as
green produced by the H3 center.
The solid laboratory-grown diamond ring was studied by GIA and determined to have VVS2 clarity basedon dark non-diamond carbon pinpoint
inclusions (growth remnants typicalof CVD-grown diamonds) with Good
polish (polishing on the interior surface of the ring precluded Very Good).
Although the ring was a near-perfectcircle (figure 15), symmetry was considered not applicable. The diamond
had an E color specification based on
GIA’s color grading scale. The ring
was laser inscribed “LaboratoryGrown” on a side facet.
The quality and size of this solid
diamond ring (see the video clip at
www.gia.edu/gems-gemology/fall2023-lab-notes-solid-CVD-ring) provide a great example of the advancing
technology in laboratory-grown diamonds and changing trends in the jewelry industry. This is the first time a
GIA laboratory has examined a colorless diamond ring fashioned entirely
from a laboratory-grown diamond.
Paul Johnson, Stephanie Persaud,and Madelyn Dragone

GEMS & GEMOLOGY

During the era when the maharajahs
and maharanis ruled much of India,
natural pearls were in great demand.
In the nineteenth and early twentiethcenturies, many merchants from theMiddle East relocated to Bombay (nowMumbai) to benefit from this demand(R. Carter, Sea of Pearls, Arabian Publishing, London, 2012, pp. 169–175).
Thus, it is no surprise that India has
been a significant center of pearl manufacturing, processing, and trading for
centuries. To showcase their highquality pearls, traders and artisans in
Bombay would sort the pearls, string
them with silk threads, and tie them
together at both ends using decorative
metallic cords of various colors. Thesebunches were in great demand in theheyday of the natural pearl market andwere sold as “Bombay bunches” (M.
Manutchehr-Danai, Dictionary ofGems and Gemology, Springer, Berlinand Heidelberg, 2008, p. 101).
Recently, GIA’s Mumbai laboratory had the opportunity to examine an 80-year-old Bombay bunch
inherited by one of the city’s established pearl families. The bunch consisted of 553 light cream round andnear-round drilled pearls, strung in
nine hanks, each containing four
strands, with the exception of one
hank with seven strands. The hanks
were tied at both ends with white
metal wires, together with silver andblue cords and tassels (figure 17). The
pearls ranged from 3.83 to 5.72 mm,
and the total weight of the bunch
was approximately 80.51 g. The
owner stated that the total weight ofthe pearls was approximately 130
chow (equivalent to 331.8 ct or 66.35
g). Chow is a system of converting
weight into volume (The Pearl Blue
Book, CIBJO, 2020).
The strung pearls were perfectly
matched in color and exhibitedsmooth surfaces with high luster.
When viewed under 40× magnification, they all exhibited typical nacreous overlapping aragonite platelets.
Their internal structures were examined by real-time microradiogra-

FALL 2023

363

Figure 17. A Bombay bunch consisting of nine hanks and 553 high-quality natural pearls ranging from 3.83 to
5.72 mm.

phy. The majority revealed fine
growth arcs and concentric ringed
structures, while others revealed tightor minimal structures, some withsmall faint cores or light organic-richcores (figure 18). It was evident thatthese were natural pearls from Pinctada-species mollusks, as their internal structures were very similar to
those observed in Pinctada radiata
pearls from the Arabian (Persian) Gulf
when compared to GIA’s researchdatabase.

All pearls showed an inert reaction
when exposed to optical X-ray fluorescence analysis. Energy-dispersive Xray fluorescence spectrometry on a
few samples revealed manganesevalues ranging from below detection
limits to 30.7 ppm and strontium
values of 1215 to 1989 ppm, consistent with a saltwater growth environment. Raman analysis using 514 nm
excitation was also carried out on thesurface of selected pearls showing adoublet at 702/705 cm–1 as well as a

Figure 18. Real-time microradiography images of eight pearls from theBombay bunch revealing fine growth arcs and concentric ringed structures typical of those internal structures observed in Pinctada radiata
pearls.

364

LAB NOTESGEMS & GEMOLOGY

peak at 1085 cm–1, indicative of aragonite. Very weak polyenic pigmentrelated peaks at 1130 and 1530 cm–1
were also observed in a few of thepearls, likely associated with theirlight cream coloration (A. Al-Alawi etal., “Saltwater cultured pearls from
Pinctada radiata in Abu Dhabi(United Arab Emirates),” Journal ofGemmology, Vol. 37, No. 2, 2020, pp.
164–179). The photoluminescence
spectra under 514 nm excitation werealso consistent with the Raman results and displayed high fluorescence
together with the aragonite peaks,
typical of most nacreous pearls. When
exposed to long-wave ultraviolet
light, the pearls showed a moderate
blue reaction.
Historically, Bombay bunches
were known to contain high-quality
Pinctada radiata pearls fished fromthe Arabian (Persian) Gulf, commonlyreferred to as “Basra pearls” in the
trade. Records indicate that the proportion of round pearls found in eachnatural pearl harvest today is lessthan 5% (H. Bari and D. Lam, Pearls,
Skira, Milan, 2010, p. 43). Natural
pearls are rare to find, and it can takedecades to match a round pair of similar size, color, and luster. Encountering Bombay bunches in today’s

FALL 2023

Figure 19. A white button-shaped
atypical bead cultured pearl
weighing 8.15 ct and measuring
11.73 × 11.35 × 9.03 mm.

Figure 21. XRF reaction of thepearl, with the sides appearing
brighter than the base and apex.

DiamondView instrument revealed aclear bluish reaction, with the surface
showing a chalky white flame-likestructure extending from the apex
center across the pearl. Raman analysis of the pearl’s surface using a 514
nm excitation showed a doublet at
701/704 cm–1 as well as peaks at 1086
and 1464 cm–1 characteristic of aragonite. The photoluminescence spectra
revealed high fluorescence, with amaximum centered at approximately
620 nm.
Energy-dispersive X-ray fluorescence spectrometry revealed a low
manganese level of 16.9 ppm and ahigher strontium level of 1577 ppm,
indicative of a saltwater environment.
However, when exposed to X-ray fluorescence (XRF) analysis, the pearl exhibited a moderate yellowish green
fluorescence with a higher intensity
toward the sides but a weaker reaction on the base and apex (figure 21).
Saltwater pearls are usually inert
when tested by this method, while
freshwater pearls and saltwater beadcultured pearls with freshwater shell
nuclei tend to fluoresce weak tostrong yellowish green. In the case ofbead cultured pearls, a thinner nacre
usually produces a stronger reaction;
conversely, the thicker the nacre, theweaker the reaction.
Real-time microradiography imaging (RTX) revealed a long, complex
linear feature in the center typicallyassociated with non-bead cultured
pearls (figure 22, left). When viewed
under X-ray computed microtomo-

GIA’s Mumbai laboratory recently received an atypical bead cultured pearl
with a unique surface appearance.
The white button-shaped pearl
weighed 8.15 ct and measured 11.73 ×
11.35 × 9.03 mm (figure 19).
Viewed under 40× magnification,
the surface exhibited typical fine nacreous overlapping aragonite platelets,
but a distinct pattern similar to theflame structures observed in someporcelaneous pearls was also noted(Summer 2021 Lab Notes, pp. 152–
153). These flame-like features wereformed due to the presence of relatively opaque, frosty white rounded
depressions on the pearl’s surface. Although rounded, they seemed to beoriented longitudinally and weremore tapered toward the base andapex and broader along the sides of thepearl (figure 20). Also observed on thesurface were small white spots withslightly elongated comet tails thatflowed tangentially to the circumference of the button. The base and apexof the pearl lacked the whitish subsurface opaque features that produced
the flame structure effect, and these
two areas appeared more translucent
than the other parts of the pearl.
Under long-wave ultraviolet radiation, the pearl showed a strong yellowish green reaction, and a similar
but weaker reaction was noted undershort-wave ultraviolet radiation. The
deep-UV (<230 nm) radiation of the

Figure 20. Rounded depressions
forming the flame-like structure.
Field of view 0.4 mm.

Figure 22. Left: RTX image of the pearl’s internal structure with a large
linear feature. Right: μ-CT image showing a freshwater-type linear featurein the center and a faint boundary (arrow).

market is a rarity, and it was a great
opportunity for GIA Mumbai to examine such an interesting and memorable pearl submission.
Pfokreni Nipuni, Abeer Al-Alawi,
and Roxane Bhot Jain

Flame-Like Surface Structure in a
Nacreous Atypical Bead Cultured
Pearl

LAB NOTESGEMS & GEMOLOGYFALL 2023

365graphy (μ-CT), the linear feature
looked very similar to those often observed in freshwater non-bead cultured pearls. A faint boundary was
also seen around the central linear feature (figure 22, right) and other growthrings. All these observations, together
with the XRF reaction of the pearl andthe saltwater chemistry, were indicative of an atypical bead cultured
saltwater pearl with a freshwater pearl
used as its nucleus (P. Kessrapong and
K. Lawanwong, “Atypical bead cultured Pinctada maxima pearls nucleated with freshwater non-bead
cultured pearls,” GIA Research News,
April 6, 2020). The nacre thickness
overlying the freshwater non-bead cultured pearl nucleus ranged from approximately 1.09 to 1.90 mm. The
thicker nacre was positioned near thebase and apex and the thinner nacre
toward the sides. This aligned with
the XRF reaction observed, as athinner saltwater nacre will allow
more fluorescence from the freshwater pearl nucleus to pass throughthe nacre layers, while thicker nacremasks the reaction.
Over the years, GIA has encountered similar atypical bead cultured
pearls with sizeable freshwater nonbead cultured pearls used as a nucleus(Spring 2023 Lab Notes, pp. 74–76).
However, the presence of flame-likestructures seen on nacreous pearls is
a very rare phenomenon. This combination of interesting culturing alongwith the unique surface features
make this pearl noteworthy.
Prasad Mane, Nishka Vaz, andAbeer Al-Alawi

Pearls in Traditional
Indian Nose Rings
The beauty of Indian jewelry lies in
the artisanship involved in creating
intricate, unique designs. One example is the classic Indian nose ring(known as the nath). It is typically
cashew shaped, with a chain to connect it to a hairpiece or earring. The
nath exemplifies traditional Maharashtrian jewelry and is usuallycrafted in yellow gold. Made famous

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LAB NOTES

Figure 23. Two traditional Indian-style pearl nose rings (nath) set with colored gemstones and pearls measuring approximately 6.63 × 6.09 mm to8.42 mm (left) and 5.38 mm to 8.02 × 7.61 mm (right).

during the Peshwa rule, the nath has
been crafted using diamonds, pearls,
rubies, and emeralds. Today, the nathis worn mainly for weddings or special occasions, while everyday noseadornments take the form of smaller,
simpler studs or rings made from goldor silver. Recently, GIA’s Mumbai laboratory received two traditional noserings for pearl identification.
Each was set with 17 drilled
pearls, near-drop and button, ranging
from light cream to cream in color.
They were skillfully strung together
with yellow metal wire and set withcolored gemstones of various shapes
and cutting styles. The pearls in thelarger nose ring (figure 23, left) ranged
from approximately 6.63 × 6.09 mm
to 8.42 mm, and the item weighed atotal of 13.96 g. Those in the smaller
nose ring (figure 23, right) ranged from
5.38 mm to 8.02 × 7.61 mm, and thispiece weighed 9.05 g overall. Whenviewed under 40× magnification, thepearl surfaces in both pieces showed
typical nacreous overlapping aragonite platelets and a medium to highsurface luster. Energy-dispersive X-ray
fluorescence (EDXRF) spectrometry
revealed manganese levels below the
instrument’s detection level and astrontium content averaging 950
ppm. In addition, the pearls were inert
to X-ray fluorescence, indicative ofsaltwater origin. They showed a moderate greenish yellow reaction underlong-wave ultraviolet light and aweaker reaction of similar color undershort-wave UV. Real-time microradiography imaging revealed internal
structures similar to those observedin natural pearl studies for various
Pinctada-species mollusks (K. Scarratt et al., “Natural pearls from Australian Pinctada maxima,” WinterGEMS & GEMOLOGYFALL 2023

Figure 24. Real-time microradiography of the internal structures observedin the larger nose ring consisting of organic-rich acicular structures andgrowth arcs (left) and a “collapsed core” near the drill hole area surrounded by growth arcs (right).

of 950 ppm) and manganese (mostly
below detection limits) consistent
with that mollusk species and below
those expected from P. maxima pearls(A. Homkrajae et al., “Internal structures of known Pinctada maxima
pearls: Natural pearls from wild marine mollusks,” Spring 2021 G&G,
pp. 2–21).
The combinations of natural
pearls made these two pieces very attractive. It is no easy task to create
traditional nose ornaments withpearls of such size. These two uniquepearl jewelry items provide an excellent example of Indian heritage.
Lubna Sahani, Abeer Al-Alawi, andRoxane Bhot Jain

2012 G&G, pp. 236–261; N. Nilpetploy et al., “The gemological characteristics of Pipi pearls reportedly from
Pinctada maculata,” Winter 2018
G&G, pp. 418–427). The radiopaqueareas visible on the microradiographsof both items correspond to the yellow metal fittings and wire, since
they are denser and prevent X-rays
from passing through.
Five pearls from the larger nath revealed an organic-rich internal structure with varying patchy light anddark gray contrasting areas, alongwith an acicular structure radiatingfrom their centers that occupied almost half of the internal structure(figure 24, left). Two pearls showed“collapsed cores” close to the drill
holes (figure 24, right); the bulk of thecores, if any, were removed as a resultof drilling. The remaining ten pearls
displayed fine concentric rings andgrowth arcs, all proving their natural
origin.
Microradiography of the pearls inthe smaller nath revealed similar internal structures. While the majorityof those pearls showed fine concentricrings and growth arcs, three of them(marked with arrows in figure 25) also
possessed organic-rich and acicularcores that occupied varying degrees of
their structure. The internal structures observed in both items contained classic natural pearl structures
similar to those observed in known
Pinctada radiata pearls in GIA’s research database. The EDXRF results
were also within the range expected
for Pinctada radiata samples, withthe lower levels of strontium (average

RUBY“Nebula” Inclusion in Ruby
Beryllium-Diffused to Heal Fractures
Recently the Carlsbad laboratory received a 4.12 ct purple-red stonemeasuring approximately 11.50 ×
7.70 × 4.72 mm (figure 26) for identification services. It featured unusualveil-like reddish color zoning wafting
throughout a purple bodycolor. The
refractive index measured 1.760–
1.770, identifying the stone as a ruby.

Figure 26. An unusual ruby withcoloring reminiscent of a nebula
found in the depths of outer space.

Figure 25. The majority of the pearls in the smaller nose ring showed faintgrowth arcs and concentric ringed structures, while three of them showedorganic-rich acicular cores (marked with arrows) typical of natural pearls.

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Figure 27. Fingerprints resulting from flux-assisted heating of a berylliumrich flux. Field of view 7.19 mm.

Microscopic examination indicated that the reddish color zoning
was associated with altered fingerprints that resembled a cosmic nebula
due to a flux-rich residue (figure 27).
These fingerprints appeared to be consistent with fractures that had been
healed using a flux-assisted heatingprocess in a high-heat environment.
Due to the indications of highheat, laser ablation–inductively coupled plasma–mass spectrometry (LAICP-MS) chemical analysis was
performed to check for the presence ofberyllium diffusion. Initially, LA-ICPMS conducted on two spots revealed
beryllium concentrations of <0.019
ppma and 60.87 ppma. This significant discrepancy prompted additional
LA-ICP-MS testing in three spots, revealing beryllium concentrations of<0.19 ppma, 1780 ppma, and <0.19
ppma. Microscopic observation of thetesting spots showed areas of high beryllium on or near the reddish colorzones, while areas of very low beryllium were associated with testingareas on the purple areas of the stone.
Therefore, it was concluded that aberyllium-rich flux was used to heal
the fractures naturally present in thestone. The ruby was likely heated inthis flux at temperatures high enoughto allow the beryllium to seep fromthe flux into the stone, but not long

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LAB NOTES

enough for the beryllium to infiltrate
its entirety.
Michaela Damba

Translucent Ruby Filled with
Zinc Glass
Recently the Carlsbad laboratory received an 8.57 ct translucent purplish
red heart-shaped mixed-cut stone for
identification service (figure 28). Stan-

dard gemological testing revealed a refractive index of 1.760–1.769, and aruby spectrum was obtained with ahandheld spectroscope. The stone
showed a medium to strong red fluorescence to long-wave UV radiation
and a very weak red fluorescence toshort-wave UV radiation. During microscopic analysis, several fracturescontaining a whitish filler and air
pockets were seen using fiber-optic
lighting (figure 29). Using reflected
light, fractures with a lower luster
than the host corundum were also observed, which confirmed the presenceof a filling material.
The stone did not show the blue
flash effect typically observed inrubies filled with lead glass (S.F.
McClure et al., “Identification anddurability of lead glass–filled rubies,”
Spring 2006 G&G, pp. 22–36). Although the filler improved the durability, it did not appear to improvethe clarity.
Glass filling treatment has beenused on heavily fractured rubies to increase their clarity and durability
since the early 1980s, and cavity filling was noted and described as early
as 1984. The original filler was silica
glass, which was easily visible since
its refractive index (~1.5) is significantly lower than corundum (R.E.

Figure 28. An 8.57 ct heart-shaped fracture-filled ruby treated with a zincglass filling.

GEMS & GEMOLOGYFALL 2023

Figure 29. A fracture with whitish filler (left) and trapped air pockets (right). Fields of view 1.58 mm (left) and2.90 mm (right).

Kane, “Natural rubies with glassfilled cavities,” Winter 1984 G&G,
pp. 187–199). In early 2004, gemologists discovered a new type of glassfiller with high lead content and amuch better clarity enhancement due
to its higher reflective index (~1.70)(GAAJ Research Laboratory, “Leadglass impregnated ruby,” March 15,2004). Since then, lead glass fillers
have become the most popular filler
for rubies, although other glass types
such as bismuth and cobalt have also
been used (T. Leelawatanasuk et al.,“Cobalt-doped glass-filled sapphire:
An update,” Australian Gemmologist, Vol. 25, No. 1, 2013, pp. 14–20;
Spring 2020 Lab Notes, p. 139).
In the heart-shaped ruby submitted
to the laboratory, energy-dispersive X-

ray fluorescence spectroscopy did notdetect lead or bismuth but did show
chromium, iron, and zinc. Additional
chemical analysis was performed withlaser ablation–inductively coupledplasma–mass spectrometry on two
spots of one larger fracture with a substantial amount of filler in order toquantitatively measure the elementsin the glass filler. The averages of thetwo analysis spots were 28750 ppmw
silicon, 1091 ppmw zinc, and 108
ppmw lead.
Chemical results and gemologicalproperties revealed that this glassfiller was not the lead or bismuth
glass the authors first suspected, butrather a silica-based glass doped withzinc. While this type of filler had visual properties similar to those ofother glass fillers in corundum, it did
not show a flash effect. This is thefirst time the authors have encountered a ruby filled with zinc glass.
Shiva Sohrabi and Amy Cooper

PHOTO CREDITS
Adriana Robinson—1, 26, 28; Matthew Hardman—2, 3; Evelina Goldort—4; Diego Sanchez—5; Barbara Whalen—6; Elina
Myagkaya—8–10; Sally Eaton-Magaña—13(right); Towfiq Ahmed—14, 15; Paul Johnson—
16 (left); Madelyn Dragone—16 (right); Gaurav
Bera—17, 19, 23; Nishka Vaz—20; Nathan
Renfro—27; Shiva Sohrabi—29

For online access to all issues of GEMS & GEMOLOGY from 1934 to the present, visit:

gia.edu/gems-gemology

LAB NOTESGEMS & GEMOLOGYFALL 2023

369

Editor: Nathan Renfro
Contributing Editors: John I. Koivula and Tyler Smith

Breyite in Diamond
The author recently examined a 0.71 ct, D-color, type IaB
diamond with an interesting inclusion reminiscent of astone splashing in water (figure 1). Raman analysis identified the well-formed colorless transparent crystal as breyite(CaSiO3), a calcium-rich silicate mineral commonly foundin sublithospheric or superdeep diamonds. It was hypothesized that this diamond formed in the lower part of thetransition zone or the lower mantle beneath the earth’s
crust.
Since the majority of diamonds in the jewelry industry
come from the lithosphere, each one provides a window
into Earth’s intricate geology. Inclusions such as this one
serve as a reminder that beauty and flaws may coexist
peacefully, whether they add to a diamond’s charm or provide scientific insight.
Aprisara Semapongpan
GIA, Bangkok

Spray of Columbite Crystals in Topaz
A probable columbite inclusion in beryl was featured ina recent Quarterly Crystal, demonstrating the striking

About the banner: Numerous epidote crystals show colorful birefringence inplane-polarized light within their rock crystal quartz host. Photomicrographby Nathan Renfro; field of view 5.75 mm.
GEMS & GEMOLOGY, VOL. 59, NO. 3, pp. 370–379.
© 2023 Gemological Institute of America

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MICRO-WORLD

Figure 1. A well-formed colorless breyite crystal reminiscent of a stone making a splash in water. Photomicrograph by Aprisara Semapongpan; field of view
1.07 mm.

form these minerals can display (Spring 2023 G&G
Micro-World, pp. 90–91). It was only fitting to follow up
with a look at the same inclusion hosted by a different
mineral.
The cluster of bladed columbite crystals in figure 2,
identified via Raman spectroscopy, was found in an 8.91 ct
colorless topaz. The thinner crystals allowed the brownbodycolor to show through, while the thicker, sword-likecenter crystal appeared black and opaque. Thin-film interference was observed between the host and inclusion whenilluminated with oblique lighting, adding some welcome
color to an otherwise featureless crystal face. While columbite has been previously described in topaz (Fall 2009 G&G

GEMS & GEMOLOGYFALL 2023

Figure 2. Bladed crystals of columbite radiate from a central pointin colorless topaz. The
mirror-like surfaces
allow for reflections ofneighboring blades, asseen in the top left crystal. Photomicrograph
by Tyler Smith; field ofview 2.90 mm.

Lab Notes, pp. 212–213; E.J. Gübelin and J.I. Koivula,
Photoatlas of Inclusions in Gemstones, Volume 2, Opinio
Publishers, Basel, Switzerland, 2005, pp. 263, 738), this is
the most dramatic example observed by the author.
Tyler Smith
GIA, New York

weapon belonging to the Hindu god Hanuman. This etchchannel was the only one observed in the diamond; thesefeatures are often created by dissolution processes (T. Lu
et al., “Observation of etch channels in several natural diamonds,” Diamond and Related Materials, Vol. 10, No. 1,
2001, pp. 68–75).
This unusual clarity characteristic showcases the variety that is possible within the natural world.

Inclusion Resembling a Gada in Diamond

Rujal Kapadia and Bhavya Maniar
GIA, Surat

Recently, the authors observed an interesting etch channel(figure 3) in a 1.00 ct L-color type Ia round brilliant diamond with SI1 clarity. It resembled a gada, a mace-like

Sally Eaton-Magaña
GIA, Carlsbad

Figure 3. This etch channel seen in the crown facets of a 1.00 ct diamond with SI1 clarity (left) resembled a gada(right), a mace-like weapon wielded by the Hindu god Hanuman. Photomicrograph (left) by Deepak Raj; field ofview 0.80 mm. Photo (right) courtesy of the Metropolitan Museum of Art.

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371

Figure 4. This iridescentferropericlase crystal
with a stress halo measuring approximately 0.7
mm was trapped inside
a light brown diamond.
Photomicrograph by
Kyaw Soe Moe; field ofview 1.58 mm.

Iridescent Inclusion in Brown Diamond
The iridescent crystal inclusion in the 1.07 ct light brown
type IIa diamond in figure 4 was identified by Raman spectroscopy as ferropericlase. Ferropericlase, (Mg,Fe)O, canoriginate from either shallow (lithospheric) or deep (sublithospheric) depths within the earth. A stress halo was developed around the crystal by temperature and pressurechanges during the ascent to the earth’s surface. The colorful iridescence may have been caused by light interference at a thin film trapped between the inclusion and thehost diamond. It is rare to see ferropericlase as an inclusion
in diamond, especially such a large crystal.
Kyaw Soe Moe
GIA, New YorkNatural Diamond with Twinning Wisps
Resembling a Whale
The authors recently examined a 0.46 ct type Ia round brilliant diamond with SI1 clarity that had surprisingly patterned twinning wisps. When viewed through the table, thetwinning wisps resembled a whale (figure 5). Twin planes
can form in diamond when the orientation of the crystal
structure changes during growth in the earth’s mantle.
Twinning wisps are atomic defects along this plane, foundin ~10% of type Ia diamonds (S. Eaton-Magaña et al., “Natural-color D-to-Z diamonds: A crystal-clear perspective,”
Fall 2020 G&G, pp. 318–335).
A termination of one of these features just below thepavilion was examined using photoluminescence (PL)mapping at various excitation wavelengths to examine the
distribution of atomic-level defects in the crystal lattice.
The PL mapping with 455 nm excitation revealed a relative increase in the defect concentrations of the nitrogenrelated centers of H3 (NVN0) with zero-phonon line (ZPL)

372

MICRO-WORLDat 503.2 nm and H4 (4N+2V) with ZPL at 495.9 nm; a weakradiation-related defect, the TR12 with ZPL at 469.9 nm
was also detected (A.M. Zaitsev, Optical Properties of Diamond, Springer-Verlag, Berlin and Heidelberg, 2001, andreferences therein). PL mapping using 633 nm excitation
revealed that the GR1 (V0; ZPL at 741.2 nm) had a higher
Raman-normalized peak area corresponding to the termination of the twinning wisp compared to diamond adjacentto the wisp. The GR1 defect is typically formed by radiation damage of a diamond lattice. Although the GR1 waselevated, we did not observe radiation stains around thetwinning wisps. Furthermore, there were no visible
changes to the color of the diamond’s fluorescence when
exposed to deep ultraviolet luminescence, which can occur
with significant radiation exposure.

Figure 5. This round brilliant diamond displays twinning wisps that resemble a whale. Photomicrographby Nathan Renfro; field of view 2.62 mm.

GEMS & GEMOLOGYFALL 2023

Figure 6. Gas bubbles
hide just beneath natural crystal inclusions inthis garnet and glass
doublet. Photomicrograph by Emily Jones;
field of view 1.58 mm.

This stone offers an example of how natural growth
processes in diamond can result in microscopic featuresthat occasionally form delightful patterns.
Taryn Linzmeyer and Roy Bassoo
GIA, Carlsbad
Jaldeep Sojitra
GIA, Surat

A Red Herring in a Red Garnet
At first glance, a 2.54 ct round mixed-cut stone examinedby the author presented itself as a natural red garnet. Microscopic inspection revealed a host of transparent crystals
and widespread epitaxial needles, both typical inclusions
for garnet. Upon closer inspection, however, some of theapparent “crystals” were recognized as gas bubbles, betrayed by their smooth spherical forms (figure 6). This distinction can be easily missed without careful examination
under high magnification. Additional observations andtesting conclusively identified the material as pyropealmandine garnet fused with manufactured glass. As withmost garnet-topped doublets, the garnet portion was cut tooccupy the crown while the glass made up most of the pavilion so that the natural inclusions of the garnet top
masked the telltale signs of manufactured glass beneath.

White, “Let’s get it right: Tajikistan heliodor,” Rocks andMinerals, Vol. 80, No. 4, 2005, pp. 285–286). The heliodor
had a large schorl tourmaline inclusion prominently under
the table facet (figure 7) and a small schorl tourmaline near
one corner. A thin band of fine fluid inclusions ran parallelto the length of the stone adjacent to the tourmaline. The
stone was clearly cut to highlight the interesting inclusion
rather than hide or remove it; aside from the two tourmaline inclusions and the thin band of fluids, the beryl had
high clarity.

Figure 7. Morphological features of the tourmaline
crystal included in the heliodor are visible; striations
and triangular growth marks are present. A band of
fluid inclusions is visible near the tip of the schorl
tourmaline. Photomicrograph by Rhiana Elizabeth
Henry; field of view 2.90 mm. Gift of Mark Mauthner,
GIA Museum no. 37772.

Emily Jones
GIA, New York

Heliodor with a Large Schorl Inclusion
An 8.33 ct long rectangular heliodor, the yellow variety ofberyl (ideally Be3Al2Si6O18), was examined by the author.
It was reportedly from the Zelatoya Vada mine in Murgab,
Tajikistan; however, it was likely from Pakistan (J.S.

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373

Figure 8. Blade-like magnetite inclusions developed from a healed fracture in a 2.08 ct Burmese peridot resemble arainy lakeside city. Photomicrograph by Makoto Miura; field of view 1.06 mm.

Both the heliodor and the tourmaline were analyzed by
Raman spectroscopy and laser ablation–inductively coupledplasma–mass spectrometry (LA-ICP-MS) to better understand the structure and chemistry of the minerals. Analyses
were conducted on the girdle of the heliodor and on the backend of the tourmaline inclusion where it intersected a pavilion facet. The heliodor had low-moderate water bound inits structural channels, as determined qualitatively by
Raman spectroscopy, which is expected for heliodor withlow sodium content (R.E. Henry et al., “Crystal-chemical
observations and the relation between sodium and H2O indifferent beryl varieties,” Canadian Mineralogist, Vol. 60,
No. 4, 2022, pp. 625–675). The heliodor fit well within the
expected chemistry; it had low overall cation substitutions,with minor or trace iron, magnesium, lithium, sodium, andcesium content. The tourmaline was confirmed to be schorl
dominant due to its predominant sodium and iron content;
however, due to high vacancy at the X structural site, thetourmaline had a high foitite component (D.J. Henry et al.,374

MICRO-WORLD“Nomenclature of the tourmaline-supergroup minerals,”
American Mineralogist, Vol. 96, 2011, pp. 895–913).
After data collection for research, this stone will be
used in the GIA GemKids program as an educational piecefor young students at the Carlsbad campus. It will represent the heliodor variety of beryl, showing fascinating inclusions as well as evidence of LA-ICP-MS laser pits on thegirdle of the stone (not shown in the photomicrograph).
Rhiana Elizabeth Henry
GIA, Carlsbad

Rainy Lakeside City in Peridot
Inclusions in gemstones provide useful information regarding their geological origin. A 2.08 ct Burmese peridot containing unique black blade-like inclusions developed fromhealed fractures (fingerprints), as shown in figure 8. Raman
spectroscopy identified these blade-like inclusions as magnetite. Tiny raindrop-like magnetite lamellae were also ob-

GEMS & GEMOLOGYFALL 2023

served throughout the stone. Such inclusions in Burmese
peridot sometimes create four-rayed asterism. According
to previous studies, the presence of magnetite lamellae
suggests the oxidation or dehydration of olivine by geological processes. This image resembles a rainy lakeside city
landscape.

ural healing process of a fracture oriented parallel to thec-axis.
These zigzag-patterned fingerprints often indicate a Sri
Lankan origin, but they can also be found in sapphires from
Myanmar and Madagascar. The undamaged rectilinear pattern could indicate that the stone is unheated.

Makoto Miura
GIA, Tokyo

Yuxiao Li
GIA, Tokyo

Zigzag Fingerprint in Sri Lankan Sapphire

Fissure with Moiré Pattern in Spinel

The author recently examined a 3.21 ct oval mixed-cut
blue sapphire. Chemical analysis and gemological observation of internal fissures suggested a Sri Lankan origin.
Under fiber-optic illumination, the fingerprints showedrectilinear zigzag-patterned films with vibrant colors that
resulted from thin-film interference (figure 9). The rectilinear structure of this fingerprint was caused by the nat-

Surface-reaching fissures commonly host precipitates of
epigenetic minerals, as was the case with a 4.22 ct purple
spinel recently examined by the author. “Islands” of unidentified birefringent inclusions occupied a near-planar
fissure. These inclusions were inert to Raman spectroscopy. Delicate depositions radiating from these islands interacted to create a complex moiré pattern (figure 10).

Figure 9. The rectilinear zigzag-patterned fingerprint in this 3.21 ct blue sapphire shows beautiful vibrant colors,
indicating a Sri Lankan origin. Photomicrograph by Yuxiao Li; field of view 4.45 mm.

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375

Figure 10. A precipitation of an unidentified
mineral in a fissure of a
purple spinel creates thishighly intricate moiré
pattern. A combinationof oblique fiber-optic
and darkfield illumination was used. Photomicrograph by Tyler
Smith; field of view 2.90mm.

Moiré patterns, named after their resemblance to a type offabric, form when parallel or concentric lines overlap. It
is unclear whether the islands formed first and were partially dissolved in a secondary event, or if they formed simultaneously with the spinel. Although moiré patterns
have been observed in partially healed fluid fingerprints
and surface-reaching fissures, it is rare to see them expressed in such a spectacular form.
Tyler Smith

Large Stellate Dislocation in Spinel
The author recently examined a 4.49 ct greenish blue spinel exhibiting a prominent stellate inclusion characterized
by a network of needle-like dislocations clustered in a sixrayed star pattern (figure 11). The inclusion was large
enough to be readily observed without magnification.
Star-like needles have been previously documented inspinel originating from Vietnam. These findings contributeto our understanding of spinel and its origins, providing

Figure 11. Stellate dislocations decorate the interior of a greenish blue
spinel believed to befrom Vietnam. Photomicrograph by Ezgi Kiyak;
field of view 4.79 mm.

376

MICRO-WORLDGEMS & GEMOLOGYFALL 2023

valuable information for gemologists. This is one of themost remarkable stellate inclusions the author has observed in a spinel.
Ezgi Kiyak
GIA, New York

Tourmaline in Emerald
The author recently examined a 3.80 ct emerald with wellformed black, opaque prismatic crystals (figure 12). The
surface-reaching crystals were identified as tourmaline
through the use of Raman spectroscopy. While tourmaline
crystals are relatively uncommon inclusions, they havebeen documented in emeralds from Zambia, Pakistan, Russia, and Ethiopia, all of which are schist-hosted deposits.
Laser ablation–inductively coupled plasma–mass spectrometry chemical analysis combined with inclusion observation revealed this emerald to be of Zambian origin.

Figure 12. Prismatic black tourmaline in a Zambian
emerald. Photomicrograph by Virginia Schneider;field of view 1.76 mm.

Virginia Schneider
GIA, New York

Turquoise Planet Earth
Armenia is not a widely known source of turquoise, but arecent donation to GIA’s colored stone reference collection
proved that the country can produce high-quality, aesthetically pleasing stones. One of them, cleverly cut into an

84.90 ct sphere measuring 23.70 × 23.88 × 23.91 mm, bore
an astonishing resemblance to planet Earth (figure 13). The
natural brown matrix material mimicked the continents,
while the greenish blue turquoise represented the oceans.
Infrared spectroscopy showed that, like most gem turquoise, the specimen had been polymer impregnated, atreatment that makes the stone more durable. Additionally,

Figure 13. An 84.90 ct sphere of Armenian turquoise measuring 23.70 × 23.88 × 23.91 mm displays a striking resemblance to planet Earth. Left: Diffuse fiber-optic lighting allows the face of the stone to be seen in full. Right:
Pinpoint fiber-optic lighting creates an appearance of day and night cast onto Earth. Gifted to GIA’s colored stonereference collection by Gemfab CJSC. Photos by Britni LeCroy.

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377

Figure 14. Dark yellowish to brownish green needle-like inclusions dominate the interior of this 21.36 ct Portuguese fluorapatite on siderite matrix. Photo by Adriana Robinson.

no dye was detected. To accentuate its likeness to ourplanet, the sphere was photographed against “Musou
Black,” marketed as the world’s blackest fabric and advertised to absorb 99.905% of all light. Precise placements of

378

MICRO-WORLDpinpoint lighting created various appearances of a sunlit
Earth suspended in space.
Britni LeCroy
GIA, Carlsbad

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Figure 15. Raman
analysis was used to
identify the acicular inclusions in the fluorapatite host crystal as
dravite tourmaline.
Photomicrograph by
Nathan Renfro; field ofview 7.37 mm.

Quarterly Crystal: Dravite in Fluorapatite
Growing from an angular matrix plate of brown siderite,
a doubly terminated hexagonal crystal weighing 21.36 ct
with a very light purplish blue color (figure 14) was identified as fluorapatite by Raman analysis. As the photoshows, the semitransparent crystal clearly hosts a numberof randomly arranged, eye-visible, translucent acicular inclusions. The fluorapatite thumbnail specimen, from thePanasqueira mine in the Castelo Branco district of Covilhã, Portugal, was acquired by author JIK from the col-lection of Dr. Vasco Trancoso at auction in June 2023.
When the specimen was examined microscopically, theacicular morphology and dark yellowish to brownish
green bodycolor of the inclusions, as well as their behavior
in polarized light, suggested they might be tourmaline.
Laser Raman microspectrometry was able to pinpoint
their identity as dravite (figure 15), a member of the tourmaline group.
John I. Koivula, Nathan Renfro, and Maxwell Hain
GIA, Carlsbad

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Editor: Evan M. Smith

Eclogite: Diamond-Encrusted Ancient Seafloor
Evan M. Smith and Matthew F. Hardman

The Link Between Eclogite and Diamonds
Earth’s surface is divided into tectonic plates, and the ones
that make up the ocean floor are destined to sink. Sheetsof oceanic crust typically bend and sink down into Earth’s
interior at so-called subduction zones. Occasionally thesebanished slivers of ancient seafloor may become a sort ofunexpected canvas to be painted with a sparkling sea of diamonds. In fact, ancient seafloor rocks that have been incorporated into old and thick parts of continents are oneof the most important mantle substrates or “host rocks”
for diamond growth (Stachel et al., 2022a). This particularhost rock, known as eclogite, can be found as distinct fragments, called xenoliths, at some diamond mines and is also
represented by mineral inclusions in diamonds (figure 1).
Our current understanding of where and how diamonds
crystallize is largely based on mineral inclusions trapped inside them. The diamonds we mine were brought up to
Earth’s surface by kimberlites and related magmatic rocks,
but they did not crystallize directly from this magma.
Rather, the diamonds formed by independent processes andare usually millions or billions of years older than the kimberlitic magmas that inadvertently swept them up to thesurface. Most diamonds (~98%) were formed at a depth ofabout 150–200 km within the continental lithosphere, essentially in old and thick parts of continents. Based on therelative abundance of mineral inclusions in a studied set of
2,844 diamonds, the three principal mantle host rocks here
are peridotite (65%), eclogite (33%), and websterite (2%)
(Stachel and Harris, 2008). Eclogitic diamonds constitute asignificant portion of the gem diamond market, and theirfeatures embody the dynamic processes unique to Earth.

What Is Eclogite?
Typically, eclogite refers to a colorful red and green metamorphic rock that is made up of two key minerals: grossular-almandine-pyrope garnet and omphacitic clinopyroxene(figure 2) (Winter, 2010). Additional minerals sometimes
present include rutile, coesite, kyanite, and iron-rich sulfides.
All of these can be encountered as mineral inclusions in diamond. Eclogite can form in several ways, but the dominant
process involves oceanic crust sinking down into the mantle(by subduction). The basalt and gabbro that make up oceanic

Figure 1. Mineral inclusions in diamond. This connected pair of green clinopyroxene and orange garnet
indicates that diamond growth occurred within an
eclogitic mantle host rock. Photomicrograph by
Nathan Renfro; field of view 2 mm.

Editor’s note: Questions or topics of interest should be directed toEvan Smith (evan.smith@gia.edu).
GEMS & GEMOLOGY, VOL. 59, NO. 3, pp. 380–386.
© 2023 Gemological Institute of America

380

DIAMOND REFLECTIONSGEMS & GEMOLOGYFALL 2023

Metamorphic rocks are sometimes classified into
groups, called metamorphic facies, based on the pressure
and temperature conditions at which they formed. One ofthese groups is known as eclogite facies. Any given metamorphic facies encompasses metamorphic rocks that form
under the same range of pressure and temperature conditions, regardless of what the exact starting rock, or protolith, might have been. Figure 3 shows where the eclogite
facies and other metamorphic facies fall within a pressure
vs. temperature diagram. When basalt or gabbro from theoceanic crust get subducted, they metamorphose throughzeolite and blueschist facies before transforming into eclogite at depths of about 50 km.
If the eclogite were carried deeper still, the clinopyroxene would begin to break down between 300 and 400 km,
followed by the appearance of a new phase, stishovite(SiO2) (Irifune and Ringwood, 1987). At this point, it would
technically not be eclogite but could be described by themore general terms metabasalt or metagabbro. With varying pressure and temperature, the mineralogy can change
drastically, even if the bulk composition of the rock remains unchanged.

Figure 2. Polished slice of an eclogite xenolith. Thisbacklit specimen shows the typical orange garnet andgreen clinopyroxene that make up eclogite. Recoveredfrom the Roberts Victor mine in South Africa, thiseclogite xenolith (sample JJG-243) is part of the Mantle Room collection curated by the University ofCape Town. Photomicrograph by Evan M. Smith; fieldof view approximately 5 cm.

Ancient Seafloor and the Process of Subduction
Inclusions (figure 1) and xenoliths (figure 2) show that
eclogite has served as a host rock for diamond growth at
depths of about 150–200 km. But that eclogite has actuallycrust undergo a transformation, known as metamorphism,
as a result of changes in pressure and temperature that causesome minerals to break down and new ones to form.

M ETAMORPHIC FACIES DIAGRAMTEMPERATURE (°C)
0

200

400

600

800

1000

Hornfels
0.2
10

Zeolite0.4

PrehnitePumpellyite

Greenschist
20

0.8

30

c
du

1.0

Granulitetio

Blueschist

n
ce

o

1.4

of

1.2

an
ic
cru
s

40
t

Eclogite

50

DEPTH (km)Amphibolite

S ub

PRESSURE (GPa)0.6

Figure 3. Metamorphic
facies diagram. Eachfield in this pressure vs.
temperature chart represents a group of metamorphic rocks thatshare similar characteristics. Eclogite is a highpressure metamorphic
facies as well as a rockname. The orange line
gives a hypotheticalpathway for the subduction of oceanic crust.
Boundaries are gradational and do not actually exist as sharp
divisions in P-T space.
Adapted from Winter(2010).

1.6

DIAMOND REFLECTIONSGEMS & GEMOLOGYFALL 2023

381

APPROXIMATE DEPTH
BELOW SEA LEVEL (km)Mid-oceanridge axis

Oceanic crust

Accretionary wedge

Volcanic arc

Continental crust

0
50
100

Kimberlite

Graphite stability
Upper mantle
upwells and melts

150

Diamond stability

Oceanic mantle

Continental
lithosphericmantleAsthenosphere(Convecting mantle)

200
250

Diamond
Continental mantle peridotite
Mantle eclogite

Figure 4. Diagram of a mid-ocean ridge and subduction zone. At mid-ocean ridges, the upper mantle upwells andmelts, and the melts crystallize as oceanic crust. The oceanic crust spreads away from the ridge and subducts
into the mantle beneath continental crust, which is more buoyant. Diamonds may be stable in the deep roots ofcontinents, within the continental lithospheric mantle. They may form in association with mantle peridotite or
eclogite. Kimberlite may rip diamond and mantle rocks from these deep roots as they erupt to the surface.

traveled a long way from its birthplace. Those sameeclogitic inclusions and xenoliths have chemical and isotopic characteristics indicating that the protolith originated at Earth’s surface as oceanic crust. Present-day ocean
floors are composed of oceanic crust with an underlying
layer of mantle peridotite, which together form a rigid plate
called lithosphere that “floats” upon the convecting upper
mantle because it is less dense (figure 4). Oceanic crust is
generated at mid-ocean ridges, where the two plates on either side of the ridge spread apart from one another. The
gap left between the diverging plates is filled by rock thatflows up from beneath and partially melts, producing themagma that will make up newly formed crust. When thesemagmas erupt at the seafloor, they cool quickly and forma dark, fine-grained rock called basalt. At deeper levels
within a mid-ocean ridge, the magma cools slowly andcrystallizes as coarser-grained rocks, chiefly gabbro.
Oceanic crust is produced continuously at mid-ocean
ridges on the ocean floor. Old crust is pushed farther fromthe ridge as new crust is produced. Old oceanic crust willcool, increasing its density. After millions of years, anddriven by continual mid-ocean spreading, the oceanic crust
will eventually collide with continental crust, which is
generally much less dense due to its different mineralogy
and elemental composition. When the oceanic crust collides with continental crust, the large difference in density
leads the oceanic crust to subduct into the upper mantle
beneath the continental crust (figure 4).
Subduction is the process by which a plate on theearth’s surface moves beneath another and sinks into themantle. During subduction, oceanic crust undergoes a progressive sequence of metamorphic reactions as pressure
and temperature increase with depth (figure 3). This in-

382

DIAMOND REFLECTIONS

creases the density of the rock, which may continue tosubduct until it transforms into eclogite. Occasionally,
subducted eclogite is incorporated into the deep roots beneath continents, where the majority of diamond formation occurs, as shown by the green and orange lenses infigure 4.
Geoscientists can inspect aspects of eclogite geochemistry and isotopic composition to reconstruct its history.
For example, when basalt forms at a mid-ocean ridge, it hasan oxygen isotope composition that is similar to the upper
mantle source region from which it was derived. When seawater interacts with oceanic crust, however, it can change
the oxygen isotope composition of the rock, shifting it
away from the original mantle-like value. Oxygen isotope
compositions of minerals in eclogite that are outside therange of the typical ambient mantle are considered evidence that the eclogite had a protolith that was exposed toseawater prior to subduction (Jacob, 2004; Korolev et al.,2018, and references therein).
Eclogites also have relatively high concentrations of
sodium compared to many other mantle rocks. Thissodium comes from plagioclase in the oceanic protoliths,
but during subduction the plagioclase becomes unstableand breaks down, with its constituent elements—including sodium—redistributed to new minerals formed duringmetamorphism.
With the proper analytical equipment, elements withextremely low abundance (e.g., concentrations at the parts
per million or billion level) can be measured in eclogite
minerals. Among these are the rare earth elements (REE),
which are a group of 17 elements including the lanthanides(lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dyspro-

GEMS & GEMOLOGYFALL 2023

REE PATTERNSCONCENTRATION NORMALIZEDTO CHONDRITE

1005). Some eclogites, however, may have a positive or negative europium “anomaly” when compared to samariumand gadolinium (figure 5). The mineral plagioclase tends tohave strong positive europium anomalies, as europium incorporates into plagioclase very strongly relative to other
REE. Consequently, an igneous rock that accumulates plagioclase (such as gabbro in oceanic crust) may inherit a positive europium anomaly. A rock that forms from a magmathat previously crystallized plagioclase (such as an oceaniclava) will have a negative europium anomaly. As plagioclase is only stable in the shallow portions of the earth (<30km, approximately), eclogites with detectable europium
anomalies are generally inferred to have protoliths thatformed at shallow depths. Seeing this signature in mantlederived rocks, such as eclogite xenoliths at a diamondmine, requires that those rocks formed near the surface andwere carried down into the mantle by subduction (and in
the case of xenoliths, these rocks have been brought backup again by kimberlites). Eclogite xenoliths really havecome a long way.

10

1

Negative Eu-anomaly
No Eu-anomaly
Positive Eu-anomaly

0.1

0.01
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Figure 5. REE patterns for three eclogites with different
europium anomalies. The concentrations of all elements are normalized to the elemental concentrations
in chondritic meteorites (McDonough and Sun, 1995).

sium, holmium, erbium, thulium, ytterbium, and lutetium)
plus scandium and yttrium. Slight differences in the behavior of these elements during geological processes can leadto changes in their relative concentrations, imparting a sortof chemical imprint of a rock’s history. REE concentrationsin igneous rocks can be inherited from their magmatic
sources and subsequently modified by melting and otherprocesses in the earth.
In many mantle rocks, REE patterns will have a smooth
trend, with the concentration of europium being intermediate to the elements samarium and gadolinium (figure

Recycled Carbon and Its Contribution to
Diamond Growth
Not all carbon atoms are identical. By paying close attention to differences in their mass (carbon isotopes), geologists have observed that diamonds formed in eclogite are
distinguished by an unusual signature (figure 6). The signature suggests that eclogitic diamonds contain “recycled”
carbon that was subducted from Earth’s surface down into
the mantle, where diamond growth occurred (figure 4).
A typical carbon atom has 6 protons and 6 neutrons in
its nucleus, which sum to give its atomic mass, 12. Carbon-

CARBON ISOTOPE HISTOGRAMS
Figure 6. Carbon isotope
histograms for peridotitic and eclogitic diamonds. While both
histograms show a mode
centered near –5‰ that
corresponds to mantle
carbon, eclogitic diamonds have a tail ofnegative values stretching beyond –40‰ that isthought to reflect subducted organic carbon.
Data are from Stachel etal. (2022b).

Peridotitic diamonds
400

FREQUENCY

200

0
300

Eclogitic diamonds

200
100
0
-40

-35

-30

-25

-20

-15

-10

-5

0

5

δ13C OF DIAMOND (‰)

DIAMOND REFLECTIONSGEMS & GEMOLOGYFALL 2023383

12 (12C) is the most common isotope of carbon. About 1%
of naturally occurring carbon atoms have an extra neutron
and therefore a mass of 13. Carbon-13 (13C) is the secondmost abundant isotope of carbon. Given that 12C and 13C
have different masses, they behave slightly differently during chemical reactions. When plants grow, for example,
photosynthesis favors 12C. As a result, plants are enrichedin 12C (and depleted in 13C) relative to the initial carbon
dioxide in the atmosphere. Scientists can gain insight into
many carbon-bearing materials by measuring the relative
proportions of 12C and 13C. For any measurement, the13
C/12C ratio is reported in terms of how different it isfrom an agreed-upon standard (a reference known as theVienna Pee Dee Belemnite). This deviation is written as
δ13C (pronounced “delta C thirteen”) expressed in parts
per thousand (per mille, ‰) and is calculated as δ13C =[(13C/12C)sample/(13C/12C)standard −1]×1000.
Figure 6 shows δ13C values for peridotitic and eclogitic
diamonds. The histograms convey information aboutsources of carbon and potentially about the chemical
process of diamond formation as well. In both histograms,
the large peak centered at –5‰ corresponds to the averagecomposition of carbon spread throughout the mantle (Cartigny, 2005). Eclogitic diamonds, however, possess a conspicuous tail of negative values or “light” isotopic
compositions. These negative values are one of the mostdebated features of diamond geology, with no firm consensus on the exact processes at play (Cartigny, 2005).
The most accepted general view is that the negative isotopic trend of eclogitic diamonds stems from subducted carbon (Li et al., 2019; Stachel et al., 2022b). Subducted carbon
is thought to mix with the ambient carbon already present
in the mantle to give the range of observed carbon isotopic
characteristics in eclogitic diamonds (figure 6). The recycled
carbon could come in the form of sediments and alteredoceanic crust containing organic matter and carbonates(biogenic and abiogenic) (Li et al., 2019). Taking into account the covariations in nitrogen concentration and isotopes suggests that eclogitic diamond formation involves
multiple subducted ingredients that mix in complex ways
during the process (Stachel et al., 2022b). Although there
are unresolved details, the evidence is strong that a portionof the carbon in eclogitic diamonds is subducted. These diamonds are a physical manifestation of the deep and ancient geological carbon cycle.

Eclogitic Diamond Ages
Through the incredible phenomenon of radioactive decay,
many rocks and minerals have a sort of built-in naturalclock. Radioactive decay is the process whereby unstableradioactive elements break down over time, with different
elements each having a predictable decay rate. Several
decay systems apply to various types of inclusions in diamonds, and their analysis has led to our understandingthat diamonds are millions to billions of years old (Smit384

DIAMOND REFLECTIONS

and Shirey, 2019). The oldest diamonds are 3.5 billion
years old, and diamonds have been forming episodicallysince that time, spanning three-quarters of Earth’s history.
Their antiquity combined with their deep mantle origin
makes diamonds a unique record of large-scale geologicalevolution.
For example, diamond ages may provide a time stamp
to mark the beginning of modern-style plate tectonics.
Many of us are at least vaguely familiar with the notionthat roughly 200 million years ago, the continents to theeast and west of the Atlantic Ocean were nestled together
in the supercontinent Pangaea. The Atlantic grew andopened up as Pangaea broke apart. The subduction zone
and mid-ocean ridge spreading center depicted in figure 4
are integral mechanisms that explain how tectonic plates
move, how new oceans form, and how old ones close andvanish. This dynamic surface structure is unique to Earth.
Our planet did not always have plate tectonics, however,
and at one point, its surface was a giant magma ocean. So
when exactly did things change and the processes we observe today take hold?
Geologists have used several methods to constrain thetiming of when plate tectonics began, including clues fromdiamonds. Comparing samples of different ages reveals thatthey are not all equal across time. Diamonds older than 3.0
billion years are exclusively peridotitic, whereas eclogitic
diamonds only become prevalent among those younger
than 3.0 billion years. This change arguably marked the firstmajor episode of subduction of oceanic crust beneath a continent and the onset of the Wilson-cycle style of movements that characterize modern plate tectonics (Shirey andRichardson, 2011). Effectively, this model suggests there areno eclogitic diamonds older than 3.0 billion years because
oceanic crust was not yet being subducted and incorporated
as eclogite into the continental lithospheric mantle.

Economic Importance of Eclogitic Diamonds
Finding an economic diamond deposit is no trivial matter.
Even after locating one or more kimberlites (figure 7) orlamproites, it is a challenging exercise to evaluate thequantity and quality of diamonds. A key variable that canmake or break a potential mine is the presence of eclogitic
diamonds. In some deposits, eclogitic diamonds are much
more abundant than might be expected based on the relatively small amount of eclogite in the mantle.
Most of the continental lithospheric mantle is madeup of peridotite (>95 vol.%; figure 4). Conversely, eclogite
has a low global abundance (<5 vol.%) (Dawson andStephens, 1975; Schulze, 1989), yet some mines can contain much higher abundances of eclogitic diamonds, to thepoint where they dominate the diamond population, suchas at Koidu (Sierra Leone) and Orapa (Botswana). One inthree diamonds have eclogitic vs. peridotitic mineral inclusions, indicating a relatively large proportion of diamonds are associated with eclogite (Stachel and Harris,

GEMS & GEMOLOGYFALL 2023

Present-day
erosive surface
Lake
Crater zone

Sedimentary rocks

Diatreme zone
Metamorphic rocks
Volcanic rocks

Root zone

Figure 7. Diagram of ahypothetical kimberlite
pipe. The kimberlite
erupts through existingrocks and geologic formations (“countryrock”). The shape, size,and distribution of rocks
within a kimberlite pipe
are highly variable fromoccurrence to occurrence. The country rockis arbitrary in this diagram and will vary
based on the local geology into which the kimberlite erupts. The
majority of kimberlites
are not diamond-bearing(Kjarsgaard et al., 2022).

Diamond
Continental mantle peridotite
Mantle eclogite
Archean basement

50 m2008). While not all eclogites are associated with diamond,small volumes of eclogite in some kimberlites could beindicative of diamond-rich deposits.
Kimberlites can be very small (as small as 50 m across)and show very little topographic relief, making them difficult to find. Over millions of years, these sites may becomecompletely covered by vegetation or water (forming lakes).
In these cases, kimberlites can be located by exploiting their
physical properties, as they erupt through existing rock andmay have different magnetic or gravimetric properties fromthe surrounding material. Aerial surveys that passively examine the landscape, searching for gravitational or magnetic anomalies, could potentially identify a kimberlite.
A more direct approach is to use kimberlite indicator
minerals (figure 8). Even in kimberlites that are diamondbearing, diamonds may be present in abundances at theparts per million level. Conversely, rocks that are associated with diamonds are generally much more abundantand easier to locate. Minerals in these rocks commonly

have elemental compositions that reflect the rock fromwhich they derived and, occasionally, an association with

Figure 8. Broken-up sample of kimberlite from theEkati diamond mine in northern Canada featuringcolorful mantle indicator mineral grains. The brightgreen diopside and purple garnets are from peridotite,and the orange garnets (indicated by arrows) arefrom eclogite. Photo by Evan M. Smith; field of view
approximately 2 cm.

DIAMOND REFLECTIONSGEMS & GEMOLOGYFALL 2023

385

diamond itself. For example, there is an observed association between diamond and mantle peridotites that are very
chemically depleted (i.e., experienced substantial melting
in the mantle, removing elemental components from theirstructure) (Gurney, 1984). The compositions of the garnets
in these peridotites are characterized by low calcium contents and high chromium contents (Grütter et al., 2004).
Garnets in eclogite are very different and instead have low
chromium contents and variable calcium (Grütter et al.,2004). Given the disproportionate number of eclogitic dia-monds compared to the global abundance of eclogite, evena small number of eclogitic garnets, such as the orange
grains in figure 8, may indicate an economically valuable
deposit. Minerals within kimberlite can be redistributedby rivers or glaciers, so if a garnet with composition similar
to those associated with diamond is found in stream sediment or glacial till, this could indicate that a kimberlite
deposit—possibly a diamond-bearing one—occurs upstream. When identified, a kimberlite deposit could one
day be developed into an operating diamond mine.

ABOUT THE AUTHORS
Dr. Evan M. Smith is a senior research scientist with GIA in New York, and Dr. Matthew F. Hardman is a research scientist at GIA in
Carlsbad, California.

REFERENCES
Cartigny P. (2005) Stable isotopes and the origin of diamond. Elements, Vol. 1, No. 2, pp. 79–84, http://dx.doi.org/10.2113/
gselements.1.2.79
Dawson J.B., Stephens W.E. (1975) Statistical classification of garnets from kimberlite and associated xenoliths. Journal of Geology, Vol. 83, No. 5, pp. 589–607.
Grütter H.S., Gurney J.J., Menzies A.H., Winter F. (2004) An updated classification scheme for mantle-derived garnet, for use
by diamond explorers. Lithos, Vol. 77, No. 1-4, pp. 841–857,
http://dx.doi.org/10.1016/j.lithos.2004.04.012
Gurney J.J. (1984) A correlation between garnets and diamonds inkimberlite. In J.E. Glover and P.G. Harris, Eds., Kimberlite
Occurrence and Origin: A Basis for Conceptual Models in Exploration, Volume Publication 8, Geology Department and University Extension, University of Western Australia, pp. 143–166.
Irifune T., Ringwood A.E. (1987) Phase transformations in primitive
morb and pyrolite compositions to 25 Gpa and some geophysicalimplications. In M.H. Manghnani and Y. Syono, Eds., High‐Pressure Research in Mineral Physics. Terra Scientific Publishing
Company, Tokyo, and the American Geophysical Union, Washington DC, pp. 231–242, http://dx.doi.org/10.1029/GM039p0231
Jacob D.E. (2004) Nature and origin of eclogite xenoliths fromkimberlites. Lithos, Vol. 77, No. 1-4, pp. 295–316,
http://dx.doi.org/10.1016/j.lithos.2004.03.038
Kjarsgaard B.A., de Wit M., Heaman L.M., Pearson D.G., Stiefenhofer J., Janusczcak N., Shirey S.B. (2022) A review of the geology of global diamond mines and deposits. Reviews in
Mineralogy and Geochemistry, Vol. 88, No. 1, pp. 1–117,
http://dx.doi.org/10.2138/rmg.2022.88.01
Korolev N.M., Melnik A.E., Li X.-H., Skublov S.G. (2018) The oxygen isotope composition of mantle eclogites as a proxy of their
origin and evolution: A review. Earth-Science Reviews, Vol. 185,
pp. 288–300, http://dx.doi.org/ 10.1016/j.earscirev.2018.06.007

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Li K., Li L., Pearson D.G., Stachel T. (2019) Diamond isotope compositions indicate altered igneous oceanic crust dominates deep
carbon recycling. Earth and Planetary Science Letters, Vol. 516,
pp. 190–201, http://dx.doi.org/10.1016/j.epsl.2019.03.041
McDonough W.F., Sun S.-S. (1995) The composition of the Earth.
Chemical Geology, Vol. 120, No. 3-4, pp. 223–253,
http://dx.doi.org/10.1016/0009-2541(94)00140-4
Schulze D.J. (1989) Constraints on the abundance of eclogite in
the upper mantle. Journal of Geophysical Research: Solid
Earth, Vol. 94, No. B4, pp. 4205–4212, http://dx.doi.org/
10.1029/JB094iB04p04205
Shirey S.B., Richardson S.H. (2011) Start of the Wilson Cycleat 3 Ga shown by diamonds from subcontinental mantle.
Science, Vol. 333, No. 6041, pp. 434–436, http://dx.doi.org/
10.1126/science.1206275
Smit K.V. and Shirey S.B. (2019) Diamonds from the Deep: How
old are diamonds? Are they forever? G&G, Vol. 55, No. 1, pp.
102–109.
Stachel T., Harris J.W. (2008) The origin of cratonic diamonds—
Constraints from mineral inclusions. Ore Geology Reviews,
Vol. 34, No. 1-2, pp. 5–32, http://dx.doi.org/10.1016/
j.oregeorev.2007.05.002
Stachel T., Aulbach S., Harris J.W. (2022a) Mineral inclusions inlithospheric diamonds. Reviews in Mineralogy and Geochemistry, Vol. 88, No. 1, pp. 307–391, http://dx.doi.org/
10.2138/rmg.2022.88.06
Stachel T., Cartigny P., Chacko T., Pearson D.G. (2022b) Carbon
and nitrogen in mantle-derived diamonds. Reviews in Mineralogy and Geochemistry, Vol. 88, No. 1, pp. 809–875,
http://dx.doi.org/10.2138/rmg.2022.88.15
Winter J.D. (2010) Principles of Igneous and Metamorphic Petrology, 2nd ed. Upper Saddle River, New Jersey, Pearson Education, 702 pp.

GEMS & GEMOLOGYFALL 2023

Knowledge a ndSkills f or Today’s
Hot Topics

Online and In-Person Seminars Designed
for Gem and Jewelry Professionals• Natural and LaboratoryGrown Diamonds• Jewelry Design• Colored Stones• Pearls• Bench Skills

Visit collective.gia.edu/seminars
for schedules and more information.

GIA Alumni receive a 10% discount.
Seminars are open to all.

C ON T INUING E DUC AT ION B Y

These seminars are avocational and are not intended to prepare individuals for entry-level employment.They are non-credit bearing and do not lead to a GIA academic credential.
They do not fall under the approval of the Accrediting Commission of Career Schools and Colleges (ACCSC) or the Distance Education Accrediting Commission (DEAC).
© 2023 Gemological Institute of America, Inc. All trademarks owned by Gemological Institute of America, Inc. and used under license.

GEM NEWS INTERNATIONAL
Contributing Editors
Gagan Choudhary, IIGJ-Research & Laboratories Centre, Jaipur, India (gagan.choudhary@iigjrlc.org)
Christopher M. Breeding, GIA, Carlsbad (christopher.breeding@gia.edu)
Guanghai Shi, School of Gemmology, China University of Geosciences, Beijing (shigh@cugb.edu.cn)

COLORED STONES AND ORGANIC MATERIALSThe special color effect in “chameleon” amber. One special
variety of Burmese amber is “chameleon” amber, named
for the bluish green color that appears to float on its surface(figure 1). This material is found in the famous Tengchongmarket in Yunnan Province, China’s largest Burmese
amber market. Its bodycolor ranges from golden brown tobrownish red or even red (figure 2A). When exposed to sunlight or strong white light against a black background, its
surface shows a uniform green color (figure 2B).
Chameleon amber is rare and expensive compared to regular Burmese golden or brownish amber, especially withthe severe reduction of exports from Myanmar.
Until recently, few studies have been conducted on
chameleon amber. X. Jiang et al. speculated that the green
surface color might be caused by the superposition of its
bodycolor and the fluorescence of amber (“Gemmologicaland spectroscopic characteristics of different varieties ofamber from the Hukawng Valley, Myanmar,” Journal ofGemmology, Vol. 37, No. 2, 2020, pp. 144–162). C.C. Shuai
et al. proposed that the “light retention” effect was related
to elevated sulfur and calcium contents (“The spectrum
characteristic research of color change effect and ‘lighttarry effect’ amber from Burma,” Spectroscopy and Spectral Analysis, Vol. 40, 2020, pp. 1174–1178). Z. Shi et al.
reported that the effect might be due to unique internalaromatic hydrocarbon structures (“Spectral characteristics
of unique species of Burmese amber,” Minerals, Vol. 13,
No. 2, 2023, article no. 151).

For this study, three Burmese chameleon amber specimens were acquired from the Tengchong market (see figures
1, 2A, and 3A). All three displayed a translucent brownish
red bodycolor, with a few bubbles and flow lines withinthem. The bluish green floating color was observed when
the specimens were rotated. This phenomenon is most obvious with a black background. Standard gemological properties and Fourier-transform infrared spectroscopy data wereconsistent with those of regular Burmese amber. While thebluish green floating color might be related to fluorescence,no such effect is seen on other types of fluorescing Burmese
amber (e.g., brownish amber). To further explore this,brownish Burmese amber beads with bodycolor similar tothat of chameleon amber were selected as a reference (figures 2C and 3C). Their bodycolor was not as vivid aschameleon amber, and it did not display the bluish green
floating fluorescence. Observation under a UV lamp showed

Figure 1. A good-quality 88.25 ct “chameleon” amberwith a red bodycolor displaying a vivid green fluorescence color that appears to float on the surface. Photoby Yan Li.

Editors’ note: Interested contributors should send information and illustrations to Stuart Overlin at soverlin@gia.edu.
GEMS & GEMOLOGY, VOL. 59, NO. 3, pp. 388–412.
© 2023 Gemological Institute of America

388

GEM NEWS INTERNATIONALGEMS & GEMOLOGYFALL 2023

A

C

B

D

that the chameleon amber in figure 3A produced strong
bluish white fluorescence (figure 3B) in long-wave UV (365A

C

GEM NEWS INTERNATIONAL

Figure 2. A and B: A
78.51 ct chameleonamber exposed to whitelight against white andblack backgrounds. C
and D: A 12.50 ct
brownish reference
sample exposed to similar lighting and backgrounds. Photos byJinfeng Yang.

nm), while the brownish amber beads in figure 3C showed
a weak bluish purple fluorescence (figure 3D).

B

Figure 3. A and B: A
71.05 ct Burmese
chameleon amber andits bluish white fluorescence under long-wave
UV light. C and D: 16mm brownish Burmese
amber beads (12.60 ct
on the left, 12.16 ct on
the right) and theirbluish purple fluorescence under long-wave
UV. Photos by Jinfeng
Yang.

D

GEMS & GEMOLOGYFALL 2023

389

A

EMISSION SPECTRA
6000

386 nm406 nm
436 nm
457 nm

5000

497

4000
3000
2000
1000

6000

INTENSITY (a.u.)

434 464 469

5000
4000
3000
2000
1000

0

EXCITATION WAVELENGTH (nm)

300

400

500

0

600

EXCITATION SPECTRA
500
455

457
436

434

406
386

403
384

400

497 nm469 nm464 nm434 nm300
469
434 464

497
6000

5000

4000

3000

2000

1000

0

EMISSION WAVELENGTH (nm)INTENSITY (a.u.)

B

EMISSION SPECTRA
1400
1200

463

1000
800

475

600

500

400
200

1200

INTENSITY (a.u.)

436

386 nm406 nm
436 nm
457 nm

1000
800
600
400
200

0

EXCITATION WAVELENGTH (nm)

300

400

500

600

Figure 4. 3D fluorescence spectra of
Burmese chameleonamber (A) and brownish amber (B).

0

EXCITATION SPECTRA

500
457
436

430

406
386

400

475
436 463

408
402

500 nm475 nm463 nm
436 nm

300

500
1400

1200

1000

800

600

400

200

0

EMISSION WAVELENGTH (nm)INTENSITY (a.u.)390

GEM NEWS INTERNATIONALGEMS & GEMOLOGYFALL 2023

The samples were then examined with 3D fluorescence
spectroscopy (figure 4) under the excitation of a continuous-wavelength excitation light source. The fluorescenceintensity of brownish amber was clearly much lower thanthat of chameleon amber. In addition, two peaks (455 and384 nm) were absent in the excitation spectrum of brownish amber. A high degree of similarity was exhibited in thesamples’ emission spectra, with all the peaks positioned at
approximately 434, 464, 469, and 497 nm. However, thebiggest difference was in the fluorescence intensity, which
was easily observed in chameleon amber.
The 3D fluorescence spectra of chameleon amber exhibited several fluorescence centers, mainly at 434, 464, 469,
and 497 nm generated by excitation wavelengths of 386,
406, 436, and 457 nm. The fluorescence color could be calculated by the fluorescence centers, and the results showedthat the spectral color was blue under 386 nm excitation andbluish green under 436 nm excitation. We believe that thebluish green floating color under sunlight is caused by thebluish green fluorescence center at 469 nm under 436 nm
excitation. This is much more consistent with observations
in daylight. Due to the low fluorescence intensity of brownish amber, its blue fluorescence color is very weak, and mostlight penetrates it without producing any special optical effect. The difference in fluorescence intensity between the
469 and 434 nm fluorescence centers corresponds with therelative spectral power distribution of the light source.
Under the excitation of a xenon lamp, the fluorescence center at 434 nm is more intense than the light-emitting centerat 469 nm. However, amber is usually observed in daylight.
The optimal excitation wavelengths for the emission centers at 434 and 469 nm are 386 nm and 436 nm, respectively.
From the relative spectral power distribution of the two lightsources, it can be seen that the spectral density of the xenon
lamp at 386 nm is greater than 436 nm, while the D65 daylight-equivalent source is the opposite. Against a dark background and under daylight, the fluorescence of chameleonamber should be dominated by the bluish green fluorescenceemitted at 436 nm.
The fluorescence center at 469 nm is very common in
Baltic, Dominican, Mexican, and Burmese amber. This fluorescence center in chameleon amber is unique for its highintensity and separation from other fluorescence colors.
Only if the fluorescence intensity is high enough will it
show as the dominant fluorescence color. The optimal excitation wavelength (436 nm) corresponding to the 469 nm
fluorescence center is far removed from the optimal excitation wavelength of other strong fluorescence centers;
therefore, the fluorescence color is not a mixture of multiple colors. Many varieties of amber can also produce strong
fluorescence at 469 nm, but often mixed with other fluorescence colors of similar intensity. For example, the fluorescence peaks of 448 and 474 nm in Dominican blue
amber always appear together, and their strong fluorescence peaks can be seen with exposure to the same excitation wavelength. In most cases, the fluorescence intensityof the former is slightly greater, which leads to the fluorescence of the blue amber being mainly blue, without a green
modifier. Some Dominican blue amber with uneven fluorescence distribution can also show a mosaic pattern (X.
Chenxing et al., “Characterisation of patchy blue and green
colouration in Dominican blue amber,” Journal of Gemmology, Vol. 37, No. 7, 2021, pp. 700–713). The fluorescence distribution of the chameleon amber tested in thisstudy was uniform, with high intensity and good separation, producing a high-quality bluish green floating fluorescence effect.

GEM NEWS INTERNATIONALGEMS & GEMOLOGY

Jinfeng Yang, Yan Li, and Youzhi Liang
Gemmological Institute
China University of Geosciences, Wuhan
A newly discovered iridescent andradite from Inner Mongolia, China. Iridescent garnet found in Chifeng City, Inner
Mongolia, was recently encountered in the Chinese market.
Iridescent garnet was first discovered in 1943 in the U.S.
state of Nevada (E. Ingeron and J.D. Barksdale, “Iridescentgarnet from the Adelaide Mining District, Nevada,” American Mineralogist, Vol. 28, 1943, pp. 303–312), and it is also
found in Mexico and Japan (M.A. Badar and M. Akizuki, “Iridescent andradite garnet from the Sierra Madre Mountains,
Sonora, Mexico,” Neues Jahrbuch für Mineralogie - Monatshefte, Vol. 1997, No. 12, pp. 529–539; T. Hainschwang and
F. Notari, “The cause of iridescence in rainbow andraditefrom Nara, Japan,” Winter 2006 G&G, pp. 248–258). To theauthors’ knowledge, this is the first report of iridescent garnet in China. Two rough samples were collected by the authors in the Lindong lead mining area (figure 5). The
refractive index was over the limits of a conventional refractometer, and the hydrostatic specific gravity values were
3.85 and 3.84. Both showed an inert reaction to long- andshort-wave ultraviolet fluorescence. These standard gemological testing results were consistent with garnet.
On the basis of energy-dispersive X-ray fluorescence, calcium, silicon, iron, aluminum, manganese, magnesium, titanium, and sodium were detected. The chemicalcomposition of the larger sample was analyzed using electron probe microanalysis (EPMA). The composition wasconsistent with that of andradite, containing major elementsof calcium, iron, aluminum, and silicon, with the derived
chemical formula of (Ca3.015Mn0.031Na0.006Mg0.003Ti0.002)(Fe1.846Al0.138)[Si2.978O12]. The backscattered electron image
showed clear growth layers (figure 6, left). Aluminum inthe darker zones was about 2 wt.% higher on average thanin the lighter zones, and the iron content decreased with increasing aluminum. Raman spectroscopic analysis involved
comparison with the RRUFF database (B. Lafuente et al.,https://rruff.info/about/downloads/HMC1-30.pdf). The peak
positions (figure 7) indicated that the samples were consistent with those of andradite, supporting the composition inferred from EPMA. Raman shifts within 300 to 400 cm–1
were assigned to SiO4 rotational vibrations. Raman shifts
within 400 to 700 cm–1 were caused by Si-Obend (bridge oxy-

FALL 2023

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(211)

(110)

(211)

G-1

G-2

Figure 5. Iridescent andradite samples (25 ct and 125 ct) from the Lindong area of Inner Mongolia, China. As
shown in the left photo, sample G-1 displays iridescence on the (211) plane but not on the (110) plane. Photos by
Jiuchang Yang.

gen) bending and asymmetric stretching vibrations. Raman
shifts at 800 to 1200 cm–1 were attributed to the characteristic absorption peak of Si-Ostr stretching vibration (B.A.

Kolesov and C.A. Geiger, “Raman spectra of silicate garnets,” Physics and Chemistry of Minerals, Vol. 25, No. 2,
1998, pp. 142–151).

Figure 6. Left: Growth layers viewed normal to the (110) crystal face in a backscattered electron (BSE) image froma polished thin section. Right: The iron and aluminum contents corresponding to each spot in the BSE image.

Fe AND Al CONTENT
35

Fe
Al

1
30

2
3
4
5

4

Fe (wt.%)

3

20
15

2

Al (wt.%)

25

6

7
8
9
10
11
12
13

5

10
1
5
10 µm

0

0
1

2

3

4

5

6

7

8

9

10 11 12 13

SPOT

392

GEM NEWS INTERNATIONALGEMS & GEMOLOGYFALL 2023

RAMAN SPECTRA
370

Andradite
R040001
G-1

874
515

INTENSITY

352
312

100

300

493

500

842
815
994

700

900

1100

1300

1500

RAMAN SHIFT (cm )–1Figure 7. Sample G-1 was identified as andradite
based on a comparison with a sample from the
RRUFF database. Spectra are offset vertically forclarity.

Figure 8. Sample G-1 displays strong iridescence with afull spectrum of colors. For certain bands, the color does
not change as the viewing angle changes. This image isof a face parallel to (211). Photomicrograph by Jiuchang
Yang; field of view 24.45 mm.

Microscopic observation revealed an attractive playof-color with a complete visible spectrum of colors onthe (211) crystal plane of sample G-1 (figure 8). We cut
three polished thin sections from sample G-2 along the(110) and (211) crystallographic axes to observe the twinning structure and extinction phenomena under a polarizing microscope. The sample displayed angled layered
growth structures and a strong grid-like pattern and ab-

normal birefringence (figure 9). A grid-like extinction
phenomenon parallel to the growth layer (figure 9A) may
have been caused by lattice-like twins, and figure 9C
shows two areas with different extinction orientations.
One study on Japanese andradite showed that the iridescence effect is caused by the alternate growth of iron-richlayers and aluminum-rich layers. These layers cause athin-film interference phenomenon in the crystal struc-

A

B

C

D

GEM NEWS INTERNATIONAL

Figure 9. Top: The polishedthin section in A and B
was cut normal to the(110) crystal face. Observation under crossed polarizers (A) showed strong
grid-like pattern, abnormal
birefringence (possibly
caused by lattice-like twinning), and angled growthbands. Bottom: The section
in C and D was cut parallel to the (110) crystal face.
Observation under crossed
polarizers (C) showed partitioned abnormal birefringence caused by contact
twinning and faint growthbands. Photomicrographs
by Jiuchang Yang; field ofview 25.05 mm.

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393

Figure 10. A 55 ct cushion-cut aquamarine exhibiting a distinct lightbluish green and deep
blue pleochroism using
a dichroscope. Photos
by Kaiyin Deng; courtesy of Farrugia Gem.

ture (Hainschwang and Notari, 2006). Therefore, we speculate that the iridescence of andradite from the Lindong
lead mining area is not closely related to twinning but
may be closely related to thin-film interference caused byits layered growth structure.
Jiuchang Yang, Quanli Chen (chenquanli@cug.edu.cn),
Fengshun Xu, and Yan Li
Gemmological Institute
China University of Geosciences, Wuhan
Xianyu Liu
College of Jewelry, Shanghai Jian Qiao University
Aquamarine with zigzag growth line inclusions. A 55 ct
cushion-cut aquamarine (figure 10, left) was recently submitted to Guild Gem Laboratories in Shenzhen for testing.
This stone exhibited a very intense blue color accompanied
by a subtle greenish secondary hue. A refractive index of
1.580–1.586 was obtained, together with a specific gravity
of approximately of 2.71. Fourier-transform infrared (FTIR)
and Raman spectroscopy confirmed its identity as beryl. The

FTIR SPECTRAUV-VIS-NIR SPECTRUM
2359

3238
3165
3113

2318
2338
2360
2731
2686
2641

3235
3162
3111

3100

2900

2700

2500

WAVENUMBER (cm–1)

394

Figure 11. Infraredtransmission spectra(left) and UV-Vis-NIR
spectrum (right) of theaquamarine sample.
The spectra on the left
are offset vertically forclarity.

2341

ABSORBANCE

TRANSMITTANCE

Natural aquamarine
This sample

3300

transmission FTIR spectrum exhibited distinct peaks at
2731, 2686, and 2641 cm–1 and a carbon dioxide–related signal at 2359 cm–1. Peaks at 3235, 3162, 3111, and 3021 cm–1
were assigned to the presence of water (figure 11, left). This
collection of FTIR peaks has been reported in natural aquamarine, but no synthetic counterpart has been reported toshow this pattern (I. Adamo et al., “Aquamarine, Maxixetype beryl, and hydrothermal synthetic blue beryl: Analysis
and identification,” Fall 2008 G&G, pp. 214–226). No organic-related peaks were found around the 2800–3200 cm–1
range, suggesting no clarity enhancement had been applied(L. Jianjun et al., “Polymer-filled aquamarine,” Fall 2009
G&G, pp. 197–199). Strong deep blue and light bluish green
pleochroism (figure 10) was observed using a dichroscope.
Similar beryl materials with a deep blue color have been reported in previous studies (Fall 2014 GNI, pp. 244–245; Fall2019 GNI, pp. 437–439). The ultraviolet/visible/near-infrared spectrum (figure 11, right) showed a broad Fe2+-relatedband centered at around 835 nm and narrow absorption
bands at 370 and 426 nm related to Fe3+ (D.S. Goldman etal., “Channel constituents in beryl,” Physics and Chemistry

GEM NEWS INTERNATIONAL

370

426

2300

300 400 500 600 700 800 900 1000

WAVELENGTH (nm)GEMS & GEMOLOGYFALL 2023

Figure 12. The thin film along the basal plane of the aquamarine under fiber-optic illumination. Photomicrographs
by Yizhi Zhao; fields of view 3.45 mm (left) and 1.72 mm (right).

of Minerals, Vol. 3, No. 3, 1978, pp. 225–235; K. Schmetzer,“Hydrothermally grown synthetic aquamarine manufactured in Novosibirsk, USSR,” Fall 1990 G&G, pp. 206–211;
Adamo et al., 2008). Using a polariscope equipped with aconoscope, the optic axis was observed parallel to the table
along the long axis of the cushion shape. Further chemical
analysis by energy-dispersive X-ray fluorescence showed ahigh iron content (around 13000 ppmw), which likely contributed to the saturated blue color (Y. Shang et al., “Spectroscopy and chromaticity characterization of yellow tolight-blue iron-containing beryl,” Scientific Reports, Vol. 12,
No. 1, 2022, article no. 10765).
Microscopic observation under darkfield illumination
revealed fluid inclusions and minute voids. Under the table,
a circular thin film with a strong light reflection was visible(figure 12). The image in figure 12 (left) was captured at anangle to the table of the aquamarine, since the thin film waseasier to see under an oblique light source. Because beryl

Figure 13. Zigzag lines were observed in the aquamarine using oblique light. Photomicrograph by Yizhi
Zhao; field of view 4.52 mm.

has imperfect cleavage along its basal plane {0001}, thespace between the basal planes caused by initial cleavage
was sufficient to form the thin film.
An interesting inclusion was observed only withoblique light: a distinct series of fluids resembling dots andbroken lines. Higher magnification (figure 13) revealed thatthese zigzag lines consisted of discrete and minute fluid inclusions. Further observation along the optic axis confirmed that the planes were parallel to the c-axis of theaquamarine.
A previous study described similar inclusions in emeralds from Colombia as resembling a DNA double helix(Summer 2019 G&G Micro-World, p. 262). The authors
have also encountered this type of inclusion before. Basedon our observations, these helical inclusions spiral along
the optic axis of the emerald. To our knowledge, this is thefirst time such helical inclusions have been reported in an
aquamarine. This finding will help advance our understanding of the beryl family.
Yujie Gao, Xueying Sun (shirley.sun@guildgemlab.com),
Yizhi Zhao, and Kaiyin Deng
Guild Gem Laboratories
Shenzhen, China
Blackish green omphacite jade from Guatemala. In recent
years, a new kind of Guatemalan jade has entered the Chinese jewelry market, where it is called yongchuliao fei cui(or yongchuliao for short). Prior to the availability of
yongchuliao, Chinese consumers had a negative impressionof Guatemalan material and preferred Burmese jade. However, the recent emergence of this new high-quality blackish green jade has attracted the attention of Chinese buyers.
The authors recently tested a rough stone and a carved
pendant of yongchuliao (figure 14). Under reflected light,
these samples appeared blackish green with a greasy luster
similar to Burmese inky black jade. The rough stone was
opaque and had numerous fine white inclusions measuring
approximately 300 μm in diameter on its surface. The

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395

Figure 14. Yongchuliao samples collected for testing. The rough sample on the left weighs 30.1 ct, and the carvingon the right weighs 12.85 ct. Photos by Shilong Xu.

carved sample was bright green with coarse texture andmedium transparency under transmitted light (figure 15).
The rough and carved samples had a refractive index of
1.67 and specific gravities of 3.342 and 3.332, respectively.
These gemological characteristics are consistent with bothjadeite-type jade and omphacite-type jade.
Qualitative analysis using energy-dispersive X-ray fluorescence spectrometry revealed magnesium, aluminum,
silicon, calcium, chromium, manganese, and iron, among
other elements. It should be noted that the ideal chemical
formula of jadeite is NaAlSi2O6 and that of omphacite is(Ca,Na)(Mg,Fe,Al)Si2O6 (S.F. McClure, “The jadeite/omphacite nomenclature question,” GIA Research News,
April 12, 2012). Jadeite and omphacite can form a complete
solid solution. The occurrence of calcium, magnesium, andother elements implies that the mineral composition of
yongchuliao is not pure jadeite.
Raman shifts at 681, 514, 411, 378, 340, and 209 cm–1
were consistent with those of omphacite (figure 16). The
Raman spectra of fibrous inclusions in the carved sample
also matched those of omphacite rather than jadeite. The
strongest Raman shift at 681 cm–1 was attributed to thesymmetrical Si-O-Si stretching vibration (B. Xing et al.,“Locality determination of inky black omphacite jades
from Myanmar and Guatemala by nondestructive analysis,” Journal of Raman Spectroscopy, Vol. 53, No. 11, 2022,
pp. 2009–2018). Raman spectroscopy was used to compare
the samples’ mineral composition against the RRUFF database (B. Lafuente et al., https://rruff.info/about/downloads/HMC1-30.pdf), revealing that yongchuliao material

Figure 15. Left: Fibrous inclusions observed in the 12.85 ct carved sample; field of view 8.94 mm. Right: The
carved sample appeared bright green rather than blackish green under transmitted light. Photos by Shilong Xu.

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RAMAN SPECTRA
Rough sample
Fibrous inclusions in carved sample
Carved sample
681
Omphacite R110038
563
1021

411 340
378

INTENSITY

514

White inclusions in the rough sample
Inclusions in the carved sample
Titanite R100136
607

209
269

538

465

911 857

313
334
423

251 142
160

1070
1030

1200

1010

556

1000

800

600

400

200 1200

1000

RAMAN SHIFT (cm –1)

800

600

400

200

RAMAN SHIFT (cm –1)

Figure 16. The mineral species were confirmed by comparison with samples from the RRUFF database. Spectra areoffset vertically for clarity. Left: The host mineral and fibrous inclusions of the samples matched with omphacite.
Right: The white dotted mineral inclusions in the rough sample matched with titanite. Titanite in the carved sample could also be observed in micro-Raman imaging.

consists almost entirely of omphacite. The fine white inclusions distributed evenly in the rough sample were coincident with titanite (figure 16). In addition, some of thesample was ground into powder for X-ray diffraction analysis to obtain the mineral composition of the whole rock.
The results showed an omphacite content of about 98.49%(with a 5% margin of error), and the rest was magnesiancalcite. The results from Fourier-transform infrared absorption spectroscopy were consistent with the Raman spectra,
Figure 17. In the UV-Vis-NIR spectrum of a piece cut
from the rough sample, absorption peaks were observed within the visible range at 437, 637, 657, and688 nm.

Shilong Xu, Quanli Chen (chenquanli@cug.edu.cn), and
Yan LiGemmological Institute
China University of Geosciences, Wuhan

UV-VIS-NIR SPECTRUM

ABSORBANCE

1.4

Xianyu Liu
College of Jewelry, Shanghai Jian Qiao University

657

437

1.3

688
637
Cr 3+

Fe 3+

1.2

1.1

400

500

600

WAVELENGTH (nm)GEM NEWS INTERNATIONAL

identifying the main mineral composition of the samples
as omphacite.
A double-sided polished piece was cut from the roughsample for testing. Its ultraviolet/visible/near-infrared (UVVis-NIR) absorption spectrum (figure 17) showed an absorption peak at 437 nm, indicating the presence of Fe3+.
Absorption peaks at 637, 657, and 688 nm were also observed
and attributed to Cr3+ (G.R. Rossman, “Color in gems: The
new technologies,” Summer 1981 G&G, pp. 61–62).
Based on testing, both samples were omphacite-typejade containing titanite inclusions. Since almost all of the
yongchuliao we have observed exhibited nearly the samecolors and textures as these samples, it is likely that otheryongchuliao may also have similar mineral compositions.

700

Shell blister on an Isognomon isognomon shell. Theoretically, all mollusks are capable of producing pearls, but onlya small number of them actually produce commerciallytraded nacreous pearls. Most come from the Pinctada andPteria genera belonging to the Pteridae family. Species that
produce the nacreous pearls regularly seen in the market,
such as Pinctada maxima, Pinctada radiata, Pteria sterna,and Pteria penguin, are well known to aficionados. However, some unusual bivalve species can also produce pearlsor shell blisters, and one example was recently submitted
to GIA’s Mumbai laboratory for scientific examination.

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Figure 18. Isognomon
isognomon shell andblister. The shell measures approximately
107.76 × 72.39 mm.
Photo by Gaurav Bera.

The shell was from the Isognomon isognomon species,
belonging to the Isognomonidae family and commonly
known as the Pacific toothed oyster. It exhibited a blisterfeature that appeared as a folded section of nacre, locatednear the center, close to the posterior muscle scar and thehinge (figure 18). The shell weighed 46.88 g and measured
approximately 107.76 × 72.39 mm, while the baroqueshaped silver to light gray blister measured approximately
8.80 × 4.21 mm. Not all instances of nacre growth on ashell are classified as pearls. While cyst pearls or whole
pearls develop independently from the shell, blisters are
nacreous concretions that appear to protrude from the shell(K. Lawanwong et al., “Natural shell blisters and blister
pearls: What’s the difference?” GIA Research News, Au-

gust 26, 2019). According to the pearl classification established by CIBJO, the concretion observed on the shell inquestion would be categorized as a shell blister.
The shell itself displayed a narrow, elongated shape, featuring a straight hinge with distinct equidistant dark brownnotches along its length (figure 19, left). These notches, also
known as resilifers or ligament pits, resemble teeth, giving
the species its common name; see I. Temkin and C. Printrakoon, “Morphology and taxonomy of Isognomon spathulatus (Reeve, 1858), a cryptic bivalve from the mangroves ofThailand,” Zootaxa, Vol. 4107, No. 2, 2016, pp. 141–174.
The interior of the shell exhibited a predominantly
nacreous aragonite surface, characterized by a discernible
spiral platelet structure visible under microscopic exami-

Figure 19. Left: Dark brown rows of notch-like features observed on the hinge of the shell. Right: A whitish subsurface dendritic network of parasite channels. Photomicrographs by Prasad Mane; fields of view 16.3 mm (left) and6.1 mm (right).

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Figure 20. Left: RTX image of the top view of the 8.80 × 4.21 mm shell blister (arrow) with a network of parasitic
boreholes and channels visible around it. Right: RTX image of the side of the shell blister with more radiopaquefolds marking its outline.

nation. This nacreous region was bordered by a non-nacreous shell margin. The central nacre displayed a light graycoloration, transitioning to a darker purplish gray toward
the margin. The center of the shell had a more translucent
appearance, revealing a subsurface dendritic network ofparasitic channels surrounding the blister (figure 19, right).
Real-time microradiography (RTX) revealed a networkof fine channels beneath the blister and throughout theshell itself (figure 20, left). These features corresponded tothe parasitic channels observed within the translucent
layer on the shell’s interior. RTX imaging of the blister revealed a void (figure 20, right), potentially linked to parasitic activity that may have initiated the blister formation.
X-ray fluorescence (XRF) imaging of both the shell andthe blister yielded an inert reaction. Energy-dispersive Xray fluorescence spectrometry of the blister indicated amanganese level below the instrument’s detection limit
and a strontium level of 1135 ppm. The inert XRF reaction,
along with the chemical composition, proved the mollusk
formed in a saltwater environment. Under long-wave UV
radiation, the shell displayed a greenish yellow fluorescence reaction, while short-wave UV radiation yielded aninert response. Raman analysis using a 514 nm ion/argon
laser excitation revealed peaks at 701/704 cm–1 and 1085
cm–1, indicative of aragonite. Photoluminescence spectracollected from both the shell and the blister showed threebroad peaks centered at 620, 650, and 680 nm, which is observed in some naturally colored pearls.
Although the Isognomonidae family and the Pteridae
family belong to the Pteroidae superfamily, pearls fromoysters such as the Pacific toothed oyster (Isognomon isognomon) are rarely seen. Therefore, it was intriguing to examine this blister. Perhaps we will someday encounter anatural pearl from this mollusk species.
Prasad Mane, Nishka Vaz, and Abeer Al-Alawi
GIA, Mumbai

GEM NEWS INTERNATIONAL

Mining basalt-related gems in southeast Vietnam. A number of gemstone deposits are known in provinces all acrossVietnam. These include ruby (Yen Bai and Nghe An), sapphire (Yen Bai, Nghe An, Binh Thuan, and Dak Nong),
spinel (Yen Bai and the Central Highlands), aquamarine(Thanh Hoa and Khanh Hoa), tourmaline and garnet (YenBai), and peridot (Gia Lai). Recently, seven gemstone occurrences have been identified within the basaltic fields insoutheast Vietnam, which are mainly found in Dong Nai
and Ba Ria–Vung Tau provinces. These gems include sapphire, zircon, garnet, augite, brown peridot, feldspar, andhyalite opal. They are found predominantly in basalt
bedrock and its regolith as xenocrysts or xenoliths deposited on hillslopes or in placer formations along
streambeds. Seven gemstone species from southeast Vietnam were collected and documented (figure 21).
The sapphires usually appeared as hexagonal crystals
or fragments, exhibiting mainly blue, deep to dark greenish
blue, and greenish yellow colors. Most were transparent tosemitranslucent and some opaque, and the luster was
greasy to vitreous. The typical rough sizes ranged from 5
to 20 mm, weighing 2–10 ct.
Euhedral tetragonal crystals of zircon were very common, while the rest of them were fragmented. Colors included orange, brown-red, gray, and colorless. These zircon
samples showed high luster ranging from adamantine tovitreous, with a transparent to semitransparent appearance. Zircons from southeast Vietnam were not found in
fresh basalt; they were found mostly in placer and in
weathered basalt instead (figure 22A).
“Red stone” is a local name for gem-quality garnet fromDong Nai. The garnets occurred in two forms of anhedralfragments: xenocrystic fragments (also called “orphan
stone”) weighing 1–50 ct, with a corroded surface, and xenolith-hosted garnet containing small corroded fragments that
produce 1–5 ct fashioned gems. Both xenocryst (figure 22B)

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Figure 21. Gem materials from southeast Vietnam. The three largest yellow and white samples on the far left are
feldspar, while the brown, orange, and near-colorless samples in the center are zircon and the two greenish whitesamples near the top right are hyalite opal. The rest of the dark stones are sapphires, garnets, augites, and brown
peridot. Photo by Le Ngoc Nang; courtesy of Tran Ngoc Vien.

and xenolith-hosted garnet were found mainly in the regolith on the hillslopes. The color ranged from deep to dark
red and occasionally deep orangy yellow, with a transparent
to semitransparent quality and a vitreous luster.
Euhedral crystals of Dong Nai feldspar were characterized by two directions of perfect cleavage. These feldspars

were colorless, gray, and rarely light yellow, with a vitreous luster and a transparent to translucent appearance.
This mineral was the most common gem material in theregion and could be found in both bedrock and its regolith.
As reported recently, the augite samples appeared
black under daylight-equivalent illumination but dis-Figure 22. Basalt-hosted gem materials from southeast Vietnam: A subhedral zircon crystal in laterite (A), a garnet
xenocryst in basalt (B), and a brown peridot nodule in basalt (C). Photos by Le Ngoc Nang.

B

A

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GEM NEWS INTERNATIONALCGEMS & GEMOLOGYFALL 2023

1%

played green and brown colors under transmitted light.
Most samples were transparent to translucent and had vitreous luster. The brown gems tended to be more transparent than the green ones (L.N. Nang et al., “Gem-quality
augite from Dong Nai, Vietnam,” Summer 2023 G&G, pp.
182–194).
Brown peridot was found as slightly corroded xenocrystic fragments measuring 0.5–5 cm. The samples were blackunder reflected light but greenish yellow-brown with transmitted light. Most samples were transparent to translucent, with a vitreous luster. The brown peridot was quiterare and frequently confused with augite due to their similar morphology and occurrence in the vesicular basalt (figure 22C).
The hyalite opal was found in Ba Ria–Vung Tau, andeach sample consisted of two parts: the inner body and theouter layer. The contact between the two parts was distinct. The outer layer was opaque white, 2–5 mm thick,and porous. The inner body was usually colorless andrarely green, brown, or light pink, transparent to opaque,with a vitreous appearance. The hyalite opals with goodtransparency were suitable for jewelry due to their glowing
green fluorescence under long-wave ultraviolet light (weakfluorescence under short-wave UV).
Several of these gem deposits in southeast Vietnam areunlicensed. Most materials were collected by locals using
artisanal methods, without any commercial mining. The
gem materials found along hillslopes or stream channels
were extracted at shallow depths ranging from 0.3 to 0.5
m. The gems appeared more frequently after rain and were
distributed by their specific gravity. For instance, light material such as hyalite opal was concentrated on the hilltop,
stored in slightly weathered and broken-up rock, whileheavier minerals, namely sapphire, garnet, feldspar, andperidot, usually accumulated along the hillslopes. Zircon,on the other hand, was often deposited in streambeds.
Some gems stored in basalt were extracted easily by homemade tools.
Most of the rough materials are sold to gem dealers in
Ho Chi Minh City, who sell fashioned products to local
markets in southeast Vietnam. Gemstones in the area havelong been gathered by locals during farming and welldrilling, but the old-fashioned mining methods led to a significant waste of gem resources. Preliminary evaluationsof gem quality have shown that gemstones from southeast
Vietnam are of economic potential. The thick basaltic hostrock covering a large area is a foundation for prospecting
numerous gem materials and requires further research tocomprehensively assess available reserves.
Le Ngoc Nang
University of Science, Vietnam National University
Ho Chi Minh City
Liu Gemological Research and Application CenterPham Minh Tien and Ho Nguyen Tri Man
Liu Gemological Research and Application Center

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4%
10%

35%

20%

30%

Aesthetics
Preciousness
Naturalness

Sustainability and ethics
Symbolism
Economic value

Figure 23. Survey participants identified their top consideration when purchasing colored stones or coloredstone jewelry.

Survey of colored stone preferences among younger generations. The global colored stone market is dynamic andconstantly evolving. To understand general perceptions
and preferences regarding colored stones among designers
and potential end consumers from the younger generations, the author conducted a survey in April 2023. The
survey targeted 300 respondents under the age of 35, all followers of Donna Jewel (@donna.jewel), an Instagram community designed to champion jewelry from artisans andemerging talents worldwide. Although not scientific, theresults of the survey are nonetheless revealing.
Demographics and background: The survey participantsconsisted of 50% Millennials (born between 1981 and1996) and 50% from Generation Z (born from 1997 onward). All respondents were over the age of 18. Forty percent were from the United States and another 40% from
Europe (specifically Italy, France, the United Kingdom, andGermany), with the remainder split between Latin America (mainly Brazil) and Asia (mainly India and Iran). Respondents identified themselves as gem and jewelry lovers(50%), designers (33%), or gem and jewelry experts (17%).
Colored stone preferences: Participants were asked tochoose their single most important consideration whenbuying colored stones or colored stone jewelry: aesthetics,
preciousness, naturalness, sustainability and ethics, symbolism, or economic value. As shown in figure 23, the top
two responses were aesthetics (35%) and preciousness(30%). Naturalness was also an important consideration (re-GEMS & GEMOLOGYFALL 2023

401

5%

10%

10%
15%
15%

70%

Prefer natural stones
Indifferent

Prefer synthetic stones
Not informed

75%

Prefer untreated stones
Prefer treated stones

Not informed

Figure 24. The preferences of survey respondents regarding natural vs. synthetic (lab-grown) gemstones and untreated vs. treated gemstones.

ceiving 20%), while sustainability and ethics was chosen
by 10% of the interviewees, symbolism by 4%, and economic value by only 1%.
Given the choice between natural or synthetic gemstones (figure 24, left), 70% chose natural. Only 10% selected synthetic, while an additional 15% expressed no
preference. When asked to explain their preferences, theparticipants who chose natural stones perceived them as
unique, rare, and authentic products of nature, capable ofevoking emotion, unlike their synthetic counterparts.
Those who chose synthetic gemstones viewed them as
more sustainable, ethical, and affordable. The topic of natural and synthetic gemstones remains extremely relevant.
Regarding treated vs. untreated natural gemstones (figure
24, right), 75% preferred untreated, which they perceive as
more beautiful for their uniqueness and originality. They
viewed natural imperfections as an added value lost during
treatment. Only 15% preferred treated stones, while 10%
acknowledged that they were not informed on the topic.
When asked whether they preferred “precious” or“semi-precious” stones, the participants proved generally
indifferent (designers and potential end consumers alike).
They associate preciousness with a broader definition more
related to uniqueness, emotion, and originality.

Designer perspectives: Designers participating in this survey revealed that they follow both trends and personal inspiration. They noted several uncommon gemstones ofparticular interest: rainbow obsidian, red beryl, sphene,
pargasite, chrysoberyl, scapolite, axinite, enstatite, andbenitoite. Fifty percent indicated that they had purchasedor used for their jewelry artificially manufactured materials
such as glass or cubic zirconia.
At the end of the survey, participants were asked what
topics they would be most interested in learning more

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about. They expressed a strong desire for more knowledge
on uncommon and unique gemstones, original colors,
treatments, history, lore, origin, and sustainability.
The survey highlighted the younger generations’ strong
interest in what is unique, original, authentic, and emotional rather than alignment with traditional notions ofbeauty and preciousness. Naturalness emerged as an important element for the interviewees, who appreciated imperfection as a mark of authenticity and originality. Youngdesigners favored uncommon and original gemstones with
distinct and extravagant colors (especially yellows, blues,
greens, and reds). While designers sought uniqueness, rarity, and extravagance, potential end customers were also
looking for a gemstone with a story to tell.
Laura Inghirami
Donna Jewel, MilanDIAMOND
Diamondiferous mantle eclogite: Diamond surface features
reveal a multistage geologic history. More than 99% of diamonds originally formed in the lithospheric mantle, thecold, rigid mantle that underlies continents, generally <300
km deep (T. Stachel and J.W. Harris, “The origin of cratonic
diamonds – Constraints from mineral inclusions,” Ore Geology Reviews, Vol. 34, No. 1-2, 2008, pp. 5–32). In the lithospheric mantle, the two major rock types—and host rocks fordiamond—are peridotite (>95 vol.%), composed predominantly of olivine, and eclogite (<5 vol.%), composed of ironmagnesium-calcium garnet and sodium-rich clinopyroxene.
Eclogite is a high-pressure and high-temperature metamorphic rock that can form when dense oceanic crust subducts
into the mantle beneath continents at convergent margins.
We cannot directly study the deep portion of the earth
where diamonds form. Instead, geologists study diamonds

GEMS & GEMOLOGYFALL 2023

Figure 25. Mantle eclogite xenolith (2.8 cm inlongest dimension) consisting of clinopyroxene(green) and garnet (redorange), with a partially
exposed octahedral diamond. Photo by Annie
Haynes. Gift of Mark
Mauthner, GIA Museum
no. 37511.

themselves, as well as pieces of mantle rock that are transported to the surface as xenoliths by volcanic magmas suchas kimberlite or lamproite. The authors have studied one
such mantle eclogite xenolith, part of the GIA Museum
collection (figure 25). This sample was recovered from akimberlite in Russia and contained a partially exposed octahedral diamond. The exposed portions of the diamond
had visible surface features, including trigons (figure 26).
Trigons are inferred to be etch features that form after
diamond crystallization, through interaction with fluids ormelts either in the mantle source region or during transport
to the surface (J.W. Harris et al., “Morphology of monocrystalline diamond and its inclusions,” Reviews in Mineralogy
and Geochemistry, Vol. 88, No. 1, 2022, pp. 119–166).
Figure 26. Partially exposed diamond embedded in an
eclogite xenolith, with positive and negative trigons
on the octahedral face. A mineral is included within
the diamond, visible through the octahedral face.
Photomicrograph by Matthew Hardman; field of view
5.74 mm.

Negative
trigons
Positive
trigons

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Trigons form exclusively on the octahedral faces of diamond and can have several different orientations. Positive
trigons are oriented in the same direction as the octahedral
face, and negative trigons have opposing orientation (figure26). Positive trigons are very rare and may relate to interaction with carbon-oxygen-hydrogen-rich fluids at temperatures between ~800° and 1000°C at near-surface pressures(Z. Li et al., “Positively oriented trigons on diamonds fromthe Snap Lake kimberlite dike, Canada: Implications for fluids and kimberlite cooling rates,” American Mineralogist,
Vol. 103, No. 10, 2018, pp. 1634–1648). Trigons with negative orientation are much more common. It has beenshown experimentally that increasing water content inmelts could be one cause for the change of a trigon frompositive to negative (e.g., A.F. Khokhryakov and Y.N.
Pal’yanov, “The dissolution forms of diamond crystals in
CaCO3 melt at 7 GPa,” Russian Geology and Geophysics,
Vol. 41, No. 5, 2000, pp. 682–687). Diamonds with coexisting positive and negative trigons are extremely rare (e.g.,Harris et al., 2022) and may indicate that the diamond experienced a complex multistage history related to thermalor fluid changes after its formation.
Using the acquired infrared absorption spectrum, theauthors identified the diamond as type IaAB, containing
nitrogen in the A- and B-aggregated forms. When the entire
xenolith was exposed to ultraviolet light, the diamond fluoresced blue due to N3 defects (three nitrogen atoms surrounding a lattice vacancy), consistent with the presenceof nitrogen impurities.
The diamond also contained an unidentified mineralinclusion with visible iridescence (figure 26). Minerals included fully within diamond (which itself is an impermeable time capsule) represent pieces of the mantle source
region present at the time of diamond formation. For thissample, the mineral was too deep within the diamond toconclusively identify it nondestructively. However, if themineral inclusion could be extracted, its chemical composition might be very useful in reconstructing the geologic

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403

Figure 27. Pearl diver collecting shells from the wild.
Photo courtesy of Pearls of Australia: Cygnet Bay
Pearl Farm, Western Australia.

history of the host diamond, as well as that of the mantle
rocks in which the diamond formed.
Matthew F. Hardman and Mei Yan Lai
GIA, Carlsbad

RESPONSIBLE PRACTICESAn environmental, social, and governance assessment ofmarine pearl farming. Farming pearl oysters is an important economic activity that, when done sustainably, canalso provide valuable environmental and social benefits.
To gain an evidence-based understanding of the sustainability challenges and opportunities facing marine pearl
farming and their intersection with environmental, social,and corporate governance (ESG), The Nature Conservancy(TNC), a U.S.-based nonprofit, conducted an ESG assessment of an Australian pearl farm. This pilot project was intended to guide the development of a sector-wide ESG andsustainability assessment framework. The project was sup-404

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ported and funded by GIA as part of its global effort focusedon education and the promotion of responsible practices.
Pearl oysters filter water at rates that are often higher
than those reported for other oyster species. This filtration
process contributes to waste treatment through the uptakeof not only nutrients (nitrogen and phosphorus) but also
heavy metals (S. Gifford et al., “Quantification of in situ
nutrient and heavy metal remediation by a small pearl oyster (Pinctada imbricata) farm at Port Stephens, Australia,”
Marine Pollution Bulletin, Vol. 50, No. 4, pp. 417–422; L.T.
Barrett et al., “Sustainable growth of non-fed aquaculture
can generate valuable ecosystem benefits,” Ecosystem Services, Vol. 53, 2022, article no. 101396). Pearl oysters arehung in baskets suspended from long lines, and these structures in which they are farmed provide habitat for a variety
of fish and invertebrates (S.J. Theuerkauf et al., “Habitat
value of bivalve shellfish and seaweed aquaculture for fish
and invertebrates: Pathways, synthesis and next steps,” Reviews in Aquaculture, Vol. 14, No. 1, 2022, pp. 54–72). If
the negative impacts of pearl farming can be mitigated andeffective sustainability strategies implemented throughwaste reduction, recovery and reuse, disease mitigation, andavoidance of overstocking, pearl farming could significantly
benefit regional economies and local marine environments.
TNC’s 2019 report titled Towards a Blue Revolution
provided comprehensive guidance for evaluating criticalaquaculture systems. Since the report’s publication, TNC
has built a science-based tool to assess industry and company operations. This tool takes into account factors related to the natural environment as well as the inherent
and unique characteristics of a sector or company’s operations. The tool has been designed to consider environmental data and the health of local ecosystems, enabling theidentification of potential environmental benefits via
restorative aquaculture, which has the potential to generate a net positive ecosystem outcome. This approach sets
it apart from other sustainability assessment frameworks,
risk assessments, and life cycle assessment methodologies.
Between February and May 2023, the assessment was
conducted on Cygnet Bay Pearl Farm (CBPF; see figures 27–29), a company that produces pearls from Pinctada maxima
north of Broome, Western Australia, using both wild capture and hatchery operations. CBPF is an operation of Pearlsof Australia (PoA). Using the TNC framework, a number ofoverarching factors were assessed:• Company capacity, ethics, and management measures• Impacts on wild stocks from source of stock, hatcheryescapes/interbreeding, and macrofauna interactions• Reliance on added feed and chemicals• Habitat impacts• Water column effects (e.g., temperature, salinity,
light penetration, and chemical characteristics ofseawater at different depths)• Disease• Use of resources: fresh water, land, and energy

GEMS & GEMOLOGYFALL 2023

cluding school, transport, and fishing. Dedicated cultural
heritage tours managed by Bardi people form part of theecotourism offered at Cygnet Bay Pearl Farm. CBPF is active in research, housing the Kimberley Marine Research
Station, and parent company PoA currently partners with
the Marine Bioproducts Cooperative Research Centre. This
research focuses on better understanding the implications
and potential positive effects on fish and invertebrates thatgrow naturally on PoA farms.
CBPF could already be providing environmental benefits to the surrounding area through restorative aquaculture
practices. Based on a general understanding of ecosystemservices associated with oyster aquaculture, CBPF does
meet the criteria for restorative outcomes, such as increased
cycling of water and nutrients and providing habitat toother species. However, the extent to which these ecosystem services positively impact broader environmental outcomes remains unknown, in part because of the limited
understanding of anthropogenic effects on water quality andhabitat loss in the area.
Overall, the assessment provided an effective means toevaluate CBPF for its management and approach to ESG, theextent to which cultural communities are considered, thecompany’s engagement with voluntary certification andcodes of practices, and its awareness of key environmental
risks and negative impacts as well as positive impacts.

Figure 28. The pearl oysters are partially opened toallow seeding. Photo courtesy of Pearls of Australia:
Cygnet Bay Pearl Farm, Western Australia.

Figure 29. Seeding Pinctada maxima pearl shell. Photo
courtesy of Pearls of Australia: Cygnet Bay Pearl
Farm, Western Australia.

The assessment showed that CBPF meets or exceeds all
essential requirements within the ESG framework, including the company capacity, ethics, and management metrics.
The company has no compliance issues associated with sustainability metrics, and an internal management team oversees sustainability topics and implementation. CBPF
adheres to the Pearling Environmental Code of Conduct, developed by Australia’s Pearl Producers Association in 2002
and updated in 2007. The farm has Marine StewardshipCouncil certification, meeting independent, third-party ecological sustainability standards. It operates within a country
that has an effective governance, legal, and enforcement system in place to manage the industry into the future.
Regarding the social component of ESG, CBPF forms
part of the Banararr Steering Committee (BSC) of the Bardi
people, the traditional owners of the area. The BSC’s role
is to establish a positive relationship between CBPF and
the Bardi traditional owners. Any amendment, use, or development of the area is discussed by the BSC together
with relevant topics affecting the community at large, in-

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Figure 30. Akshay Poddar, Melizza and Jennifer Tanpoco, and Julia Hackman Chafé are some of GIA’s featured
graduates on the Alumni Collective website.

At the same time, additional needs for TNC’s ESG Assessment Framework have been identified. Brood stock
sourcing and handling, biosecurity, and genetic risks are allareas that require additional research. One hundred percent
utilization of the oysters farmed should be an explicit goal
and assessment factor. The assessment should also consider the legacy effects of aquaculture equipment (e.g., recovery, reuse, and recycling), as well as the origin andspecies status of shells utilized to manufacture nuclei.
Once these considerations are addressed, the TNC’s ESG
Assessment Framework should provide an effective sectorwide approach.
Heidi Alleway
The Nature Conservancy
Adelaide, Australia

ANNOUNCEMENTS
GIA Alumni Collective. Discover the GIA Alumni Collective, a networking and knowledge hub for GIA graduates.
The online community at collective.gia.edu is a diverse
group who all share a passion for the gem and jewelry industry. Users can access both live and self-paced Continuing Education seminars, join virtual chapters, connect with global
alumni, and more. The site also highlights some of the GIA
alums who uphold GIA’s highest standards (figure 30).
Akshay Poddar earned his Graduate Gemologist
diploma at GIA’s Mumbai campus. A third-generation diamantaire, Poddar was driven to start his own business, deal-

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ing in both wholesale trade and retail. The diamond jewelry
designer believes that while nature creates the diamond, his
job is to add beauty to it.
Jennifer Tanpoco launched Jaune Pearls in 2002, later
enlisting her daughter Melizza as CEO. The Tanpocos,
both GIA Pearls Graduates, focus on sustainability andlong-term environmental impact in their business, educating clients on how the pearls are sourced.
Julia Hackman Chafé used her marketing backgroundand her Colored Stone Essentials Certificate from GIA tocreate a strong digital presence after taking over social
media for her family’s wholesale colored stone business.
Now a successful influencer under the name JewelsWithJules, Chafé shares photos and videos of gems, jewelry, andinternational travel, providing her followers with a uniquelook at the colored stone trade.
Visit https://collective.gia.edu/meet-the-collective.html
to read stories from these alumni and more.
Gübelin Gem Museum opens in Switzerland. In July 2023,
the Swiss firm Gübelin, known for its luxury timepieces
and jewelry and its gemological laboratory, opened the Gübelin Gem Museum in Lucerne, Switzerland (figure 31).
The collection allows visitors to explore the formation, origin, and diverse colors of fascinating gemstones (figure 32),
as well as the history of the family-owned business,founded in 1854.
The opening of the museum commemorates the 100th
anniversary of the Gübelin Gem Lab, which became aworld-renowned institution in the field under the leader-

GEMS & GEMOLOGYFALL 2023

Figure 31. A look inside
the new Gübelin Gem
Museum in Lucerne,
Switzerland. Photo courtesy of Gübelin.

ship of Dr. Edward J. Gübelin. Recognized for his gemstone
inclusion research, Dr. Gübelin’s groundbreaking work isa focal point of the museum, including many of his instruments and gemstones he collected during his travels worldwide. Visitors will discover 174 gemstones selected fromthe company’s reference collection in addition to selectedGübelin watches and jewelry pieces from various eras.
The compact space not only houses historical exhibits
but also provides a window to the future. Visitors will learn
about Provenance Proof, the company’s blockchain technology for colored gemstones, and Gemtelligence, a col-ored gemstone analytical tool that uses artificial intelligence to increase consistency in data interpretation, origin
determination, and identification of heat treatment. It isalso the new home of the Gübelin Academy, offeringcourses to inspire students in the museum setting.
Through digital and interactive elements, the exhibits will
be continuously updated and expanded, ensuring new experiences for years to come. For more information, visit
www.gubelin-gemmology.com.
Erica Zaidman
GIA, Carlsbad

Figure 32. A display
case featuring emerald
at the Gübelin Gem
Museum. Photo courtesy of Gübelin.

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CONFERENCE REPORTS
Chicago Responsible Jewelry Conference. The seventh annual Chicago Responsible Jewelry Conference (CRJC) was
held August 11–13, 2023, in Rosemont, Illinois. Susan
Wheeler, Responsible Jewelry Transformative (RJT) founderand Virtu Gem cofounder, opened by highlighting efforts tofurther the United Nations Sustainable Development Goals(SDGs) and the UN Women’s Generation Equality campaign. The first of the 17 SDGs is to end poverty, which disproportionately affects the roughly 40 million artisanal andsmall-scale miners worldwide. Wheeler said poverty posesa risk to the jewelry industry because consumers do notwant to perpetuate it. Responsible supply chains can help
lift miners from poverty, she noted, and the industry canmake great strides toward the UN’s goals.
Anna Samsonova (Samsonova Consulting) moderated
a panel on opening markets for artisanal gemstone miners
in East Africa. Pauline Mundia (Virtu Gem Zambia), Jessica
Hudson (Virtu Gem), Sejal Karavadia (Brilliant Earth), andCristina Villegas (PACT and Moyo Gems) discussed theimportance of information exchange with miners and cutters, developing trust with the help of local associations,creating sourcing standards, and training and education.
Karavadia said customers are very receptive to a gem’s
mine-to-market story. Sixty percent of the miners in
Moyo’s supply chain are women, Villegas said, and they
have begun incorporating young woman cutters. “Formany years, Africa was exporting raw unprocessed materials,” she said. “Now it’s high-end beautiful faceted gemstones, and we should celebrate that.”
Deep-sea biologist Lisa Levin (Scripps Institution of
Oceanography, University of California, San Diego) explained how deep-sea mining of minerals, including goldand silver, would disrupt ecosystems. The International
Seabed Authority (ISA) has approved 31 mining exploration
contracts in international waters as of January 2023 and isdeveloping regulations. Deep-sea ecosystems host abundant life, with interdependent species that depend on specific hydrologic and geochemical properties, she said, andprovide many benefits to the broader ocean. The deep sea
is also home to bacteria that consume methane, a driver ofglobal warming. Levin said mining could cause extinctions
and risks destruction of deep-sea biodiversity. Additionally,
scientists believe that post-mining restoration is not possible on the seafloor. Since the definition of serious harm
remains unclear and much about these ecosystems is still
unknown, she said, there is insufficient knowledge for evidence-based decision making by the ISA.
Holly McHugh (Mejuri, Inc.) moderated a panel on responsible silver sourcing. Panelists noted an increase inhigh-level guidance for responsible sourcing. Examining
this market is complex because more than 70% of silver
production is a byproduct of other mining, panelists said,
while artisanal and small-scale mining is challenging because silver requires so much mining effort. Torry Hoover

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said Hoover & Strong’s refined metals are now 100% recycled, and Will Nevins-Alderfer’s W.R. Metalarts uses
Fairmined silver for its alloys. Many of the Richline Group’s
larger customers now ask about sourcing, said Mark Hanna,and the company has begun tracking silver withblockchain. He emphasized knowing your supplier and customer to ensure ethical products.
Susan Wheeler and Virtu Gem’s Chiko Manda (viavideo message) discussed Malawi’s gem trade. The country
exports hundreds of kilograms of high-quality aquamarine
annually but is one of the poorest in the world. Manda saidvery few gems mined in Malawi are processed there due tolack of expertise and equipment, compounded by smuggling problems, and data indicate that less than 1% of its
aquamarine is legally exported. He asked jewelry companies to allocate a small percentage of their budgets to buy
cut stones from source countries to motivate cutters to improve their skills. Wheeler added that this would improve
traceability. “If you support the gem processing industry
in Africa, you empower a lot of people,” Manda said.
In a virtual presentation, Mkhululi Nkosilamandla
Ncube (African Minerals Development Center, AMDC) introduced the African Mining Vision (AMV), a project of theUN. The AMV aims to transform the paradox of mineral
wealth and dire poverty through equitable mineral exploitation. The AMDC’s methods to implement the AMV include
community engagement, training, technology, and financial
support. As part of the approach, the Africa Gemstones andJewellery Exhibition and Conference (AGJEC) series promotes the trade of African gemstones within the continent.
The first event was held in Zambia in July; upcoming events
will take place in Nigeria and Ghana this fall.
Brecken Branstrator (GemGuide) moderated a panel on
working with source country gem traders (figure 33). Eric
Braunwart (Columbia Gem House), Ola ErogbogboOyeniyi (Deinte Designs Limited), Pauline Mundia, Stuart
Pool (Nineteen48), and Susan Wheeler shared key takeaways, including the importance of understanding the local
market and legal system, supply chain analysis, and working with local partners. Erogbogbo-Oyeniyi said that helping a community and weaving that into a gem’s story is agreat benefit.
Anna Bario described founding jewelry company Bario
Neal with fellow designer Page Neal in 2008 and developinga responsible supply chain. Early on, they connected with
Ethical Metalsmiths, the Alliance for Responsible Mining(ARM), and artisanal miners in Tanzania and Peru. They
built relationships with ethical suppliers over the next
decade and in 2019 hired a consultant to help establish aprocess based on the UN SDGs and OECD Due Diligence
Guidance for Responsible Business Conduct. This includes
a code of conduct they share with suppliers, supplier interviews, and an eight-step partner and material evaluation
process. In 2020, the company released its first biannual sustainability report, possibly the first by a small independent
jeweler. Their next goal is Climate Neutral certification.

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Figure 33. Pauline
Mundia, Susan Wheeler,
Eric Braunwart, andOla Erogbogbo-Oyeniyi
discuss working with
gem traders in source
countries. Photo courtesy of RJT.

A panel on sex trafficking in Marange, Zimbabwe,
moderated by Brandee Dallow (Grandview Klein DiamondGroup), began with a documentary clip of survivors’ stories. The area has been fraught with conflict and violencesince the discovery of diamonds there in 2006. The U.S.
has banned imports of Marange diamonds due to reportsof forced labor. According to Abigail Sibanda (Marange
Women’s Alliance, MWA) (figure 34, left), the military, police, mine guards, and informal miners collaborate tosmuggle diamonds out and exploit women and girls. Vul-

nerability to trafficking stems from Zimbabwe’s highpoverty, and many victims are lured by the promise ofmining work. The community questions where the diamond revenue goes because there is no development,
Sibanda said. Susan Wheeler noted that RJT and MWA
have facilitated support for survivors through court advocacy, but prosecution is still very limited. MWA helps survivors with peer-to-peer counseling and other support andempowers all women to speak out, holding weekly meetings on WhatsApp. The group has been planning to pur-

Figure 34. Left: Abigail
Sibanda describes efforts by the Marange
Women’s Alliance tohelp women and girls in
Zimbabwe. Right: Laura
Galvis (ARM) speaks
about Fairminedcertified projects combining gold mining andagriculture in Colombia.
Photos courtesy of RJT.

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chase a chicken coop to develop consistent income andpay for their cellular data, which RJT currently covers.
MWA is also creating a billboard campaign for Marange tohighlight the issue. “We protect the diamonds. Why don’t
we protect the women?” remarked Sibanda, who has relocated outside of Zimbabwe because her activism has put
her at risk.
Roy Maconachie (University of Bath, England) described the relationship between mining and agriculture in
Sierra Leone based on his research there since 2003, after
the civil war. Most mining occurs in the dry season, withincome used to farm in the rainy season. Over the last
decade, artisanal miners have shifted to gold because mostalluvial diamond deposits are mined out. Artisanal gold
mining here is dominated by women and provides a reliable income. The interconnected nature of farming andmining can help rebuild agricultural institutions destroyed
during the war, he said, but miners still need basic infrastructure, access to markets and credit, and technology.
Laura Galvis (ARM) (figure 34, right) detailed three
Fairmined-certified projects—the Íquira cooperative, La
Gabriela, and the Chede mine—that support almost 200
workers and show how small-scale gold mining and agriculture coexist in Colombia. The Fairmined label certifies
gold, silver, and platinum from small-scale mining organizations using responsible practices, and it comes with the
Fairmined Premium, an economic incentive. The Chedemine committee will use this year’s premium to sponsor a
coffee brand sourced by miners farming coffee; they have5,600 Fairmined credits and need 20,000 for this project.
Businesses can buy Fairmined credits to support the projectof their choice.
Sarah Yood (Jewelers Vigilance Committee) and Andrea
Hill (Hill Management Group) discussed U.S. sanctions
against Russian diamonds due to the war in Ukraine. Yood
explained that because diamonds are considered a productof the country in which they are cut—with 90% cut in
India—Russian diamonds can still reach the U.S. She acknowledged India’s difficult position and the issue of protecting jobs there. The EU and the G7 countries areworking to stop the flow of Russian diamonds. (At press
time, an announcement was expected shortly.) Yood said
that starting in 2024, G7 countries will likely require diamonds of one carat or larger to carry proof they were notmined in Russia (the size threshold will be progressively
lowered). Hill said that in theory, it will eventually be clear
if a diamond originated in Russia or Zimbabwe.
Saleem Ali (University of Delaware and UN International Resource Panel, IPA) presented an IPA report analyzing resource rushes in Africa, which found that mineral
resources can lead to migration within Africa and prevent
migration to Europe. A key message of the report was thatproperty rights to resources can reduce forced migration.
Ali suggested jewelry companies partner with the UN
High Commission on Refugees to train refugees in jewelry
making.

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Natasha and Eric Braunwart (Columbia Gem House),
Megan Cochran (Megan Cochran Jewelry Design), and Hannah Smythe (Toast Fine Jewelry) discussed how responsibly
sourced designs can engage consumers. The MJSA 2023 Design Challenge, sponsored by Columbia Gem House, asks
for designs based on a set of gemstones and a fictional client
story. The Jewelry for Wildlife raffle of selected pieces benefits CRRIFS, a wildlife rescue, rehabilitation, and research
center in Guaymas, Mexico.
Shannon Kurzyniec and Chelsea Rowe of the Ethical
Metalsmiths Student Committee introduced 2023’s So
Fresh + So Clean online responsible jewelry exhibition,
which asked artists to consider the life cycle of their creations. This year’s awards went to Rashele Alradaideh (Virginia Commonwealth University), Amy Beeler (BowlingGreen State University), and Maria Hammond (Manchester
Metropolitan University, England), with cash prizes sponsored by Hoover & Strong, Rio Grande, and Earthworks.
The conference also included a collaborative session on
advancing the UN SDGs through jewelry supply chains.
Sunday offered a Responsible Gem Boutique with eightsuppliers, co-located with the InStore Show. After Saturday’s sessions, Anna Samsonova announced that attendee
donations had fully funded the Marange Women’s Alliance’s chicken coop project.
Erin Hogarth
GIA, Carlsbad
Turquoise United Conference 2023. The second annual
Turquoise United conference was held August 10–12, 2023,
at the Albuquerque Convention Center in New Mexico.
The vision of the conference is to bring together membersof the turquoise industry at a single event for discussion
and reunion. With this goal in mind, the conference caters
to every aspect of the turquoise industry, attracting miners,
lapidaries, collectors, dealers, artists, treatment experts, andresearchers. A year ago, GIA representatives attended theinaugural Turquoise United conference (Fall 2022 GNI, pp.
390–394).
This year, there were additional symposium sessions
across a range of topics and new competitions for registeredconference participants. The first session was hosted by
Turquoise Museum executive director Jacob Lowry, witha panel consisting of Kenneth Van Wey (Indian Arts andCrafts Board), Matthew Wernz (Federal Trade Commission), and Sean Hyrons (U.S. Fish and Wildlife Service). The
panel discussed the laws surrounding the sale and purchaseof turquoise jewelry and gave examples of situations inwhich their respective agencies may become involved (figure 35). The panelists offered advice to consumers purchasing turquoise jewelry, noting the importance of a written
receipt stating the authenticity and nature of the productin as much detail as possible.
The turquoise grading system proposed by Joe Dan
Lowry at last year’s conference has been put into practice.
Attendees could attempt to grade sample stones or stones

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Figure 35. From left:
Jacob Lowry, Sean Hyrons, Matthew Wernz,
and Kenneth Van Wey
engage in a panel discussion titled “Laws:
Liabilities and Protections.” Photo courtesy of
the Turquoise Museum.

of their own using the system based on turquoise masterstone sets and grades of color, matrix, and “zat” (boldness
and dynamism). Application of the grading system requires
detailed knowledge of certain turquoise features present at
different mines. For example, author AG did not recognize“waterweb” matrix as being a valuable pattern, and therefore gave a lower matrix score on a stone with a high value.
The grading system continues to be developed with feedback from the conference, with a goal of publicly accepted
use by 2027.
The conference also offered courses for attendees. A“Turquoise Basics” course taught by Jacob Lowry described
the types of turquoise and turquoise imitations in the market to better inform new buyers of the stone. Joe Dan
Lowry taught a class called “Collecting 101,” offering guidance to consumers beginning their own turquoise collection. A follow-up course, “Collecting 102,” is planned for
Turquoise United 2024.
Some attendees were rewarded for their impressive
specimens and extensive turquoise knowledge. A highestgrade turquoise stone contest received numerous entries
from multiple mines, ranging from natural cabochons withstriking color and matrix to hefty rough turquoise nuggets.
Best in Show winner Mark Baca (American Turquoise Online) received a monetary prize and trophy (figure 36). An
identification contest over both days of the conference allowed attendees to test their skills at identifying the natureand mine origin of turquoise specimens. Michael Turano(Michael Turano Lapidary) won the contest, with Dye
Masaki (Native Spirit) placing second, and Ty Gibson (Gibson Lapidary, LLC) in third.
Author AG presented his research on the Mona Lisa
mine, located in western Arkansas, in a session on August
11. He provided details about the history, geology, mineralogy, and current mining operations at the site. Discussion
about the methods used and Mona Lisa turquoise’s potential
place in the market followed the presentation. Material fromthe mine was also on display at the Turquoise Museum,
where the day ended with the Collectors’ Circle Gala.

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Figure 36. Mark Baca (American Turquoise Online)
holds his winning piece (right) and trophy for Best in
Show. Photo courtesy of the Turquoise Museum.

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Figure 37. Left: A conference participant examines turquoise cabochons. Right: Iranian turquoise cabochon with abrown “spiderweb” matrix pattern. Photos by Aaron Palke.

The following morning, author AP presented GIA’s
methods for identifying turquoise in the gemological laboratory. Attendees expressed their interest in learning
more about these techniques and shared their views on terminology in the turquoise industry.
Next, Joe Dan Lowry presented a session on the challenges of fingerprinting turquoise mine sources, especiallyin archaeological contexts. The heterogeneity of
turquoise, the multitude of sources, and the stone’s formation in near-surface, sedimentary environments create
unique challenges when trying to determine a stone’s
mine origin.
Throughout the conference, a gem show focused solely
on turquoise was open to the public. Roughly a dozen dealers participated, offering a wide variety of turquoise material (figure 37), including high-end jewelry pieces, naturalrough material, untreated cabochons, stabilized cabochons,and beadwork. Many American turquoise localities wererepresented, while turquoise from Iran, China, Egypt, andMexico was also available. A silent auction allowed con-

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ference attendees to bid on natural turquoise and turquoise
jewelry pieces donated by the event sponsors.
The conference concluded with a banquet honoring thefirst inductees into the Turquoise Hall of Fame: Joseph
Pogue, Leonard Hardy, Robert Zachary, Philip Chambless,and James Elquist. These honorees were nominated fortheir impact on the turquoise industry. The event sponsors
reiterated their desire to grow Turquoise United, with confirmation that the annual event will be hosted in Albuquerque in 2024.
Alex Goodsuhm and Aaron Palke
GIA, Carlsbad

ERRATUMIn the Summer 2023 article on augite from Vietnam (pp.
182–194), the carving sizes in figure 1 were incorrectly presented in centimeters. These were the millimeter sizes of
the two carvings.

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