FEATURE ARTICLESMEASUREMENT AND CHARACTERIZATION
OF THE EFFECTS OF BLUE FLUORESCENCEON DIAMOND APPEARANCE
Yun Luo, David Nelson, Troy Ardon, and Christopher M. Breeding
In this study, the effects of blue fluorescence on the appearance of diamonds—including table-down color, faceup color, brightness, and transparency—are quantitatively characterized using customized measurement systems.
The results show that UV intensity in the light source affects table-down color, face-up color, and brightness of
diamonds. A standardized lighting environment containing a fixed UV component is essential for accurate andconsistent color evaluation that fully incorporates the effects of blue fluorescence. The “hazy” appearance that
often impacts apparent diamond transparency is mainly attributed to light scattering from structural defects, but
our results also indicate that strong fluorescence causes minor contrast loss in the face-up patterns of some polished diamonds. The presence of both strong fluorescence and light-scattering structural defects increases theapparent haziness. The transparency changes induced by structural defects, fluorescence, or both can be characterized by a bulk contrast evaluation method using the diamond’s face-up pattern. Fluorescence is an intrinsic
property of diamond that can improve the color of some stones when they are exposed to a lighting environment
with significant UV content, such as daylight, but fluorescence alone does not noticeably reduce transparency.

Fluorescence in diamond is a subject that has
generated lively discussion in the trade for
decades. It is believed that diamonds with D to
F color grades (i.e., colorless), which do not possessenough color to offset the presence of fluorescence,are prone to appear hazy when combined with strong
or very strong blue fluorescence. Lower-color diamonds with medium to very strong blue fluorescence may appear up to one color grade better due tothe blue fluorescence “neutralizing” the yellow
color. The trade has expressed concerns that the true
color of diamonds is not represented under light containing UV (<400 nm) and considers diamonds color
graded under such light as over-graded (Cowing,2010). Conversely, some global markets believe thatfluorescence may have a beneficial impact on color
and thus lower-color diamonds with fluorescence
will sell at a slight premium (table 1). These mixed
perceptions on the effect of fluorescence on overall
diamond appearance have impacted the value of dia-See end of article for About the Authors and Acknowledgments.
GEMS & GEMOLOGY, Vol. 57, No. 2, pp. 102–123,
http://dx.doi.org/10.5741/GEMS.57.2.102© 2021 Gemological Institute of America

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BLUE FLUORESCENCE IN DIAMOND

monds with fluorescence since the late 1970s but
have been amplified in recent years (table 1).
Based on visual observations under different lighting conditions, several studies have investigated the effect of blue fluorescence on the color and overall
appearance of diamonds. Moses et al. (1997) concluded
that strongly blue fluorescent diamonds were per-In Brief• Fluorescence is an important property of gem diamonds
that impacts their value and pricing in the marketplace.
• Analysis indicates that a gem diamond’s visible color isonly occasionally impacted in those with strong to verystrong fluorescence and depends on the amount of UV
in the light source.
• Modulation transfer function (MTF) is a very useful toolfor examining contrast loss as a measurement of haziness in gem diamonds.
• The hazy appearance sometimes observed in gem diamonds is primarily caused by atomic-scale light scattering defects or nano-inclusions. Strong fluorescence byitself does not cause haziness, but it may produce more
contrast loss in some diamonds that already have ahazy appearance.

GEMS & GEMOLOGYSUMMER 2021

A

B

C

D

E

Figure 1. The 127.01 ct
Portuguese diamond
was graded by GIA as
M color and VS1 clarity(A,B) with very strong
blue fluorescence (C);photos courtesy of theSmithsonian Institution. The Portuguese diamond has been cited
as a classic example of astone being “over blue”
and exhibiting a noticeably oily or hazy appearance (D). It is also
shown being worn by
American actress Peggy
Hopkins Joyce (E). Photos by NMNH Photo
Services (A), Chip Clark(B), and Harold & Erica
Van Pelt (D).

ceived to have a better color appearance when viewed
face-up, with no discernible trend table-down, and that
there was no observable relationship between fluorescence and transparency. Bouman et al. (2018) foundthat diamonds observed table-down in outdoor conditions showed improved color grades, whereas fluorescence did not directly correlate with the color grade in
the face-up position. Despite no clear evidence of fluorescence negatively affecting diamond color and appearance in these studies, pricing adjustments basedon fluorescence continue in the trade (tables 1 and 2).
Historically, three major challenges needed to be
overcome in order to understand better the effect offluorescence on diamond appearance: the lack of astandard methodology to quantify color and fluorescence, the fact that color and fluorescence correspondto a range of values, and the lack of an accepted illumination standard.
The absence of a standardized characterization system, together with the fact that both color grades andfluorescence evaluations correspond to a range ofmeasurable values, makes consistent and accurate visual comparison difficult. For example, if we choose ahigh K color (closer to J) diamond with medium-low
blue fluorescence to compare with a low K (closer toL) stone with medium-high blue fluorescence, the effect of medium fluorescence will be different because
the two samples fall at opposite ends of continuous yet
independent ranges for both color and fluorescence.
Even with carefully aligned visual observations andspectroscopy-based measurement techniques for colorand fluorescence, we still need image-based measurements that more accurately reflect how the stones appear in “real life” and can be directly linked to humanvisual perception. No previous study has utilized imaging systems to demonstrate and quantify the effectsof fluorescence on diamond appearance. Even if we canvisually identify the effect of fluorescence on face-up
color by comparing very strongly fluorescent stones
against inert stones under certain lighting conditions,these differences are not readily obvious in images
taken under different lighting environments.
Figure 1 shows the famous 127.01 ct Portuguese
diamond, housed at the Smithsonian Institution. It
has been cited as a classic example of a very strong
blue fluorescent diamond displaying a noticeably oilyor hazy appearance. Photos of this legendary diamond taken under different lighting setups show various aspects of its appearance, including clarity, color,and haziness. It is not easy to find the right combination of illumination and camera resolution to accurately and precisely display these effects with
sufficient reproducibility.
The last but perhaps most important challenge isthe standardization of illumination for color measurement and visual observation. In King et al. (2008),
the GIA team concluded that a standard light sourcefor diamond color grading should possess key daylight elements—including a UV component to truly
and accurately represent how a diamond appears toBLUE FLUORESCENCE IN DIAMONDGEMS & GEMOLOGYSUMMER 2021

103

TABLE 1. Approximate percentage pricing differential from nonfluorescent diamonds.
Clarity Grade

Color FL Intensity
Grade(blue)
Very Strong
D-F

StrongMediumFaint
Very Strong

G-H

StrongMediumFaint
Very Strong

I-K

StrongMediumFaint
Very Strong

L-M

StrongMediumFaint

IF to VVS2017
–10 to –15%
–7 to –10%
–3 to –7%–1%
–7 to –10%
–5 to –7%
–1 to –3%
–1%
0 to +2%
0 to +2%
0 to +2%
0
0 to +2%
0 to +2%
0 to +2%
0

VS

2020
n/a
–18 to –25%
–14 to –21%
–9 to –15%n/a
–13 to –17%
–10 to –14%
–7 to –11%n/a
–6 to –12%
–5 to –9%
–4 to –6%
n/a
n/a
n/a
n/a

2017
–6 to –10%
–3 to –5%
–1 to –2%0%
–3 to –5%
–2 to –3%
0 to –2%
00 to +2%
0 to +2%
0 to +2%
0
0 to +2%
0 to +2%
0 to +2%
0

SI

2020
n/a
–12 to –22%
–9 to –17%
–6 to –12%n/a
–10 to –15%
–8 to –12%
–5 to –9%n/a
–5 to –11%
–4 to –8%
–3 to –5%
n/a
n/a
n/a
n/a

2017
0 to –3%
0 to –1%
0
0
0
0
0
0
0 to +2%
0 to +2%
0 to +2%
0
0 to +2%
0 to +2%
0 to +2%
0

2020
n/a
–8 to –16%
–6 to –13%
–4 to –8%n/a
–7 to –11%
–5 to –8%
–3 to –5%n/a
–4 to –10%
–3 to –7%
–2 to –4%
n/a
n/a
n/a
n/a

Data source: Rapaport price lists (https://www.diamonds.net/Prices/DetailedInfo.aspx)
n/a: data not available from Rapaport 2020 update

the human eye. For the results of this study to ultimately have significance for the diamond trade and its
practices and regulations, we require an illumination
standard that includes UV content and thus accounts
for the effects of fluorescence on diamond appearance.
Taking advantage of recent technological advances,
the focus of this study was to use customized, imagebased measurement systems to quantitatively characterize the effect of fluorescence on the color,brightness (i.e., the internal and external white light
return), and transparency of gem diamonds viewed inboth table-down and face-up orientations under wellcharacterized lighting conditions. Our goal is to pro-vide a better understanding of the effect of blue fluorescence on diamond appearance and quantify it instrumentally. We believe this will help to reduce theconfusion and biases in the industry and serve as asolid scientific foundation to ensure public trust withrespect to diamond fluorescence.

MATERIALS AND METHODSPolished Diamond Samples. Four sets of round brilliant diamonds, color graded by GIA as D, F, G, and H,
were used to measure table-down and face-up color.
Each set was assembled with the same color grade and

TABLE 2. Average price for one-carat, VS -clarity, triple EX (a trade term for Excellent cut,polish, and symmetry), round brilliant cut diamonds with different fluorescence intensities.
1

Color
Grade

NON

FNT

MED

STR

VST

FNT

MED

STR

VST

D

$8613

$7860

$7425

$7171

$6748

–9%

–14%

–17%

–22%

E

$8082

$7463

$6816

$6389

$6282

–8%

–16%

–21%

–22%

F

$7689

$7007

$6527

$6432

–9%

–15%

–16%

G

$6877

$6536

$6047

$5941

–5%

–12%

–14%

H

$6675

$6220

$5917

$5544

$5346

–7%

–11%

–17%

–20%

I

$5705

$5395

$5226

$4942

$4628

–5%

–8%

–13%

–19%

J

$4933

$4314

$4411

$4209

–13%

–11%

–15%

Data source: Diamond prices posted on Blue Nile for October 26, 2019 (www.bluenile.com)

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BLUE FLUORESCENCE IN DIAMONDGEMS & GEMOLOGYSUMMER 2021

TABLE 3. Four sets of round brilliant diamonds examined in this study.
Sample
ID

Shape

Color

Fluorescence Weight
Intensity
(ct)

Cut

Symmetry

Polish

Table
(%)

Depth
(%)

Crown
Angle
(º)

Pavilion
Angle(º)

GirdleThickness
(%)

x0237

Round

D

NON

x4632

Round

D

FNT

0.53

EX

EX

EX

58

61.3

33.6

41.3

3.4

0.61

EX

EX

EX

59

61.0

32.4

41.3

x0517

Round

D

MED

4.4

0.74

EX

EX

EX

58

61.3

34.0

41.2

3.6

x8912

Round

D

STR

0.61

EX

EX

EX

x8789

Round

D

VST

0.63

EX

EX

EX

59

60.9

33.4

41.4

3.3

58

60.9

33.4

41.2

3.5

x0059

Round

F

NON

0.56

EX

EX

EX

x7519

Round

F

FNT

0.50

EX

EX

EX

58

61.7

34.9

41.2

3.4

57

61.7

34.7

41.4

x8316

Round

F

MED

0.52

EX

EX

EX

2.8

58

61.9

34.5

41.3

3.5

x7510

Round

F

STR

0.51

EX

EX

x8814

Round

F

VST

0.62

EX

EX

EX

58

61.9

34.6

41.3

3.3

EX

58

61.6

34.2

41.3

3.5

x2150

Round

G

NON

0.51

EX

EX

x2008

Round

G

FNT

0.52

EX

EX

EX

57

62.1

34.5

41.2

3.6

EX

57

62.3

35.0

41.2

x1977

Round

G

MED

0.41

EX

EX

3.6

EX

57

62.5

34.9

41.1

4.0

x2109

Round

G

STR

0.51

EX

EX

EX

x2356

Round

G

VST

0.50

EX

EX

EX

57

62.1

35.0

41.0

3.7

57

62.3

35.1

40.8

4.0

x7596

Round

H

NON

0.53

EX

EX

EX

x0271

Round

H

FNT

0.50

EX

EX

EX

57

61.6

34.3

41.0

3.7

57

61.5

34.0

41.3

x7635

Round

H

MED

0.51

EX

EX

EX

3.3

57

61.3

33.7

41.0

3.4

x8812

Round

H

STR

0.61

EX

EX

EX

x8937

Round

H

VST

0.54

EX

EX

EX

58

61.2

33.9

41.2

3.5

57

61.4

34.4

41.0

3.4

similar sizes and proportions, but with fluorescenceintensities varying from None to Very Strong (table 3).
All faceted diamonds used in this study carried GIA
reports and represented a range of diamond types.
TABLE 4. The 13 diamond plates used in this study, sorted
by fluorescence intensity according to lightness values fromfluorescence measurements.
Sample ID Thickness (mm) Lightness

StrainDiamond Type5358_A

1.709

3

MediumIaB

CBP-0368

4.314

3StrongIaAB (low)

CBP-0069

2.205

6MediumIaAB

CBP-0632

3.057

7StrongIaAB

5359_A

0.876

12MediumIaB

CBP-0413

2.661

37

Weak

IaAB

CBP-0067

2.761

39

None

IaAB

CBP-0098

2.225

43

None

IaAB

CBP-0929

3.745

44

StrongIaB>A

CBP-0151

2.437

46

None

IaAB

CBP-0033

2.768

47

None

IaAB

CBP-0361

2.864

49

StrongIaB>A

CBP-0926

2.467

62

StrongIaB>AHigher lightness value corresponds to stronger fluorescence intensity.

BLUE FLUORESCENCE IN DIAMOND

Diamond Plates. Eleven rough and two faceted diamonds were cut into plates with parallel windows(table 4). All samples were analyzed by ultraviolet/
visible/near-infrared (UV-Vis-NIR), Fourier-transform
infrared (FTIR), and photoluminescence (PL) spectroscopy to provide additional information about the
diamond type and atomic structural defects present.
In addition, the birefringence of each sample was examined under crossed polarized light in the microscope to assess internal strain. Color and fluorescenceintensity were characterized for correlation with thetransparency and contrast studies.
Color Measurement Systems. Two customized imaging systems were set up to measure the table-down
and face-up color of diamond samples in this study.
The table-down color measurement system consistsof a sample chamber with a rotational stage, a lightintegration hemisphere, a lens-camera assembly, anda light source with tunable UV content (figure 2A).
The emission spectra from the light source show four
steps of UV intensity (0, 25, 50, and 100% LED powerwith UV emission at 368 nm, corresponding to calculated UV content of <1, 2, 3, and 5%, respectively).
For the purposes of this study, UV content refers to

GEMS & GEMOLOGYSUMMER 2021

105

EMISSION SPECTRA

2A

0.3
2.0

LED Controller
with Tunable UV

0.2

Teflon Hemisphere Telecentric Lens

0.1
1.5

Ring LED ArrayINTENSITY

Camera

0.0
360

370

380

390

400

1.0

Rotating Teflon Stage0 power (<1% UV)
25% power (2% UV)
50% power (3% UV)
100% power (5% UV)Halogen

0.5

Stepper Motor
0.0
360

410

460

510

560

610

660

710

760

WAVELENGTH (nm)

2B

70000

0 power (0% UV)
10% power (3% UV)
20% power (5% UV)
40% power (7% UV)
60% power (10% UV)
80% power (13% UV)
100% power (16% UV)

60000

Dichroic Filter

Diffuser

Teflon Lid

Fiber-Optic Ring

White LED

Glass StageUV LED

Teflon Hemisphere

LED
Controllers

Telecentric Lens

INTENSITY

50000

40000

30000

20000

10000

Camera

0
350

400

450

500

550

600

650

700

750

WAVELENGTH (nm)Figure 2. Color measurement experiment setups used for this study include both table-down and face-up systems.
2A is the table-down color measurement system (left), together with its LED light spectra at different UV intensities compared to a halogen light source with daylight filter (right). 2B shows the face-up color measurement system (left) and the spectra of the light source with different UV intensities (right).

the percentage of the total emission that occurs in
the UV range (300–400 nm) and is defined as follows:% UV=

area under UV part of emission curve at 300–400 nm
×100
area under complete emission curve at 300–850 nm

The face-up color measurement system was configured using a sample chamber, lens-camera assembly,
and a light source with tunable UV intensity (366 nm
emission) at increments of 0, 10, 20, 40, 60, 80, and100% LED power, corresponding to calculated UVcontent of ~0, 3, 5, 7, 10, 13, and 16%, respectively(figure 2B). Both of these color measurement systems
were based on image processing technology. In the

106

BLUE FLUORESCENCE IN DIAMOND

table-down measurement system, the numerical values of lightness, chroma, and hue (LCH; described inbox A) are used to characterize the effect of fluorescence on table-down diamond color in precise increments. Note that reported LCH values have beennormalized to light background values. Due to camera software differences, values for hue, saturation,and lightness (see box A) were used in the face-up
measurement system to correlate color grades andbrightness with fluorescence.
Fluorescence Measurement System. A fluorescencemeasurement system consisting of a sample cham-

GEMS & GEMOLOGYSUMMER 2021

UV LED

Telecentric Lens
Shutter

CameraRing LED Array

Rotating Teflon Stage
Stepper Motor

Figure 3. The fluorescence measurement experiment
setup used in this study.

ber with a rotational stage, a lens-camera assembly,
and a 365 nm UV-LED source was set up to measure
the fluorescence intensity of diamond samples inthis study (figure 3).

Microscope Imaging. A darkfield imaging system
was designed to produce images that represent how
diamonds would be viewed during grading. The system consists of a Nikon D800E, a Nikon SB-900
Speedlight, a Sigma 150 mm macro lens, a Stackshot
macro rail package, and custom mounting fixtures.
Software to control the camera and Stackshot hardware were used to automatically capture and stack aseries of images for each diamond through a range of
focal planes. The final processed images were usedin this study to evaluate face-up appearance.
We set up two different microscopes to help understand the effect of blue fluorescence on diamondappearance under magnification. As shown in figure
4, the standard GIA microscope contains a UV component in the fluorescent overhead lamp and fromthe halogen bulb in the recessed light chamber used
for darkfield illumination. The other testing microscope, an LED-based system, has no UV peak but
does have some emissions in the wavelength rangeof 400 to 420 nm from the overhead lamp. The darkfield illumination of this testing microscope contains
absolutely no UV or observable wavelengths below
420 nm (i.e., it is completely lacking a UV component). Photos of stones under the microscope weretaken by camera through the ocular lens.

Figure 4. Light spectra collected from a standard GIA microscope overhead fluorescent lamp and darkfield illumination in the recessed light chamber (halogen) show that both contain a UV component (left). For comparison,
spectra from an LED testing microscope with no UV component (either in the overhead lamp or in darkfield illumination) are shown on the right.

510

560

610

660

710

760

360

410

460

510

560

610

660

710

760

360

WAVELENGTH (nm)BLUE FLUORESCENCE IN DIAMOND

410

460

510

560

610

660

710

760

410

460

510

560

610

660

710

760

Darkfield

460

Darkfield

410

INTENSITY360

Overhead

Testing Microscope

Overhead

GIA Microscope

GEMS & GEMOLOGY360

WAVELENGTH (nm)

SUMMER 2021

107

BOX A: MEASURE WHAT YOU SEE—COLOR AND FLUORESCENCE
Whenever an object is viewed, the color seen is a result of
the interaction of the light source, the viewer, and the object. In attempting to achieve an accurate and consistentcolor and fluorescence intensity evaluation, it is important to use a standardized lighting environment that creates reproducible measurement results that characterize
color and fluorescence intensity in a quantitative fashion.
Light Sources and Illuminants. A light source is a physicaldevice that emits light with relative energy distribution
in the ultraviolet to visible spectral range. It can be turnedon and off and used in visual color evaluation.
The color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of acolor comparable to that of the light source, expressed inkelvins (K). There are three common color temperature
ranges: warm light (2700 K to 3000 K), cool white (3000 K
to 5000 K), and daylight (5000 K to 6500 K).
An illuminant is an emission spectrum defined mathematically by a relative spectral power distribution that
may or may not be physically achievable as a source.
By CIE definitions, the commonly used “illuminantA” represents incandescent light from a tungsten filament and the “illuminant F” series represents a range offluorescent lamps. Both can be readily reproduced. The D
series illuminants represent natural daylight and include
D65 (which refers to average zenith daylight at noon witha correlated color temperature of approximately 6500 K)
and D50 (which typically refers to horizon daylight in theearly morning or late afternoon with a correlated color
temperature of approximately 5000 K). Unlike illuminant
A and F series, the D series illuminants were defined ashaving specific correlated color temperature, chromaticity, and spectral power distribution, without corresponding standard light sources (visit https://cie.co.at/ formore information). Although natural daylight is commonly referred to as a standard light, the appearance andspectral characteristics of natural daylight are affected byvarious conditions such as the season, time of day, atmospheric conditions, and altitude. All of these factors affect
how we can accurately and consistently evaluate color
appearance under natural daylight. Thus, the use of simulated daylight has become the standard and accepted
method for accurate color quality evaluations in many industries such as paints, plastics, textiles, graphic arts, andcomputer technology. It is important to note that various
common sources available in the market to simulate daylight may be quite different in spectral power distribution
from each other and even from any real natural daylight.
Sunlight at the earth’s surface typically has a UV component between 3% and 5% (calculated from “Reference
Solar Spectral Irradiance: Air Mass 1.5,” National Renewable Energy Laboratory, www.nrel.gov/grid/solarresource/spectra-am1.5.html). Therefore, a standardized
daylight spectrum containing a UV component is essential to providing realistic, accurate, and consistent colorand fluorescence intensity evaluations for diamond.

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BLUE FLUORESCENCE IN DIAMOND

Reflection

Refraction

Absorption

Fluorescence
Scattering

Transmission

Figure A-1. Light can interact with a faceted diamond inseveral different ways, as shown here.

Light Interaction with Diamonds. When light strikes a diamond, a small fraction of the light is reflected while therest is transmitted through the stone. As the light passes
through, it may be refracted, absorbed, or scattered byatomic-scale defects in the diamond structure or inclusions, and it can be internally reflected by speciallyarranged facets in a well-cut diamond. Ultimately, the resultant wavelengths of light that exit the stone and returnto the observer create the color of the diamond (figure A1). Scattering occurs when the transmitted and internallyreflected light interacts with microscopic foreign particles
or atomic-scale structural imperfections, causing the lightto change directions. In most cases, scattering is thought
to be responsible for an apparent reduction in contrast anda hazy appearance. When the particulate matter that isoften responsible for light scattering is the same size or
smaller than the wavelength of light in the visible range(400–700 nm), the scattering will cause a milky opalescence phenomenon known as the Tyndall effect. While
scattering does affect appearance, absorption of the lightby atomic-scale defects tends to have the greatest impact
on diamond color by preventing particular wavelengths
from being transmitted to our eyes. Additionally, when
the UV component of light is absorbed by certain diamond
defects, additional light of a different wavelength in thevisible range is often emitted, and this is known as fluorescence. In this paper, we specifically examined the impact of blue fluorescence (by far the most common
fluorescence color in natural diamond) on the appearanceof faceted diamonds.
Color Space. There are different color spaces defined by thecolor science community to communicate and express thecolor of objects. GIA describes color in terms of hue, tone,
and saturation (King et al., 1994). Hue refers to the diamond’s characteristic color, tone to the color’s relative lightness or darkness, and saturation to the color’s depth orstrength. In this study, we use CIE L*C*H color space (figure

GEMS & GEMOLOGYSUMMER 2021

Hue: The attribute of color perception by means of whicha color is judged to be red, orange, yellow, green, blue, purple, or intermediate between adjacent pairs of these, considered in a closed ring (ASTM, 1996).

Lightness

Due to system limitations, HSL color space was used
for color evaluation in the face-up measurement system.
While broadly comparable to LCH, the HSL color spaceconsists of H indicating hue angle, S representing saturation, and L indicating lightness (H and L are defined above,
while S correlates with chroma).

ChromaHue

Figure A-2. Conventional color space includes three attributes: lightness, chroma, and hue.

A-2) according to GIA convention to evaluate color attributes and accurately express color and fluorescence intensity
measurements in numerical terms. In this color space, L indicates lightness (also referred to as tone), C represents
chroma (also known as saturation), and H is the hue.
Lightness (tone): The attribute by which a perceived color
is judged to be equivalent to one of a series of grays ranging
from black to white (ASTM, 1996).
Chroma (saturation): The attribute used to indicate thecolor’s degree of departure from a gray of the same lightness (ASTM, 1996). It typically refers to a color’s purity,
intensity, or saturation.

Color and Fluorescence Measurements. To help readers better understand the correlation of color terminology withthe visual appearance of diamonds, figure A-3 shows imagesof four GIA research stones with numerical values for lightness, chroma, and hue using the color measurement experiment setup with a halogen light source with daylight filter.
All of the research stones are in the D-to-Z yellow hue
range, and chroma measurements are indicative of the GIA
color grades D, H, J, and L.
Figure A-4 shows a different set of four researchstones under UV light (365 nm). Their blue fluorescenceintensities are indicated by the lightness measurement
under UV, which corresponded to Faint and Strong fluorescence descriptions as reported by GIA. These are useful examples to show that each fluorescence description
covers a range of actual fluorescence intensities. Samples
A and B are both described as having Faint fluorescence,
while samples C and D are reported as Strong, despite
the obvious differences in lightness and apparent fluorescence intensities.

Diamonds under halogen light withdaylight ﬁlter
Lightness = 0.98
Chroma = 0.6
Hue = 93
D colorLightness = 0.97
Chroma = 2.3
Hue = 96
H colorLightness = 0.96
Chroma = 4.1
Hue = 96
J colorLightness = 0.96
Chroma = 5.6
Hue = 96
L colorBLUE FLUORESCENCE IN DIAMOND

Diamonds under UV LEDA
FL lightness = 14.8
Faint fluorescence

Figure A-3.
Examples ofcolor measurements on
a set of GIA
researchstones show
the variancein color space
attributeswith color
grades.

B
FL lightness = 25.5
Faint fluorescenceCFL lightness = 50.1
Strong fluorescence

Figure A-4. Fluorescence measurements on aset of GIA research stones
show the correlation between
fluorescence intensity andlightness.

D
FL lightness = 70.9
Strong fluorescence

GEMS & GEMOLOGYSUMMER 2021

109

2.0

0.980

1.8

0.975

1.2
1.0
0.8
0.6
0.4

STR

20
0

0.955

-20

0.950

-40
-60
-80
-100

NON

FNT

MED

STR

VST

F COLOR SET

2.3

0.975

100

0.970

95

0.965

90

1.7
1.5
1.3

0.9
0.7

HUE

0.980

1.9

0.960

NON

FNT

MED

STR

80

0.950

75

0.945

70

FNT

MED

STR

VST

G COLOR SET

2.8

0.975

100

0.970

95

0.965

90

LIGHTNESS

2.2
2.0
1.8
1.6
1.4
1.2

HUE

0.980

2.4

0.960

NON

FNT

MED

STR

80

0.950

75

0.945

70

FNT

MED

STR

VST

H COLOR SET

3.3

0.975

100

0.970

95

0.965

90

LIGHTNESS

2.7
2.5
2.3
2.1
1.9
1.7
1.5

NON

FNT

MED

STR

A

VST

HUE

0.980

2.9

FNT

MED

STR

VST

NON

FNT

MED

STR

VST

NON

FNT

MED

STR

VST

105

3.5

3.1

NON

65

NON

VST

VST

85

0.955

0.940

1.0

STR

105

3.0

2.6

MED

65

NON

VST

FNT

85

0.955

0.940

0.5

NON
105

2.5

LIGHTNESS

CHROMA

0.960

VST

1.1

CHROMA

40

0.940

2.1

CHROMA

60

0.0

MED

80

0.965

0.945

FNT

100

0.970

0.2

NON

120

HUE

1.4

LIGHTNESS

CHROMA

1.6

D COLOR SET

0.960

85

0.955

80

0.950

75

0.945

70

0.940

65

NON

FNT

MED

STR

VST

B

C

Figure 5. Trends of chroma, lightness, and hue can be seen under different UV intensities using the table-down color
measurement system on D, F, G, and H color sample sets. The four consecutive data points in each color and fluorescence value (from left to right in each grouping) represent data collected at UV increments of <1%, 2%, 3%, and 5%.

110

BLUE FLUORESCENCE IN DIAMONDGEMS & GEMOLOGYSUMMER 2021

Without UVWith UVF color, NON FL

F color, VST FL

Lightness

Chroma

Hue

No UV

UV

No UV

UV

No UV

UV

0.96

0.96

1.6

1.7

99

100

0.96

0.96

2.3

0.8

95

82

Figure 6. Stones in the F color set with fluorescence descriptions of None and Very Strong are seen without UV andwith high UV intensities from the table-down color measurement system, along with corresponding attributes.

Table-Down Color. Similarly sized diamonds representing a cross section of color grades and fluorescence descriptions were measured on the table-downcolor measurement system (table 3). Each sample
was measured under the same lighting environment
with UV content of <1%, 2%, 3%, and 5% at 368
nm. Figure 5A shows how chroma values changewith different UV intensities for D-, F-, G-, and Hcolor diamonds with fluorescence descriptions ofNone, Faint, Medium, Strong, and Very Strong. Diamonds with fluorescence descriptions of None or
Faint show a negligible difference in chroma values
as the UV content increases. Those with Medium
fluorescence start to show a slight decrease inchroma values as UV content increases. Diamondswith Strong or Very Strong fluorescence descriptions
show obvious reductions in chroma values with increasing UV content. The color can change as muchas two color grades for diamonds with Very Strong
fluorescence as UV content increases from <1% to

5%. The D-color diamond with Very Strong fluorescence shows a sharp reduction in chroma between<1% and 2% UV content, followed by a sharp increase in chroma as UV content increases to 3% and
5%. The reduction in chroma corresponds to a desaturation of the subtle yellow color component as it
becomes “neutralized” by blue fluorescence; the increase in chroma is caused by the fluorescence emission, which begins to dominate the color and shifts
the hue from yellow toward blue.
Figures 5B and 5C show how lightness and hue
values for these diamonds change under different UV
intensities. For lightness, D- and F-color diamonds
do not show an obvious change as UV content increases, whereas nearly all G- and H-color diamonds
show a consistent decrease in lightness values acrossfluorescence descriptions. Diamonds with fluorescence descriptions of None or Faint show negligible
changes in hue value as UV content increases. Diamonds with Medium fluorescence start to show
some change in hue value with higher UV content,
and those with Strong or Very Strong fluorescenceshow sharp changes in hue value as UV content increases. The D-color diamond with Very Strong fluorescence shows a significant change in hue value,
from 100 to –80, as UV content increases to 3%. This
dramatic change in hue value represents a huechange from yellow to blue.
The diamonds in the F color set with fluorescencedescriptions of None and Very Strong are shown infigure 6 in lighting environments with no UV con-BLUE FLUORESCENCE IN DIAMONDGEMS & GEMOLOGY

Modulation Transfer Function Measurement System.
A modulation transfer function (MTF) measurement
system was configured using a sample chamber, alens-camera assembly, a light source with tunable UV
content, and a micro slide with a sinusoidal array. The
MTF measurement system uses image processing
technology to produce percentage contrast values
from the sinusoidal array target. Box B explains how
MTF can be used to assess diamond transparency.

RESULTSSUMMER 2021

111

Without UVWith UVLightness

Chroma

Hue

No UV

UV

No UV

UV

No UV

UV

0.97

0.97

0.3

0.3

58

59

0.92

0.93

0.3

0.4

139

163

F color, NON FL

F color, VST FL

Figure 7. The same stones in the F color set from figure 6 are seen here without UV and with maximum UV intensities from the face-up imaging system, along with corresponding attributes.

tent (<1%) and with relatively high UV content (5%)
when table-down color measurements were made.
When we add UV to the lighting environment, there
is no obvious color change in the diamond with a fluorescence description of None. The diamond with
Very Strong fluorescence, however, shows a desaturated color as UV is added to the lighting environment. This demonstrates how blue fluorescence canneutralize a yellow color in some diamonds by thecomplementary color principle. The hue angle
moved from normal yellow (95) to blue (82), and thechroma value decreased from 2.3 to 0.8.
Face-Up Color. Faceted diamonds (table 3) with arange of fluorescence descriptions were imaged using
light with incrementally higher UV content. For eachface-up image, a square region confined to the tableof the diamond was selected for measurement. The
average hue, saturation, and lightness for the array of
pixels in the selected area were calculated for each
image. A diamond’s face-up color is affected by thepattern and reflections from light return, which depend mainly on cutting parameters. Thus, the quantified hue, saturation, and lightness values in thisprocess are not comparable to the color derived fromthe table-down color measurement system. Using
the stone with a fluorescence description of None ineach color grade as the baseline, the resulting values
are compared to the level of UV content for the related images to examine the effect of UV on the face-

112

BLUE FLUORESCENCE IN DIAMOND

up color and appearance of diamonds with different
fluorescence descriptions.
The color of diamonds with weak or no fluorescence is shown to be unaffected by UV, while the colorof diamonds with stronger fluorescence is systematically altered. The stones in the F color set with fluorescence descriptions of None and Very Strong fromthe face-up imaging system show that for diamondswith little to no fluorescence, the color and appearance do not change with the addition of UV, but for
diamonds with fluorescence, the addition of UV
causes the hue and lightness values to increase andsaturation to decrease. This makes sense if we consider that the mechanism of fluorescence converts invisible UV light into visible light, thereby increasing
the observed light return. Saturation values for diamonds with fluorescence are reduced with exposure
to UV as the blue fluorescence neutralizes their normally yellow hue. However, if the measured huechanged from the yellow to blue range, we may see aslight increase in saturation with increasing UV (figure
7, bottom). Our results are consistent with the faceup trends visually observed by Moses et al. (1997),
which suggested that the strongly fluorescent diamonds were perceived to have a better color appearance when viewed face-up.
Transparency. Microscope Imaging. Figure 8 showsa D-color diamond with Very Strong fluorescence (left)
next to a nonfluorescent D-color diamond (right)

GEMS & GEMOLOGYSUMMER 2021

D color, VST FL

D color, NON FL

Figure 8. The stones
with Very Strong fluorescence and None in the Dcolor set are shown
under the same microscope setup for transparency evaluation.

Microscope Imaging

under the same lighting condition (samples from the
D color set in table 3). With the same color grade andsimilar proportions, the diamond with Very Strong
fluorescence appears slightly more “milky white”
than the diamond with no fluorescence; the patternof the diamond with Very Strong fluorescence also
looks softer than the pattern in the diamond with nofluorescence, which is very crisp and sharp. We haveexamined many diamonds with the same color grade,similar proportions, and fluorescence descriptions
ranging from None to Very Strong that do not have ahazy appearance (figure 9A). We also observed two diamonds, one with a fluorescence description of Noneand the other with Strong fluorescence, that showed

Figure 9. Diamond images with different degrees of haziness/transparency are plotted by fluorescence intensity.

VSTFL Intensity

STR

MED

FNT

NON

Haziness
A

BLUE FLUORESCENCE IN DIAMOND

B

C

GEMS & GEMOLOGYD

SUMMER 2021

113

With UV

Without UV

A

STR

NON

STR

NON

B

VST

NON

VST

STR

NON

Figure 10. Sets of diamonds with different
UV intensities were
examined and imaged
under microscope
darkfield illumination,with and without UV
content.

STR

C

STR

the same degree of slight haziness (figure 9B). We
have seen diamonds with Medium and Strong fluorescence that have very obvious haziness (figure 9C),
and a diamond with Faint fluorescence that was somilky it received a Fancy White color grade (figure
9D). Given that there is no significant correlation between the occurrence of haziness and fluorescenceintensity from our observations, we believe there isno inherent link between the two.

114

BLUE FLUORESCENCE IN DIAMOND

STR

In order to distinguish between haziness created bylight scattering and contrast loss due to fluorescence,pairs of diamonds with pertinent differences werecompared microscopically in two different darkfieldlighting environments (figure 4): one containing UVand one without UV. With UV, the diamond with afluorescence description of None appears quite clear,
while the diamond with Strong fluorescence looks
very hazy (figure 10A, left). Without UV, the diamond

GEMS & GEMOLOGYSUMMER 2021

NORMALIZED PERCENT CONTRAST

is hazy. With UV content, we observe the same thing.
However, it appears that the haziness of the diamond,
together with the fluorescence, makes the stone lookbrighter and whiter.

MODULATION TRANSFER FUNCTION

120

CBP-0151 MED

100
80
60

No UV
With UV

40
20
0
4

1

16

64

256

TARGET RESOLUTION (lp/mm)

Figure 11. MTF measurement is shown for a diamondwith Medium fluorescence. The reduction in percentage contrast values under UV excitation indicatescontrast loss due to fluorescence.

CONTRAST DIFFERENCE (White% – White & UV%)with Strong fluorescence is still hazy, though possibly
to a somewhat lesser degree (figure 10A, right). Thus,
it appears that fluorescence is not the main contributor to the haziness of this diamond. Figure 10B shows
two G-color diamonds with no apparent haziness.
Viewed under the microscope lighting with UV content (left), it is easy to see blue luminescence from thediamond with Very Strong fluorescence. However,this diamond does not show obvious haziness underlighting environments with or without UV content.
The last pair shown (figure 10C) have Strong fluorescence. Without UV, one appears clear while the other

Modulation Transfer Function. Contrast reproduction and optical resolution of diamond samples witha range of fluorescence intensities were measured
using modulation transfer function analysis as described in box B. Measurements were made while thesamples were exposed to UV and while they were notexposed to UV. By comparing measurements of samples with different fluorescence intensities, the effectof fluorescence on contrast reproduction and opticalresolution can be observed. For this study, contrast
measurements collected with and without UV are
normalized to a single maximum measured contrast.
Figure 11 shows differential percentage contrast
measurements for one diamond plate with Medium
fluorescence. In this figure, the x-axis units are linepairs per millimeter and the y-axis units are normalized percentage contrast. Because the contrast values
for the series collected with UV exposure are less thanthe contrast values for the series collected without UV
exposure, a reduction in contrast due to UV exposureis measured in this fluorescent diamond plate.
The differences in contrast between measurements made with and without UV exposure for the
13 diamond plates included in this study are shownin figure 12. Fluorescent samples are shown to have
reduced contrast when exposed to UV, while inert
samples are unaffected. The summed contrast difference for each sample is plotted against fluores-

MODULATION TRANSFER FUNCTION
5

0

-5

No fluorescence
Medium fluorescence-10Strong fluorescence

Figure 12. Contrast loss
under UV excitation
was determined on 13
diamond plates.

-15

-20
1

2

BLUE FLUORESCENCE IN DIAMOND

4

8

16

32

64

128

256

TARGET RESOLUTION (lp/mm)

GEMS & GEMOLOGYSUMMER 2021

115

BOX B: MEASURE WHAT YOU SEE—TRANSPARENCY
measurable contrast, using the formula in figure B-2, thepercentage contrast can be determined. Percentage contrast measurements using a range of line-pair frequencies
describe the transparency of the lens. We adapted thisconcept to evaluate the transparency of diamond plateswith different fluorescence intensities under lighting environments with and without a UV component. An optical target from Edmund Optics with line-pairfrequencies ranging from 2 to 256 lp/mm (line-pairs per
millimeter) was used for this study.

When looking at a gem diamond, an observer usually focuses on color, clarity, and cut. Rarely does transparency
come to mind, as most people assume that all gem diamonds are completely transparent. This is not always
the case, however, and reduced transparency can have asignificant effect on a diamond’s appearance. Whiletransparency can be assessed visually, that approachtends to be subjective and difficult to quantify. Evaluating the contrast in a stone is a more objective approachto this problem.
Transparency: The degree of regular transmission, andthus the property of a material by which objects may beseen through a sheet of that material (ASTM, 1996).

Diamond Plates. MTF was used to measure the transparency of the diamond plates, which are placed between
a lens-camera assembly and an optical target (figure B-3,
A). An image is captured and cropped to include only thearea of interest. A custom application calculates the average lightness value for each column of pixels in theimage and uses those values to identify the locations ofa series of local maxima and minima that correspond to
the location of line-pairs in the optical target (figure B-3,
B). Lightness values at these locations are paired together
across each row of pixels, and their differences in value
are assembled into a density plot to observe their distribution. The average value in the data set is used to calculate the normalized percentage contrast for that target(figure B-3, C). The percentage contrast values are calculated using an image of a plate taken with no UV stimulation against an image taken with UV stimulation;
therefore, the plate is being measured against itself.
Doing this removes concerns about path length differ-

Haziness: An appearance produced by the scattering oflight in a specimen, responsible for the apparent reduction
in contrast of objects viewed through it (ASTM, 1996).
In optical science, a modulation transfer function(MTF) is routinely used to measure the ability of an optical component to transfer contrast at a given resolution—i.e., its optical transparency. Optical components(a lens, for example) can cause resolution and contrast to
be reduced (figure B-1, top) when used in an optical system. This effect can be characterized by measuring lightness across a series of equally spaced, alternating black
and white lines imaged through the lens (figure B-1, bottom) and calculating contrast as the difference in lightness between the black and white lines. Then, bycomparing this contrast measurement to a maximum

ObjectObject

1.0

0.8
0.6

1.0 – 0.0

0.4
0.2
0.0

0.8 – 0.2

Image

0.8

Figure B-1. An object imaged through a lens isshown to have reduced
contrast and resolution.

0.6
0.4
0.2
0.0

100% CONTRAST

116

Image

LIGHTNESS

LIGHTNESS

1.0

Lens

BLUE FLUORESCENCE IN DIAMOND

60% CONTRASTGEMS & GEMOLOGYSUMMER 2021

Pixels

Light

Figure B-2. Contrast wascalculated as shown here
for the modulation transfer function (MTF) study.

Contrast

Dark(Imax – Imin)% Contrast =(Imax + Imin)
where I is Intensity

× 100

ences, which allows us to compare bulk fluorescencevalues.
Polished Diamonds. Patterns of bright and dark areas
produced by light interacting with the facets of a wellpolished diamond create a mechanism by which an ob-

server can gauge the transparency of that diamond.
When a diamond is very transparent, these patterns areperceived to be high in contrast, while for less transparent stones the perceived contrast is reduced—dark areas
are less dark, bright areas less bright. The effects of poor
polishing were not investigated in this study.

A
30

C

25

Figure B-3. An opticaltarget is imaged
through a diamondplate to conduct MTF
measurements. Custom
software analyzes a select area of the image
and produces the percentage contrast value.

DENSITY

20

LIGHTNESS

0.6

B

15

10

0.4
5
0.2
0

0
0

50

100

PIXEL (X)BLUE FLUORESCENCE IN DIAMOND

150

0

10

20

30

40

50

60

70

80

90

100

PERCENT CONTRASTGEMS & GEMOLOGYSUMMER 2021

117

CONTRAST LOSS UNDER UV EXCITATION
05358_A
CBP-0368
CBP-0069

CONTRAST DIFFERENCE (White% – White & UV%)-10

CBP-0069
CBP-0069
CBP-0632-205359_A
CBP-0413
CBP-0067-30

CBP-0098
CBP-0929
CBP-0151
CBP-0151-40

CBP-0151
CBP-0033
CBP-0361-50

CBP-0926
CBP-0926
CBP-0926

-60
0

10

20

30

40

50

60

70

FLUORESCENCE (Lightness)

Figure 13. Correlation was observed between fluorescence intensity and contrast difference from measurements
made with and without UV.

cence intensity in figure 13 and shows a correlation
between these two factors. Replicated measurements
for three samples are included in this figure to demonstrate measurement repeatability. The observedchanges in contrast difference do not correlate withstrain intensity or diamond type, strongly suggestingthat the contrast loss is primarily due to fluorescence.

DISCUSSION
Illumination Standardization. Fluorescence is an intrinsic property of more than 30% of the diamonds
seen by GIA’s laboratory (Moses et al., 1997). Some inthe diamond trade argue that because typical officelighting lacks 365 nm and 385 nm UV content, it reveals the “true color” of diamonds. Based on figure 14,

118

BLUE FLUORESCENCE IN DIAMOND

we can see that office lighting does in fact containsome amount of UV light. We also need to keep inmind that blue fluorescence caused by the N3 defectcan be activated by wavelengths up to 420 nm (Luoand Breeding, 2013). If a lighting environment includes
light in the 390–420 nm range, a diamond’s color will
be viewed with some amount of blue fluorescencefrom the N3 defect. While most commercial lightsources do contain light in the 400–420 nm range (bluelight) but little in the UV range (<400 nm), our work
investigates the effects of varying the amount of UV
light (and thus the fluorescence produced) on a diamond’s appearance. Consider, for example, a fluorescent lamp with no emission below 390 nm (figure 15).
A diamond with strong fluorescence (left) glows blue

GEMS & GEMOLOGYSUMMER 2021

UV COMPONENT OF VARIOUS LIGHT SOURCES NORMALIZED TO 560 NM
0.50
0.45

NORMALIZED INTENSITY

0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
365

360

370

375

380

385

390

395

Figure 14. A comparison of the UV content
of common lightsources, including thecolor measurement system (CM) used for thisstudy, the GIA DiamondDock, the GIA
microscope overhead
and well light, officelight, and daylight. All
spectra have been normalized at the 560 nm
value by convention(King et al., 2008).

WAVELENGTH (nm)
CM_UV=0% Power

CM_UV=100% Power

Sunny Day

GIA Diamond Dock

Office Light

Cloudy Day

GIA Microscope (Overhead)

GIA Microscope (Well)in this light, while a nonfluorescent diamond (right)
does not. This illustrates the importance of using standardized illumination with fixed UV content to define
the color grade of a diamond.
UV content is typically reported as a peak intensity output percentage normalized to a certain wavelength. However, depending on the shape andposition of the UV peak, the fluorescence reaction
will differ and consequently the visual appearance ofthe fluorescence and its effect on diamond color will
also be different (Luo and Breeding, 2013). In order toconsistently evaluate fluorescence and its effects on
overall appearance, we again emphasize that anagreed-upon standard of illumination for color meas-

Figure 15. Two G-color stones with similar size were imaged for comparison under the overhead lamp from thetesting microscope with no UV peak at 365 nm but with wavelength in the 390–420 nm range.

INTENSITY

Overhead

Testing Microscope

G Color,STR FL
360

410

460

510

560

610

660

710

G Color,NON FL760

WAVELENGTH (nm)BLUE FLUORESCENCE IN DIAMONDGEMS & GEMOLOGYSUMMER 2021

119

N3 FLUORESCENCE RESPONSE
1.2

Sample 0455
Sample 3977

NORMALIZED N3 AREA

1.0

0.8

0.6

0.4

0.2

0
200

250

300

350

400

EXCITATION WAVELENGTH (nm)Figure 16. Two samples containing N3 were excitedwith narrow bands of light and the emission spectra
were recorded. The area under the N3 zero-phononline was calculated for each spectrum and plottedagainst its excitation wavelength, with both plots
being normalized to one. Data obtained from Luo
and Breeding (2013).

urement and visual observation with a fixed UV content of specific known wavelength is essential.
Fluorescence N3 Center Excitation Curve. An important metric when considering a light source is
how much UV light it contains. However, quantifying the amount of UV contained in a light source is
not a straightforward thing to do in a way that is useful in understanding the effect of fluorescence intensity. Different wavelengths of UV light will excite
the N3 defect at different efficiencies, meaning different excitation wavelengths of a given intensity
will produce different intensities of emission.
To get an idea of how the different wavelengths
affect the N3, we used data from a previous study
where two samples were run on a luminescence
spectrometer that excited the sample with a narrow
band of light and recorded the emission spectrum(Luo and Breeding, 2013). Figure 16 shows the normalized area under the N3 zero-phonon line against
the excitation wavelength. We can see that from 225
to 275 nm, there is essentially no emission for theexcitation used. From 275 to about 340 nm, there isa slow increase, and then from 340 to 395 nm there

120

BLUE FLUORESCENCE IN DIAMOND

is a rapid increase. Above 395 nm, the excitation
band overlapped too much with the N3, so accurate
area measurements were not possible. The shape ofthese plots tracks with the previously reported photoluminescence excitation (PLE) spectrum of the 440
nm side band of the N3 (Luo and Breeding, 2013).
A light source used to inspect or grade diamonds
may contain a distribution of UV light intensities asa function of wavelength. Each of these wavelengths
will interact with the N3 defect and, based on the efficiency of absorption and fluorescence reaction, contribute varying amounts to the total visualfluorescence. If we use the area under the N3 zerophonon-line (ZPL) as a proxy for visual fluorescenceintensity, we can see that the wavelength of the UV
source will affect the intensity of the fluorescence.
In theory, based on the chart in figure 16, we would
expect a stone containing only N3 excited with agiven intensity of pure 350 nm light to be approximately one-third as bright as the same stone excitedwith an equal intensity of pure 375 nm light.
Figure 14 shows the UV component of a numberof common light sources, all normalized to the full
spectrum’s value at 560 nm. If we take these spectra
and multiply them by the normalized N3 area
curves and integrate that total area, we can compare
the relative fluorescence brightness we would expectfrom each light source. To compare different lightsources, we calculated the integrated area under thenormalized N3 area excitation curves (N3) multiplied by the UV component of the listed light source(normalized to 560 nm) from 360 to 395 nm. In table
5, we see some surprising results. The expected fluorescence brightness for the table-down color measurement system with 0% UV LED power is not 0,
but is over half the value of the system at 100% UV
LED power, due to a slight UV component of theLEDs used to generate the “white light” portion of
the spectrum. This is important to note becauseeven a system labeled as a “white light” source may
contribute to fluorescence.
Blue Fluorescence Effect on Color and Brightness.
GIA has consistently used approximately 2% UVcontent in the GIA DiamondDock. Using the GIA
DiamondDock as a standard for UV content (table5), if diamonds with similar color but different fluorescence intensities are brought into an environment with higher UV content—such as outdoors
under sunlight—we would not expect any observable change in color or brightness among diamondswith fluorescence descriptions of None, Faint, or

GEMS & GEMOLOGYSUMMER 2021

Medium. In contrast, we should see changes in colorand brightness in diamonds with Strong or Very
Strong fluorescence that make them appear morecolorless in sunlight. If we view these same examples in an environment with reduced UV content(such as most office lighting), the diamonds with fluorescence descriptions of None, Faint, or Medium
will appear unchanged while those with Strong or
Very Strong fluorescence will appear more yellow
and less bright.
Blue Fluorescence Effect on Transparency. When weview a nonfluorescent diamond, we typically see avery distinct pattern with sharp boundaries between
dark and light areas (figure 8, right) created by the
facet arrangement. If we look at this pattern in a diamond with Very Strong fluorescence, in a lighting
environment with some UV content (figure 8, left),
we see that the fluorescence makes the stonebrighter; however, because the dark areas are lessdark and the light areas gain a blue hue from the fluorescence, contrast is reduced. For this study, we focused on the round brilliant cut; other shapes or
cutting styles were not evaluated and may be affecteddifferently. From our MTF studies, contrast loss wasmeasureable at line-pair frequencies less than 20
lp/mm for plates with Medium, Strong, or Very
Strong fluorescence. Five senior diamond graders at
GIA were given the same micro slide used in the
MTF measurements and asked to note the highest
frequency of line-pairs they could resolve in a standard grading environment with a 10× loupe. The
graders reported anywhere between 32 lp/mm and 48
lp/mm as the limit of their visual resolution. Therefore, any contrast difference measured at frequencies

TABLE 5. Comparison of N3 excitation curves by various
UV light sources.
Light Source

Total Area
Under Curve

GIA microscope overhead light

11.09

Office light

16.41

Color measurement system with no UV

18.18

GIA microscope well light

18.71

Color measurement system with UV

30.95

GIA DiamondDock

36.62

Daylight – cloudy day

40.22

Daylight – sunny day

73.96

less than 40 lp/mm could potentially be noticeable
to a diamond grader under controlled conditions.
Several studies (Gu et al., 2019; Eaton-Magaña etal., 2019) have suggested that a milky or hazy appearance observed in Fancy white, type IaB diamonds isdue to nano-inclusions or complex structural defects
known as dislocation loops. Light scattering (see the
Tyndall effect in box A) caused by these imperfections creates the milky or hazy appearance in these
diamonds and affects their transparency. When observed with a diamond in the face-up orientation,
scattering will cause a contrast loss in the facet pattern similar to that caused by fluorescence. When adiamond is strongly fluorescent and also contains
light-scattering structural defects, these producers ofcontrast loss seemingly combine to cause a noticeably milky or hazy appearance.
Characterization of Contrast Loss Caused by Fluorescence and Haziness. Both fluorescence and lightscattering cause contrast loss when looking at a diamond face-up, and it can be difficult to distinguishbetween their effects. We designed a bulk contrast
evaluation system and explored its ability to measurecontrast loss. In this evaluation, no considerations
were made regarding the cause of haziness in thesample, which could be fluorescence, nano-inclusions, or dislocation loops—we only measured thecontrast in the face-up pattern. The x-axis units infigure 17 are lightness, where 0 is darkest and 100 islightest. The y-axis units describe the quantity of pixels in the image that have the corresponding lightness value. For a relatively clear diamond (top), we
see the largest distance between the majority of dark
pixels and the majority of the lightest pixels. As the
sample gets hazier (middle and bottom), we see contrast loss (and a shrinking of this distance between
the majority of the lightest and darkest pixels)—pixels that make up the dark areas in the facet pattern
are lighter, and the lightest areas in the facet pattern
are darker. The authors consider this bulk contrast
evaluation method a relatively easy and consistent
way to evaluate haziness and its impact on clarity
grading of diamonds.

CONCLUDING REMARKS

Total integrated area = ∫Among the D-to-Z color gem diamonds that fluoresce, our results strongly suggest that the color will
be noticeably impacted only in those few that show
Strong to Very Strong blue fluorescence. Of course,
the degree of impact depends largely on the amountBLUE FLUORESCENCE IN DIAMONDGEMS & GEMOLOGY

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N3 × UV dλ
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BULK CONTRAST EVALUATION
60

Darkest Areas

Lightest Areas

HazierDENSITY

40

20

0
10

30

50

70

90

LIGHTNESS

Figure 17. The bulk contrast measurement used in our study provides a relatively easy way to evaluate the face-up
appearance of diamonds.

of UV component in the lighting source, meaningthat a standardized lighting environment with afixed amount of UV is critical for consistency in thecolor grading of diamonds. The GIA DiamondDock’s
UV content of ~2% provides an appropriate compromise between the UV content in sunlight (3% to5%) and that in most fluorescent and LED illumination sources used indoors (less than 1%).
We observed that stronger fluorescence producessome contrast loss in gem diamonds. However, ourresults show that this contrast loss from strong fluorescence does not by itself cause the milky or hazyappearance observed in some diamonds by the trade.
Atomic-scale defects in the diamond structure or
nano-inclusions appear to be the main causes of themilky or hazy appearance. The occurrence of strong

ABOUT THE AUTHORS
Dr. Luo is a director of metrology and quality control, Mr. Nelson
is a senior metrologist, Mr. Ardon is a research associate, andDr. Breeding is a senior manager of analytics, at GIA in Carlsbad, California.

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fluorescence in combination with these features may
cause a diamond to appear even milkier or hazier, but
we saw no evidence that strong fluorescence alone
produces noticeable haziness in diamonds that did
not already contain light-scattering structural defects
or nano-inclusions. The bulk contrast method presented here may also serve as a reliable way to quantitatively evaluate the effect of contrast loss on
apparent transparency in future diamond grading
processes. We are reviewing these quantitative andsemi-quantitative results to see how they might be
included in GIA grading reports. We believe this new
information will help to create more accurate information in the trade and ultimately allow consumers
to select diamonds based on unbiased scientific andaesthetic factors.

ACKNOWLEDGMENTS
The authors thank GIA’s Tom Moses, Wuyi Wang, Tsung-Han
Tsai, and Jennifer Hwang for many helpful discussions. Assistance from Mitchell Lyn and Curtis Berry in collecting image
data at GIA is also greatly appreciated. The authors are gratefulfor the constructive comments from the three peer reviewers of
this article.

GEMS & GEMOLOGYSUMMER 2021

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