FEATURE AR ICLESTHE VERY DEEP ORIGIN OF THE
WORLD’S BIGGEST DIAMONDS
Evan M. Smith, Steven B. Shirey, and Wuyi Wang

Large and relatively pure diamonds like the historic 3,106 ct Cullinan, found in South Africa in 1905,
have long been regarded as unusual based on their physical characteristics. For example, they often exhibit exceptional color and clarity, while routinely qualifying as type IIa, a rare designation of chemical
purity. A new research discovery about these Cullinan-like diamonds is that they contain heretofore unknown, deeply derived inclusions that originate below the continental mantle keel and are thus known
as “superdeep” diamonds (Smith et al., 2016). Originating from a depth between 360 and 750 km, they
reveal information about the conditions within the convecting mantle, beneath the earth’s rigid tectonic
plates. Here we review the previously published findings, compare the Cullinan-like diamonds to themore abundant lithospheric diamond population, and offer evidence from some additional diamondsamples that further verifies their superdeep origin. Cullinan-like diamonds contain minute and rare silicate and iron-rich metallic inclusions surrounded by a fluid jacket composed of methane and hydrogen.
The inclusion compositions suggest that this deep mantle environment contains small pockets of oxygen-deficient metallic liquid out of which the diamonds crystallized. This new and unexpected observation made on the world’s most expensive diamonds is important for understanding the chemical
reactions between mineral assemblages in the deep earth. It shows that deep regions of the mantle contain metallic iron, as opposed to the shallower, more oxidized mantle rocks actively participating inplate tectonics and its associated volcanism.

Diamonds, besides being the most valuable
gemstones for jewelry, are some of the mostscientifically valuable samples of the deep
earth. Interpretation of their included minerals andcrystallization history provides a snapshot of otherwise inaccessible geological processes from morethan 100 kilometers underground and often from billions of years in the past (e.g., Shirey and Shigley,2013). No other mineral sample can provide information for research from such depths and from so longago. Until recently, exceptional gem diamonds suchas the famous Cullinan or Lesotho Promise, with aset of physical characteristics that distinguish themfrom other kinds of diamonds, were enigmatic andhad an unknown origin (Bowen et al., 2009; Gurneyand Helmstaedt, 2012; Moore, 2014). How did they
form? Certainly not like 99% of other gem dia-monds. New research carried out in the last two
years at GIA, the Carnegie Institution for Science,and the University of Padua reveals that these Cullinan-like diamonds form in a geologically special way,
involving metallic liquid and originating from extreme depths in Earth’s mantle well below that ofother gem diamonds (Smith et al., 2016).

RECOGNITION OF A NEW DIAMOND VARIETY

See end of article for About the Authors and Acknowledgments.
GEMS & GEMOLOGY, Vol. 53, No. 4, pp. 388–403,
http://dx.doi.org/10.5741/GEMS.53.4.388© 2017 Gemological Institute of America

As a family, these diamonds tend to be large, inclusion-poor, and relatively pure (usually type IIa), andin their rough state they are irregularly shaped andsignificantly resorbed (figure 1) (Bowen et al., 2009;
Gurney and Helmstaedt, 2012; Moore, 2014; Smithet al., 2016). These characteristics are combined in
the acronym “CLIPPIR” (Cullinan-like, Large, Inclusion-Poor, Pure, Irregular, and Resorbed). Figure 2
shows a broad dataset of assorted diamonds, where
the CLIPPIR population displays these characteristics. Among larger diamonds, there is a striking increase in the prevalence of D color grades and the
proportion of type IIa diamonds, chiefly due to their

388

GEMS & GEMOLOGYEXCEPTIONAL CULLINAN-LIKE DIAMONDSWINTER 2017

Figure 1. Rough CLIPPIR diamonds have physical features akin to the historic Cullinan diamond, which forms the“C” of the CLIPPIR acronym. The size distribution for this variety of diamond is skewed toward large sizes, withthe examples shown here ranging from 14 to 91 carats. In rough form, CLIPPIR diamonds are irregularly shaped
rather than well-formed crystals and sometimes appear to be broken fragments of once larger diamonds. Photo by
Robert Weldon/GIA; courtesy of Gem Diamonds Ltd.

lack of nitrogen. As expected, these larger type IIa diamonds have very few inclusions and consequently
high clarity grades.
Historically, it has been difficult to study these diamonds because of their rarity and high value as
gemstones; they simply cannot be purchased for indepth analysis on a research budget. It is only
through the research program of GIA, where thousands of high-quality gem diamonds were available
to be examined by non-destructive methods such as
Raman spectroscopy and X-ray diffraction, that theircommon inclusion paragenesis and hence their origin was revealed. Here we review in more detail the
key features of diamonds reported by Smith et al.
(2016) as well as some additional CLIPPIR samples
examined since.
An important key to the origin of these diamonds
lies in the special inclusions they contain, composedof iron-nickel metal with minor sulfur and carbon.
At the ambient pressure and temperature underwhich the CLIPPIR diamonds formed, these inclusions were metallic liquids. But now, under surface

conditions, they have cooled and crystallized to a distinctive, rarely seen mineral assemblage. This assemblage includes the rare mineral cohenite, an iron-richcarbide with the formula (Fe,Ni)3C, and the relatively
common iron sulfide mineral pyrrhotite.
Metallic iron-nickel alloy is thought to be createdand stabilized in parts of the deep mantle (Frost et al.,
2004; Rohrbach et al., 2007). Although the normal
amount of metal in these regions of the mantle islikely about 1% or less, it is expected to regulate andlimit the local activity of oxygen (Frost et al., 2004).
Oxygen availability is a key planetary parameter resulting from the earth’s large-scale geologic evolution(Rohrbach and Schmidt, 2011). It controls, along withtemperature and pressure, the basic mineral composition of the planet’s interior, how this varies withdepth, and the composition of melts generated. At
the pressure and temperature conditions near and at
Earth’s surface, rocks almost never contain metallic
iron. Also, these shallower rocks are affected by magmatism associated with plate tectonics and by interaction with Earth’s atmosphere and hydrosphere,

EXCEPTIONAL CULLINAN-LIKE DIAMONDSGEMS & GEMOLOGYWINTER 2017389

Prevalence of D color grade

Clarity grade distribution:
Largest 1000 of 3.5 million diamonds

Proportion of type IIa

1650

D color surpasses E
for larger diamonds

4

Type I

40
30

COUNT

8

% TYPE IIA

D/E RATIO

200
12

2.0

20

150

Type IIa

100
50

10

1.3 %

m
3. ple5 om f
di illio
am n
on
La
ds
rg
es
La t 10rg00e
La s t 500rg
es
La t 1
rg 00
es
t5
0

Sa

c
2– t
3
c
3– t
4
c
4– t
5
5– c t
10
10 ct
–2
0
>2 ct
0
ct

1–
2

ct

0

<1

0

0

I3

I2

I1

SI2 SI1 VS2 VS1 VVS2 VVS1 IF

FL

Fewer inclusions/blemishes

Figure 2. Systematic deviations of properties suggest that CLIPPIR diamonds become more prevalent among largersized diamonds. Left: Diamonds over 5 carats show an increasing incidence of D color grades in a random sample
of 4.2 million diamonds graded by GIA; shown is the ratio of D color (colorless) to E color (not quite perfectly colorless) grades awarded. Center: In a similar dataset of 3.5 million samples, the proportion of type IIa diamonds (nitrogen-deficient, high-purity) also increases dramatically when considering only the larger diamonds. Right: Among a
set of large diamonds, all over 15 carats, those qualifying as type IIa have markedly higher clarity and dominate theflawless grade category. (GIA clarity abbreviations: FL = flawless, IF = internally flawless, VVS1/2 = very very slightly
included 1/2, VS1/2 = very slightly included 1/2, SI1/2 = slightly included 1/2, and I1/2/3 = included 1/2/3)

which occur at relatively oxidizing conditions. These
conditions completely mask the ambient conditionsin the earth’s deep interior, which can be revealed in
the mineralogy of CLIPPIR inclusions. It is coincidental but agreeably fitting that the world’s mostvaluable diamonds as gemstones have now taken on
exceptional scientific value.

Nitrogen is all around us every day, making up 78%of the air we breathe and forming an essential element for life on Earth. Nitrogen behavior inside theearth, especially in minerals and fluids deep in the
mantle, is less understood, but nitrogen is thought
to be recycled into the mantle by plate tectonic activity. In the mantle, it is an ingredient in the diamond-forming fluids for the most common
diamonds, known as type I. As such it is the mostabundant trace element in diamond, where it substitutes for carbon atoms in the crystal structure.
Since the advent of studies to understand the geological conditions for diamond growth and formation, diamonds have become one of the important
tools that can help geologists trace the deep cycling
and behavior of nitrogen.

The nitrogen content of diamond can vary greatly
for several reasons, including variable supply of nitrogen in diamond-forming fluids, variable partitioning
into the diamond, and variable partitioning into acompeting nitrogen-bearing phase. Diamonds with
no appreciable nitrogen-related absorption in the 800–
1400 cm–1 range in infrared spectroscopy are classically defined as type IIa (Wilks and Wilks, 1994). Notethat the abundance of nitrogen in diamond is a continuous range without a sharp division. The continuous range means that the 5–20 ppm nitrogenconcentration threshold that marks the boundary inclassification between type IIa (nitrogen-deficient) andtype I (nitrogen-bearing) has no basis in mineralogy
but rather is based on analytical sensitivity. Analytical sensitivity has shifted to lower thresholds overtime, as instrumentation has improved.
The type IIa designation of chemical purity is usually regarded as a desirable gemological trait that is
associated with high clarity and color, and it has
proven convenient for scientific classification of diamond (see review by Breeding and Shigley, 2009).
With one exception, the CLIPPIR diamonds of thisstudy are type IIa. The relationship between CLIPPIRand type IIa diamonds likely applies to larger unstudied populations, as shown by the disproportionate

390

GEMS & GEMOLOGYNITROGEN AND THE DISTINCTION BETWEENTYPE IIA AND CLIPPIR DIAMONDSEXCEPTIONAL CULLINAN-LIKE DIAMONDSWINTER 2017

number of large, high-quality diamonds that are typeIIa (figure 2). It is important to bear in mind, however,that not all type IIa diamonds are classified as CLIPPIR, and vice versa. Some lithospheric diamonds
from peridotitic and eclogitic host rocks, for example, can also have very low nitrogen concentrations
and be considered type IIa (e.g., Stachel and Harris,
2009; Gurney et al., 2010; Moore, 2014).

2013, in Moore, 2014). However, a search of the literature reveals that the only consistently reported inclusion is visually identified graphite. In fact, theCullinan diamond itself was described as having asmall round, black inclusion in the center, which
was intentionally bisected with the first cleave of therough diamond by Joseph Asscher in 1908 (Crookes,
1909). Of course, the finished Cullinan gemstones
are essentially flawless and free of inclusions (Scar-ratt and Shor, 2006). What few inclusions are to befound in rough CLIPPIR diamonds are usually eliminated during cutting and polishing, making thesearch for inclusions all the more difficult among
polished goods.
Our recent study (Smith et al., 2016), comprising atargeted subset from hundreds of thousands of faceted
diamonds as well as a small number of offcuts
trimmed from rough diamonds during cutting andfaceting, documented 53 inclusion-bearing CLIPPIR diamonds that establish a specific inclusion assemblage
that deviates from known lithospheric or sublithospheric suites. Thirty additional faceted stones havesince been studied and now yield observations on a
total of 83 diamonds, summarized in table 1 (see also
data supplement at www.gia.edu/gems-gemology/
winter-2017-worlds-biggest-diamonds; for methods,
see Smith et al., 2016). Past visual descriptions of inclusions in these types of diamonds mention flat,
black features, interpreted as graphite. Our work has
shown that these are graphitic rosette fractures thatsurround and sometimes conceal a much smaller
non-graphitic inclusion (figure 3) somewhat reminiscent of sulfide inclusions.
In the center of the black fractures, the inclusion
nucleus itself is different from that of a typical sulfide
inclusion. In contrast to lithospheric diamonds,
where black fracture rosettes are typically associatedwith sulfide inclusions, CLIPPIR diamonds haveblack fractures surrounding many inclusions, suchthat even silicates can have a superficial resemblance
to “common” sulfide inclusions (figure 4A; see also
figures S2 and S3 in Smith et al., 2016). The fractures
are likely due to the higher stresses that build up
around most superdeep inclusions when they are exhumed from the mantle.
The most common inclusion identified in CLIPPIR
diamonds is a metallic iron-nickel-carbon-sulfur mixture. While most of these inclusions can only be studied remotely through the diamond by Raman
spectroscopy, all inclusions that can be examined directly by other microanalytical techniques such as
scanning electron microscopy (SEM), electron microprobe analysis, and/or X-ray diffraction (XRD) are similar metallic Fe-Ni-C-S mixtures. In our experience,these metallic inclusions differ from sulfide inclusions,which also have metallic luster but are smoother, moreequant cuboctahedral shapes, isolated rather thangrouped, and far less magnetic. The sulfide inclusions
sometimes have brassy colors and identifiable sulfide
mineral Raman features, and they have not yet beenfound to have coexisting fluid. In contrast, the Fe-Ni-

EXCEPTIONAL CULLINAN-LIKE DIAMONDSGEMS & GEMOLOGYSOLIDIFIED IRON-NICKEL METAL:A NEWLY RECOGNIZED INCLUSION TYPE
Although inclusions are scarce in CLIPPIR diamonds, with the right approach they can be sought
and studied. By leveraging a high-volume diamond
grading service like that of GIA to find prospective
samples for examination, it was possible to cull out
enough of the rare inclusion-bearing CLIPPIR diamonds to look at their features as a group. In the past,
the only reported inclusions in CLIPPIR diamonds
were flat black inclusions with round boundaries,
previously assumed to be flecks of graphite (Bowenet al., 2009; Moore, 2014). Rarely, some inclusions
have been suspected to be sulfides, as they can resemble common pyrrhotite and pentlandite inclusions in lithospheric diamonds (Bowen, pers. comm.,

In Brief• Unusually large and pure diamonds like the 3,106 ct
Cullinan have a special geological origin, much deeper
in the earth’s mantle than most other gem diamonds.
• Many of the world’s biggest high-clarity, D-color diamonds belong to this special variety known as CLIPPIR
diamonds.
• Newly recognized iron-rich metallic inclusions in these
diamonds provide insight into the makeup of the deep
earth.
• There is still much to be learned from the study of diamonds, which are an invaluable “window” into the unseen world deep beneath our feet.

WINTER 2017391

C-S metallic inclusions often have step-faced surfaces,
strong magnetism, and elongate and irregular shapes,
grouping into chains, and routinely possess a coexisting fluid phase (again, see figure 3).
Specifically, these inclusions are accompanied bya thin, invisible fluid layer of methane (CH4) andlesser hydrogen (H2) trapped at the interface between
the solid inclusion and surrounding diamond (figure5). Detailed analysis of the inclusions reveals thatthey are made up of cohenite ((Fe, Ni)3C), interstitial
Fe-Ni alloy, segregations of Fe-sulfide, and minor occasional Fe-Cr oxide, Fe-oxide (likely wüstite), and Fephosphate (Smith et al., 2016). We interpret the minoroxides as evidence of small amounts of oxygen evenin these relatively oxygen-poor conditions, because
all the inclusions examined were carefully selectedfrom unaltered diamonds that had no surface-reaching fractures. This assemblage of minerals is inter-

preted to have crystallized during transport to the surface from the molten metallic liquid that was trapped
during diamond growth at depth. The metal contains
very limited amounts of oxygen, which indicates that
CLIPPIR diamonds form under reducing conditionsin the mantle. Metallic Fe-Ni-C-S melt inclusions
such as these have not been described previously, although rare, isolated occurrences of native iron, ironnickel alloy, and iron carbide inclusions have beenfound in other diamonds (Sharp, 1966; Jacob et al.,
2004; Kaminsky and Wirth, 2011; Mikhail et al.,2014). Coincidentally, the German mineralogist Emil
Cohen, for whom cohenite is named, might have encountered one of these Fe-Ni-C-S melt inclusions inan 80 ct rough diamond in South Africa 140 years ago,
though he described the inclusion as “specular iron”
or hematite based on color, luster, and form (“Iron indiamonds,” 1877).

Figure 3. Examples of the typical appearance of metallic Fe-Ni-C-S inclusions in CLIPPIR diamonds. A: Two parallel <111> oriented chains (field of view 2.34 mm). B: Backlit inclusion chain (field of view 1.42 mm). C: Elongate
metallic inclusion with a round black fracture of {111} orientation that obliquely cuts the <111> elongation direction of the inclusion (field of view 1.42 mm). D: A less elongate metallic inclusion with a small irregular tail on
the lower left (field of view 1.42 mm). Photos by Evan M. Smith.

A

B

C

D

392

EXCEPTIONAL CULLINAN-LIKE DIAMONDSGEMS & GEMOLOGYWINTER 2017

TABLE 1. Summary of inclusions observed in 83 CLIPPIR diamonds.
No. ofdiamonds

Largest in group

Inclusion assemblage

Inclusion properties

Mineralogy

60

30.13 ct

Metallic Fe-Ni-C-S (inferred to
be primary melt inclusions)

Magnetic, silver/black color, opaque,grouped in <111> chains, CH4 fluid
jacket (22 also had detectable H2 in
Raman); associated with healed
cracks; alters to red-brown (hematite).

Cohenite (Fe,Ni)3C + Fe-Ni
alloy + pyrrhotite + minor
phases15

32.88 ct

Calcium silicate phases withoriginal perovskite structure
inferred (CaPv)

Colorless, transparent, often withdroplet-like satellite inclusions
defining lobes of localized healed
cracks; one diamond had inclusions
with CH4 + H2 fluid jackets but no
indication of metal.

CaSiO3-walstromite ± larnite(β-Ca2SiO4) ± CaSi2O5-titanite ±wollastonite ± perovskite(CaTiO3)

4

10.21 ct

CaPv + metal

Colorless, transparent, with opaque,magnetic, metallic-luster droplet-like
satellite inclusions with detectable
CH4; note that the silicate and metal
co-occur rather than appearing as
discrete separate inclusions.

CaSiO3-walstromite ± larnite(β-Ca2SiO4) ± CaSi2O5-titanite +Fe-Ni-C-S phases

2

10.21 ct

Majoritic garnet + metal

Pale orange, transparent, with opaque,magnetic metallic phase partly
occupying inclusion space, with
detectable CH4 ; note that the garnet andmetal co-occur rather than appearing as
discrete separate inclusions.

Low-Cr majoritic garnet ± FeNi-C-S phases ± retrograde
clinopyroxene, plagioclase,
olivine1

4.41 ct

CaPv + majoritic garnet + metal

Pale orange, transparent garnet
inclusions with distinct metallic
opaque regions exhibiting strong
magnetism, with detectable CH4; onecolorless, transparent calcium silicate.

Majoritic garnet with metallic
component + CaSiO3walstromite

1

10.17 ct

CaPv + majoritic garnet

Two colorless transparent calcium
silicate inclusions; one pale orange,transparent garnet inclusion.

CaSiO3-walstromite + majoritic
garnetCombines results from 53 diamonds in Smith et al. (2016) and 30 additional diamonds (see supplement at www.gia.edu/gems-gemology/winter-2017-worldsbiggest-diamonds). Average diamond mass is 6.1 ct, with 13 of them >10 ct. Note that most recorded masses are for the cut diamond and original mass of rough is
unknown (also see data S1 in Smith et al., 2016).

The metallic inclusions (figures 3 and 5) have asilvery metallic luster with variable coating of patchy
black graphite (as identified with Raman spectroscopy—a technique that can help identify minerals using a focused laser). They are bound by
sharp-edged, flat cuboctahedral faces as well as complex stepped faces, and are often elongate in one dimension corresponding to a <111> vector withrespect to the host diamond crystal. Inclusions near
the surface of the diamond are noticeably magnetic
when brought near a small rare-earth magnet suspended from a thread. Magnetism can be helpful forrecognizing these inclusions but is not definitivesince some other kinds of inclusions can be magnetic, such as sulfides or the metallic flux trapped in
HPHT (high-pressure, high-temperature) synthetic
diamonds. Raman spectroscopy, while of limited usefulness in identifying these inclusions since they are

dominated by metals and carbides (both of which areinactive or weakly active in Raman), is good at revealing sulfides, which yield distinctive Ramanpeaks. We therefore take the general lack of Raman
response from the inclusions with metallic luster as
additional evidence that they are multi-phase metallic assemblages in which iron sulfide is a partial component rather than wholly sulfide mineralinclusions, which would have a distinctive Raman
response.
Taken together, the metallic inclusions’ consistent shape, texture, grouping, magnetism, lack ofclear Raman response, and other features justify extending the microprobe and XRD results (Smith etal., 2016) to the majority of inclusions in the faceted
samples that have been studied. It is unlikely thatsome of the metallic inclusions were misidentified
sulfide inclusions. Furthermore, it is important to

EXCEPTIONAL CULLINAN-LIKE DIAMONDSGEMS & GEMOLOGYWINTER 2017393

R AMAN SPECTRA
larnite (ȕ-Ca2SiO4)
CaSiO3-walstromite

660

32.88 ct

859
890

981

1042

1112
1086 1178

845

125
150

A

CaSi2O5-titanite

610

5 mm

RELATIVE INTENSITY

100 μmB
669

10.21 ct

999

CaSiO3-walstromite
128
1059
153

966
100 μm

C

921

5 mm

10.17 ct

Majoritic garnet
860

360
510

564

1051

644

100 μm
200

400

600

800

1000

5 mm

1200

RAMAN SHIFT (cm–1)

Figure 4. Raman spectra representative of the silicates in CLIPPIR diamonds. Spectra are offset vertically to avoid
overlap. A: The principal silicate phase CaSiO3-walstromite is accompanied by Ca2SiO4 and CaSi2O5 in a small
colorless inclusion with a large graphitic fracture in a 32.88 ct D-color type IIa diamond. The middle spectrum
shows CaSiO3-walstromite alone, with peak positions noticeably shifted due to high remnant pressure inside theinclusion, on the order of several gigapascals (Anzolini et al., 2017). B: This CaSiO3-walstromite inclusion in a
10.21 ct light pinkish brown type IIa diamond is a good example of the main part of the inclusion being accompanied by many smaller satellite inclusions, presumably from inclusion rupture/expansion during inversion from
CaPv. C: Majoritic garnet in the bottom spectrum was observed in a 10.17 ct D-color type IIa diamond. Note this10.17 ct sample contained both majorite and CaSiO3-walstromite, and the bottom two spectra were actually bothcollected from this diamond. Photomicrographs by Evan M. Smith, face-up diamond photos by Jian Xin (Jae) Liao.

note that sulfide (pyrrhotite) appears to have been exsolved from the Fe-Ni-C-S mixture, suggesting thatthe original metallic liquid would have dissolved any
sulfide minerals in this geologic environment, making it unlikely that discrete sulfide mineral grains
would be available to be trapped as inclusions.
The fluid jacket with hydrogen and methane surrounding these inclusions is detectable with Raman
spectroscopy, although the fluid is not uniformly distributed over the inclusion interface (figure 5).
Methane (CH4) is most easily detected and has a peakat 2918 cm–1, while hydrogen (H2) is generally lessabundant but has its main peak at 4156 cm–1 (Smithet al., 2016). Peaks may be shifted by a few wavenumbers due to residual pressure inside the inclusions(Frezzotti et al., 2012). Methane is present at the inclusion-diamond interface in all metallic and mostsilicate inclusion assemblages in every CLIPPIR diamond and is often accompanied by hydrogen. The
fluid is not thought to have existed as a free fluid
phase while the diamond grew. Rather, it is interpreted to have formed as a result of hydrogen atoms
exsolving out of the metallic liquid, and perhaps tosome extent the silicates as well, forming CH4 and H2
by reacting with the surrounding diamond (Smith andWang, 2016).

394

GEMS & GEMOLOGYEXCEPTIONAL CULLINAN-LIKE DIAMONDSSILICATE INCLUSIONS IN CLIPPIR DIAMONDS:EVIDENCE FOR DEEP ORIGIN
The second most abundant inclusion is a mix of calcium silicates interpreted as retrogressed CaSiO3-perovskite (CaPv), a perovskite-structured high-pressuremineral stable at depths beyond about 360 km (seebox A and Brenker et al., 2005). The most common

WINTER 2017

BOX A: POST-ENTRAPMENT RECRYSTALLIZATION OF INCLUSIONS
Diamond excels at preserving trapped inclusions byvirtue of its physical strength and chemical inertness.
Even if they are protected and isolated, however, someinclusions may nevertheless recrystallize in response to
the ambient temperature and pressure, especially whena diamond is transported to surface. This is true of themetallic Fe-Ni-C-S inclusions in CLIPPIR diamonds,
which are thought to be trapped as a homogenous melt
but later unmix into multiple phases (figure A-1). Textures within these metallic inclusions are essentially igneous, reflective of solidification and crystallization
processes.
In addition to the crystallization of trapped melt inclusions, solid mineral phases can also undergo postentrapment changes. Some minerals are only stable deepin the earth, at elevated pressure and temperature, andbecome unstable under conditions at the earth’s surface.
This is an important concept for inclusions in diamondbecause it explains how mineral inclusions can change
after being trapped in a diamond. A mineral inclusion
trapped very deep in the earth can undergo retrogression
or inversion on its way to surface, meaning the inclusions we observe might be the breakdown products ofsome original high-pressure phase. When minerals break
down, they are said to have undergone retrogression if
the breakdown results in multiple phases, or inversion
if the result is a single mineral phase. For example, thehigh-pressure mineral CaSiO3-perovskite could undergo
retrogression to produce two complementary minerals,
larnite (β -Ca2SiO4) and CaSi2O5-titanite, or inversion to
produce a single mineral, CaSiO3-walstromite. In eithercase the bulk composition remains unchanged. Often,

high-pressure phases are known mainly from experimental work and are referred to by their crystal structure. For
example, the mineral perovskite (CaTiO3) has a closely
packed cubic crystal structure that is emulated by calcium silicate at high pressures, and this phase is called“perovskite” (broadly) or CaSiO3-perovskite.

breakdown product of CaPv is CaSiO3-walstromite(figure 4). Other calcium silicate phases encounteredare larnite (β-Ca2SiO4), CaSi2O5-titanite, and, lesscommonly, wollastonite (CaSiO3). All four phases are
sometimes found together (metastably) in a single inclusion. If the inclusion contains sufficient titanium,
it can be present as perovskite (CaTiO3). Although it
is still unclear, there may be two possible CaSi2O5
polymorphs in retrogressed high-pressure calcium
silicate inclusions, corresponding to monoclinic andtriclinic structures that have been observed in highpressure experimental products (e.g., Kubo et al.,1997). Two different sets of Raman features havebeen interpreted as CaSi2O5-titanite. One signatureencountered, as in figure 4, resembles monoclinic titanite (CaTiSiO5) with its prominent 890 cm–1 band(see also database S3 in Smith et al., 2016). Another

Raman signature corresponding to a synthetic
CaSi2O5 sample (figure 2 in Nasdala et al., 2003) has
also been observed in CaPv inclusions (Nasdala et al.,
2003; Anzolini et al., 2016; Smith et al., 2016).
The CaPv inclusions are colorless and transparent, with surface textures ranging from flat-faced,sharp cuboctahedra to more ragged-looking irregularsurfaces, especially for larger inclusions. Some inclusions have small opaque portions that are magnetic,
interpreted as traces of metallic liquid trapped alongwith the silicate inclusion. Many CaPv inclusions
possess small droplet-like satellite inclusions, all of
the same mineralogy, grouped in one or more tabular
lobes that appear to have been introduced from themain inclusion outward along localized cracks (figure
4B). The satellite inclusions are interpreted as the
byproduct of pressure buildup sufficient to rupture

EXCEPTIONAL CULLINAN-LIKE DIAMONDSGEMS & GEMOLOGY

Fe-sulÿde (pyrrhotite)
Fe-Ni metal alloy

cohenite (Fe-carbide)

Fe
Ni
S

50 µm

Figure A-1. An exposed polished metallic Fe-Ni-C-S inclusion. This inclusion was originally a homogeneous
molten metal, but as it cooled and solidified the mixture crystallized into three separate phases: cohenite, FeNi alloy, and Fe-sulfide. It is shown here as anenergy-dispersive X-ray elemental map from an electron
microscope, with the three phases colored according towhere iron, nickel, and sulfur are more strongly concentrated. Fe (red) is concentrated in cohenite, Ni (blue) in
Fe-Ni alloy, and S (yellow) in Fe-sulfide. The host diamond is a 2.14 ct offcut from a larger type IIa diamondfrom the Letšeng mine, Lesotho (sample Letseng_889, inclusion F). For methods, see Smith et al. (2016).

WINTER 2017395

16.36 ct

9.56
6 ct

30.13 ct

200 ȝm

5 mm

5 mm

100 ȝmINTENSITY (ARB. UNITS))

30,000DDD

588, H2

15,000200 ȝm

2916, CH4

D
25,00020,0005 mm
m4155, H2x10

10,000
5000
0
0

500

1000

1500

2000

2500

3000

35
500

4000

4500

–1

RAMAN SHIFT (cm )Figure 5. Metallic Fe-Ni-C-S inclusions with a representative Raman spectrum. All three CLIPPIR diamonds
shown are type IIa. The faceted diamonds (left and center) are D-color and Fancy yellow-brown, respectively. The
rightmost sample is a more heavily included offcut trimmed from an even larger, potentially D-color diamond.
The Raman spectrum shows the typical lack of diagnostic mineral peaks (within the 100–1200 cm–1 range) coupledwith methane (CH4 ) and molecular hydrogen (H2 ). Diamond Raman features are marked “D.” Face-up faceted diamond photos (16.36 ct and 30.13 ct) by Jian Xin (Jae) Liao; other photos by Evan M. Smith.

the surrounding diamond, followed by injection ofinclusion material out into the crack. A large volume
increase of about 30% is estimated to accompany the
inversion from CaPv to CaSiO3-walstromite (Anzolini et al., 2016). The cracks have healed, and the
diamond between individual satellite inclusions is
optically continuous. Although this initial crack ishealed, the main inclusion and sometimes even thesatellite inclusions are each surrounded by agraphitic black fracture (figure 4B), an indication offurther inclusion expansion during exhumation fromthe mantle by the erupting kimberlite. As a diamondis transported to the surface, the confining pressure
acting on it gradually decreases and there is less pressure pushing in on its inclusions. In this situation,some inclusions push out against the diamond host,potentially causing a buildup of elastic strain andbrittle rupturing around the inclusions. It is also possible that the surrounding diamond can deform plastically, thereby reducing the tendency for brittle
failure. Such inclusions demonstrate the complex
histories of brittle, elastic, and plastic deformation

the host diamond can endure in response to inclusion pressure during ascent to surface.
An additional inclusion phase found was lowchrome majoritic garnet, a high-pressure form of garnet containing excess silica (figure 4C). The garnets
are a pale orange color, again surrounded by large
graphitic fractures, and sometimes have a coexisting
magnetic, metallic component. The garnet provides
a convenient maximum depth of derivation, since it
is not stable deeper than about 750 km (Wijbrans etal., 2016). Silicate inclusion phases therefore bracket
the depth between 360 and 750 km, overlapping theregion of the mantle known as the mantle transition
zone, which marks the transition between the upper
mantle and lower mantle.
Examples of former CaSiO3-perovskite (CaPv) andmajoritic garnet inclusions coexisting with a metallic component and CH4±H2 demonstrate a close association between the metal-only inclusions and thesilicate-dominated inclusions and confirm that they
are derived from a common, very oxygen-poor settingin the mantle. The best-studied example, since it

396

GEMS & GEMOLOGYEXCEPTIONAL CULLINAN-LIKE DIAMONDSWINTER 2017

could be polished and examined in cross section withelectron microscopy, is in one of the large diamond
offcut samples from the Letšeng mine in Lesotho. In
it, two majoritic garnets were observed, each trappedwith a Fe-Ni-C-S metallic component alongside thegarnet (fig S4 in Smith et al., 2016). The metallic accompaniments were a mixture of iron-carbide, Fe-Ni
alloy, and Fe-sulfide, similar to the metallic inclusions seen in other diamonds containing no silicates.
Again, CH4 was present.
The silicate inclusions found in CLIPPIR diamonds are comparable to inclusions that have been
described before in other sublithospheric diamonds(see summaries by Stachel et al., 2005; Kaminsky,
2012; Harte and Hudson, 2013). Here, these silicate
inclusions are consistent with a mafic (eclogitic) hostrock affiliation in the sublithospheric mantle. In the
CLIPPIR diamond suite, however, the inclusion assemblages are somewhat different because they areassociated with metal and the reduced volatiles
methane and hydrogen.

A

B

z

<1

11

>
Inc

lus

ion

s

y
x

5 mm

C

D
Subsurface inclusion

>

<111

CRYSTALLOGRAPHIC ORIENTATION OF
METALLIC INCLUSIONS IN CLIPPIR DIAMONDS

0.5 mmIn addition to their surprising metallic nature, themost common inclusions in CLIPPIR diamonds also
have a spatial distribution that is striking. The metallic inclusions are often spatially grouped in one ormore chains, all parallel to a single <111> vector (figure 6). This is a line oriented perpendicular to an octahedral {111} plane. Individual chains can waver
slightly, with jogs and discontinuities, but can reachseveral millimeters in length. When multiple metallic inclusion chains are present, they are often paralleland define a pervasive linear fabric with no tendency
to radiate from the center or otherwise align to any
obvious diamond growth pattern. Instead the chains
define a lineation within the diamond. The uniform,
parallel, and crystallographically controlled orientation of the chains suggests they are like an overprinted fabric established after diamond growth.
Cathodoluminescence or UV luminescence imaging
sometimes reveals bright curviplanar features where
the linear extension of an internal chain pierces thediamond surface. Figure 6 shows an example of thesefeatures extending radially from the point where an
inclusion chain vector meets the polished surface.
These otherwise invisible features are interpreted ashealed cracks with roughly {110} orientation, of
which there are three potential {110} plane orientations that intersect along the chain-defining <111>

Figure 6. Metallic inclusions in CLIPPIR diamonds often
occur in linear chains. A: Long, discontinuous inclusion
chain in a 2.05 ct D-color type IIa diamond. B: Sketch
showing a volume of diamond with multiple parallel<111> inclusion chains. Healed cracks of imperfect {110}orientation (blue) can be seen on the surface, at points
where chain-defining vectors exit the diamond. C: DiamondView UV luminescence image revealing healed
cracks related to a subsurface metallic inclusion in a
15.25 ct D-color type IIa diamond. Also visible are weblike dislocation networks. D: The same area shown in visible light, with blue traces drawn over the invisible healed
cracks and the <111> vector extending from the inclusion,
circled where the vector pierces the diamond surface. E:
DiamondView image showing a <111> inclusion chain
just below the surface, within the plane of the page, andtwo related healed cracks of {110} orientation running parallel to the inclusions. Two sets of finely spaced {111} slip
traces are also seen, overprinting and cross-cutting the<111> vector and healed cracks. Photos by Evan M. Smith.

EXCEPTIONAL CULLINAN-LIKE DIAMONDSGEMS & GEMOLOGYE

Healed cracks
parallel to chain<111> Inclusion chain

0.5 mmWINTER 2017397

BOX B: DIAMOND DEFORMATION AS A WAY TO MAKE <111> CHAINS
Metallic inclusions in CLIPPIR diamonds frequently
occur as chains of a few inclusions, aligned in a <111>
crystallographic orientation (figures 3A, 3B, and 6). These
chains are challenging to explain, but here we consider
that they may be a consequence of diamond deformationin the mantle. At the elevated temperatures in theearth’s mantle, diamond can deform plastically, though
it can still develop cracks under high stress. If the 3-Dstress is non-uniform and causes plastic deformation
within the diamond, there will be a concentration ofstress at the liquid metallic inclusions, where dislocation
movement is interrupted and the inclusion itself is astrengthless fluid. Under these conditions, cracks mightdevelop locally at the inclusion site. Given that thegeometry of a <111> vector can be produced by the intersection of two or three {110} planes, the next step isto consider whether or not the diamond could crack on
a {110} plane in a systematic way. In fact, brittle failureon {110} planes has been experimentally produced in diamond at elevated temperatures, concurrent with plastic
deformation on {111} planes (Brookes et al., 1999). In thisdeformation experiment, the {110} cracks were parallelto the compressional axis.
Bulk plastic deformation of the diamond may have
led to stress concentration and brittle failure localized at
the metallic melt inclusions. The single <111> vector expressed by the chains could correspond to the directionof maximum compressive stress, resolved to the diamond lattice. At the inclusion site, {110} cracks can develop parallel to the compressional axis, like thoseobserved by Brookes et al. in their deformation experiment. It would be possible to develop two or three symmetrically equivalent {110} cracks, such that theintersection of the cracks defines a short-lived dilatant<111> channel along which metallic melt inclusions canredistribute. In other words, the chains may be a redistribution of primary liquid metallic Fe-Ni-C-S inclusions, along a strain-controlled <111> direction, during
brittle-plastic deformation of the diamond in the mantle.
Redistribution of melt inclusions provides immediate
local stress relief conducive to crack closure and healing.
In two diamonds, the metallic inclusions have been observed as a planar group arranged in a {110} orientation,interpreted as a rare case of metallic liquid redistributed
into a single planar {110} crack rather than the linear intersection of two or more cracks. Compared to the solid
silicate inclusions, the metallic inclusions may be prone
to this deformation-induced redistribution simply because they were in a liquid state. Inclusions containing
both Fe-Ni-C-S metal and silicate trapped together do
not exhibit the linear spatial distribution, possibly because the solid silicate portion strengthened the inclusion. It is very unlikely that the metallic liquid could
have originated outside the diamond and penetrated into
fine, parallel <111> channels. The graphitic {111} fractures currently visible around the Fe-Ni-C-S inclusions
postdate this deformation.
The interpreted scenario of melt inclusion redistribution suggests these diamonds were deformed deep
enough that the metallic inclusions were still liquid andhad not unmixed. Development of the {110} healed
cracks in the diamond likely predates the development
of dislocation networks because dislocation networks
appear to conform to the cracks rather than being crosscut by them. These two observations suggest that the development of <111> chains may have occurred in a
discrete deformation episode at depth in the sublithospheric mantle. This deformation could be related tomantle convection and deep plate tectonic processes, orsome other physical mechanism.

vector (see box B). The observations suggest that primary metallic melt inclusions were rearranged into
chains during deformation of the host diamond at
depth in the mantle. The significance is difficult tointerpret without more detailed knowledge of the deformation behavior of diamond in the mantle, but
likely speculation would relate diamond deformation
to the pervasive deformation of mantle host rockthat accompanies mantle convection.

Much of our knowledge of the earth comes fromstudying rocks near its surface, yet the deeper interiorremains wholly inaccessible. While the continuous

action of plate tectonics and the magmatism causedby it allows geologists to relate plate deformation andvolcanism to Earth’s deeper interior, magmas react,
crystallize, and degas on their way to the surface,which changes their composition. The inevitably
modified magmas make it difficult to discern the true
nature of solid deeper mantle except by inference. It
is well known that the increase in temperature andpressure with depth in the earth causes some minerals
to break down and new minerals to form that are only
stable at the more extreme pressure and temperature
conditions. When considering the deep interior of theearth, this effect is fundamental. High-pressure experiments are used by scientists in the field of mineral

398

GEMS & GEMOLOGYMETAL IN THE MANTLE: WHAT DOES IT MEAN?

EXCEPTIONAL CULLINAN-LIKE DIAMONDSWINTER 2017

LithosphereDiaamond-bearing kimberlite eruption

osphereContinental lith
-

Cor
e

CLIPPIRdiamondg
pwellingUptlemantle

-

-

-

-

-

- -

here
osp
lith
- /ic - - - - ~-2-50- k-m-

O
ce
an

Mant
le

Amountof metal(wt.%)
0.5

1

Oxygenfugacity
(∆IW)
0 +2

/

/

660 kmMetal
saturation
depth

Metal

Figure 7. Cross section of the uppermost 1,000 km of the earth (center) illustrating the origin of CLIPPIR diamonds, shown as large blue diamond symbols. Smaller black diamond symbols indicate other varieties of diamond formed mainly in the continental lithosphere. Oceanic lithosphere subducted into the mantle provides therocks necessary to explain the CaPv and majoritic garnet inclusions. CLIPPIR diamonds are thought to grow fromliquid metal. Following growth, they may be carried upward with thermally or chemically buoyant upwelling
mantle material and entrained in a kimberlite eruption to the surface. Profiles at the right show how the amountof metal in the mantle is expected to increase with depth starting from 250 km down, reaching up to approximately 1 wt.% below 660 km and buffering oxygen fugacity to reducing conditions (ΔIW, log units relative to theiron-wüstite buffer). Profiles after Rohrbach and Schmidt (2011).

physics to predict what kinds of mineral assemblages
will occur even within the lower mantle. One important prediction to come out of these considerations ofmineral physics is the idea that there is a depth,
around 250 km, below which metallic iron might become one of the stable phases within deep mantle
rocks (Frost et al., 2004). This prediction for metallic
iron is fundamental to understanding the basic properties of the mantle. We know that at the bottom of
the mantle it is in direct contact with the iron-nickel
metal of the core, whereas at the top of the mantle,
iron metal is not even stable during mantle melting.
What happens, mineralogically speaking, in between?
The mechanism thought to be responsible is the
progressive increase in the capacity for silicate minerals to host Fe3+ (iron with a 3+ oxidation state) relative
to Fe2+ at high pressures (Ballhaus, 1995; Frost et al.,2004). The preference for Fe3+ in certain minerals
under extreme pressure induces a disproportionation
reaction, whereby valence electrons in the existing
Fe2+ atoms are redistributed according to the reaction
3Fe2+ → 2Fe3+ + Fe0. The Fe3+ is partitioned into the silicates, but the Fe0 separates into its own metallic
phase. Iron disproportionation is thought to generate
up to about 1 wt.% metal in the lower mantle (Frostet al., 2004; Rohrbach et al., 2007). Nickel, carbon, sul-fur, and other elements are expected to dissolve into
this metallic phase. This prediction is based on experiments (Bell et al., 1976; Ballhaus, 1995; Frost et al.,
2004; Rohrbach et al., 2007; Rohrbach and Schmidt,
2011) but has never before been verified by actual observation of mineral grains.
The Fe-Ni-C-S melt inclusions in CLIPPIR diamonds are interpreted to be samples of such a metallic liquid, thereby confirming the fundamental
process of Fe2+ disproportionation at depth (Smith etal., 2016) and providing evidence for a metallic liquidmechanism for diamond formation. This is the reason why CLIPPIR diamonds are so significant forearth science. The metal has large-scale implications
for the behavior and evolution of the mantle over geologic time because it will influence the storage andcycling of the many elements that dissolve into it,such as oxygen, carbon, nitrogen, and hydrogen. The
tendency for metallic iron to react with oxygen is expected to regulate or buffer the oxygen fugacity (thechemical availability of oxygen in the mantle) andmaintain reducing conditions in the ambient mantle
below about 250 km depth (figure 7). The projected
low oxygen fugacity below 250 km, shown in figure
7, is a direct consequence of Fe2+ disproportionation(Rohrbach and Schmidt, 2011).

EXCEPTIONAL CULLINAN-LIKE DIAMONDSGEMS & GEMOLOGYWINTER 2017

399

GEOLOGICAL ORIGIN OF CLIPPIR DIAMONDS
The consistent association of the mineral assemblages found in CLIPPIR diamonds with metal andmethane, and its limitation to the CLIPPIR category,indicate that these diamonds form in a unique way.
The CaPv and majoritic garnet inclusions, as well as
the light carbon isotopes of the diamonds themselves, imply an association with subducted eclogite
at extreme depths, likely within 360–750 km in theconvecting mantle (Smith et al., 2016). This is much
deeper than other gem diamonds, nearly all of whichcontain much more nitrogen (type I) and form in thelowermost parts of thick, old continental lithosphereat depths of 150–200 km. This extreme depth is,
however, within the range of other rare “superdeep”
diamonds in the sublithospheric mantle (figure 7).
The source of the carbon that crystallized into diamond is also important. Carbon has two stable isotopes, 13C and 12C, whose ratio varies according to thesource, such as ambient mantle carbon or subducted,
organic carbon. Variably light carbon isotopes in
CLIPPIR diamonds, having 13C/12C ratios lower thanthe convecting mantle (Smith et al., 2016), suggest
derivation from organic carbon (e.g., Kirkley et al.,
1991; McCandless and Gurney, 1997), which implies
the carbon was originally sourced at Earth’s surface
and then subducted.
CLIPPIR diamonds, as described above, appear togrow in close association with metallic liquid in the
sublithospheric mantle. In this deep environment, themetallic Fe-Ni-C-S melt inclusions are hypothesized
to represent small retained traces of the actual growth
medium (Smith et al., 2016). Experiments and routine
HPHT synthetic diamond growth processes show thatiron-nickel mixtures are effective media at elevatedtemperatures and pressures, similar to the conditionsin the deeper mantle. Diamond growth from metallic
liquid could help explain the large size of CLIPPIR diamonds and their near-complete lack of nitrogen. Interestingly, the CH4 ± H2 fluid jackets surrounding
inclusions in natural CLIPPIR diamonds are a feature
also seen in metal/carbide inclusions in HPHT synthetic diamonds (Smith and Wang, 2016), further supporting their interpretation as primary melt
inclusions. Natural CLIPPIR diamonds may havecrystallized inside small pockets of molten metallic
liquid in Earth’s deep mantle (figure 7). Small dropletsof this metallic liquid would have occasionally been
trapped within the diamonds as they grew, preserving
the inclusion assemblages that we now sample.
As shown in figure 7, the eclogitic host rocks responsible for the silicate inclusions in CLIPPIR dia-monds likely represent a basaltic crustal layer of theoceanic lithosphere that was subducted into thedeeper underlying mantle. Below a depth of approximately 250 km, iron disproportionation can lead tometal saturation in mantle rocks. Both eclogite andthe surrounding peridotite, including the mantle portion of the oceanic lithosphere, have the potential togenerate metal by disproportionation. The source ofthe metal can be constrained by its bulk composition,roughly estimated as Fe0.61–0.75Ni0.10–0.13C0.15–0.20S0.05–0.12
based on the measured composition and relative
cross-sectional area of cohenite, Fe-Ni alloy, and Fesulfide exposed in four polished inclusions (Smith etal., 2016). The modest Ni/(Ni+Fe) ratio (<0.2) is lower
than the 0.3–0.5 range proposed for metal compositions precipitating in upper mantle peridotites(Rohrbach et al., 2014). Combined with the notionthat the metallic liquid should be nearly immobile,
tending to become trapped as disconnected dropletsin the intergranular spaces of silicate mantle rocks(Rohrbach and Schmidt, 2011; Rohrbach et al., 2014),
it is tempting to infer that the metal was generated
locally, within the eclogitic host rocks where the diamonds grew. However, the observed Ni/S andNi/(Ni+Fe) ratios in the inclusions are significantly
higher than expected for equilibrium with eclogite(e.g., Ni/S=0.3 and Ni/(Ni+Fe)=0.02 reported for a
metallic inclusion found in eclogitic garnet [Jacob etal., 2004]). To explain this discrepancy, the metal
could have formed in equilibrium with peridotite(e.g., Zhang et al., 2016) and subsequently migrated
into the eclogite before diamond crystallization occurred, but this scenario would imply significant
metal precipitation, segregation, and liquid metal mobility in order for large diamonds to ultimately crystallize. If metal in the mantle is typically distributed
along grain boundaries in silicates at low abundancesof around 1%, then segregation and mobilization of
the liquid metal may rely on deformation of surrounding rocks. However, other rock types or localized irregular rock compositions within the proposed
growth setting of subducted oceanic lithosphere
could also play a role in the metal and diamond formation. Further inclusion analyses and experimental
constraints are needed to clarify the exact provenanceof the metal.
Metallic iron has already been suggested to play arole in the formation of other types of superdeep diamonds, by its proposed ability to reduce subductedcarbonate species (CO32–) to diamond (C), also called“redox freezing” (Rohrbach and Schmidt, 2011). It is
not clear how or even if carbonate would be involved

400

GEMS & GEMOLOGYEXCEPTIONAL CULLINAN-LIKE DIAMONDSWINTER 2017

in CLIPPIR diamond growth since an oxygen-deficient
mantle region is a prerequisite for metallic inclusions.
But the expected high calcium content of carbonate
might be one explanation for the abundance of CaPv
inclusions as a reaction byproduct (Walter et al., 2008;
Bulanova et al., 2010; Harte and Richardson, 2012;
Thomson et al., 2016). Alternatively, other forms of
subducted carbon such as abiogenic, organic carbon
could be involved where carbonate does not play a significant role. The main point here is that metallic FeNi-C-S inclusions clearly show that the metallic
iron-rich phase plays the key role for CLIPPIR superdeep diamond formation.
Once formed, CLIPPIR diamonds, like other sublithospheric diamonds, require some mechanism totransport them upward to be sampled by kimberlite
magmatism and brought to surface where they aremined. The mechanism is uncertain but may involve
thermochemically buoyant, ascending solid mantle
that is part of the general upwelling associated withmantle convection (figure 7) prior to being entrainedin erupting kimberlite. The dynamic action of the
mantle may be cryptically recorded in the crystal
strain in CLIPPIR diamonds. There is a remarkably
strong association between CLIPPIR diamonds anddislocation networks (De Corte et al., 2006) that canbe better understood in light of the present study. This
web-like feature, rare in other diamonds, is visible incathodoluminescence or UV fluorescence imaging inalmost all 83 CLIPPIR samples examined (e.g., figure
6C). The network is interpreted as a dislocation structure arising from plastic deformation followed by recovery (Hanley et al., 1977). The high temperatures
afforded by a sublithospheric origin as well as the minimal nitrogen contents in CLIPPIR diamonds are conducive to the development of dislocation networks.
These diamonds have witnessed severe conditions in
the deep mantle—a likely consequence of mantle convection. Earth’s mantle convects in the solid form, introducing strain on the scale of the mineral lattice at
extremely high pressure. Despite their often exquisite
gem quality, these diamonds have an internal structure that preserves evidence of this strain and theirjourney from the deep mantle to the surface.

It has long been recognized that a curiously high proportion of large, gem-quality diamonds are type IIa,an otherwise rare designation. But now we havelearned that these special gems, termed CLIPPIR diamonds, actually form in a unique way, in a different
part of the mantle compared to other kinds of dia-monds. Many inclusion-free, nitrogen-deficient diamonds, especially those with dislocation networks,
are expected to belong to the CLIPPIR category (figure 2). The “superdeep” (i.e., sublithospheric) originof CLIPPIR diamonds, which leaves them free of inclusions associated with shallower, lithosphericmantle diamond host rocks, helps explain why exploration geologists and miners have never been able
to correlate these diamonds with traditional indicator minerals (Bowen et al., 2009; Gurney and Helmstaedt, 2012). The rarity and extraordinary value thathave made research study prohibitive over the years
also contributes to this fact. Indicator minerals suchas chrome-rich pyrope and ilmenite, the fragmentsof diamond host rocks from the lithospheric mantle
keels of Archean continents, are useful for finding
lithospheric diamonds. CLIPPIR diamonds, however,originate deeper in the mantle, far below the lithosphere where these indicator minerals are not stable
and cannot be included. Finding CLIPPIR diamonds
is all the more difficult. Predicting their ore grade,even in promising mines, requires large bulk sampling and carries great uncertainty.
Large and pure diamonds are nevertheless ahighly sought prize. Prior to the study by Smith et al.
(2016), there was no accepted model describing how
these large type IIa diamonds form in nature. We can
now appreciate that these exceptional diamonds arecoincidentally some of the most scientifically valuable samples of our planet’s interior. They are a window into the deep convecting mantle, providing a
glimpse through the shroud of Earth’s tectonic plates
and surface magmatism.
Previous experiments and theory have predicted
for many years that parts of the deep mantle below
about 250 km contain small amounts of metallic
iron and have limited available oxygen. Now, themetallic inclusions and their surrounding methaneand hydrogen jackets in CLIPPIR diamonds provide
physical evidence to support this prediction. This isa key observation for our understanding of planet
Earth, having broad implications for its geologic evolution through time, such as the behavior and cyclingof carbon and hydrogen, which are the primary fluxes
for rock melting and the generation of new crust.
Confirming the existence of metallic iron containing
nickel, carbon, sulfur, and other elements as a discrete liquid also impacts our understanding of theseismic velocity and thermal and electrical conductivity of the mantle, as well as the way it deforms
and flows. This result is a sign that there is still muchto be learned from diamonds and their inclusions.

EXCEPTIONAL CULLINAN-LIKE DIAMONDSGEMS & GEMOLOGYCONCLUSIONSWINTER 2017401

ABOUT THE AUTHORS
Dr. Smith is a research scientist, and Dr. Wang is vice president ofresearch and development, at GIA in New York. Dr. Shirey is asenior scientist in the Department of Terrestrial Magnetism of theCarnegie Institution for Science in Washington, DC.

ACKNOWLEDGMENTS
The authors thank GIA’s Ulrika D’Haenens-Johansson and PaulJohnson for assistance with sample selection, Alex Balter for help
handling data for figure 2, and Jian Xin (Jae) Liao for acquiring
face-up images of the diamonds in figures 4 and 5. This work
was completed with the support of GIA’s Liddicoat Postdoctoral
Research Fellowship program.

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