CUT
WEIGHING IN
Weight Ratio within the GIA Diamond Cut Grading System
BY ILENE REINITZ,
PROJECT MANAGER, GEMOLOGICAL INSTITUTE OF AMERICA, NEW YORK;
RESEARCH AND DEVELOPMENT MANAGER, GEMOLOGICAL INSTITUTE OF AMERICA, BELGIUM,
AND AL GILBERTSON, RESEARCH ASSOCIATE, GEMOLOGICAL INSTITUTE OF AMERICA, CARLSBAD
RON GEURTS,
iamonds are sold by weight and not by
visible size or measured diameter. Given
two diamonds of the same average diameter,
quality factors and visual appearance — but different weights
— the heavier diamond will sell for more.This design aspect,
called Weight Ratio, is used in the Gemological Institute
of America (GIA) Diamond Cut Grading System to assess
hidden weight. If a round brilliant possesses more than a
specified tolerance of hidden weight that does not contribute to a larger appearance or better performance, the
diamond will receive a lower cut grade.
The GIA Diamond Cut Grading System calculates a
Weight Ratio value for each round brilliant diamond graded
by the laboratory.The volume of the diamond to be graded,
calculated from the rounded, reported proportions of the
stone, is compared to the volume of a specific theoretical
reference diamond using the same average diameter as the
submitted stone. Because both volumes are for diamond,
and have the same specific gravity,the value for this volume
ratio is identical to a weight ratio.
This reference diamond (Figures 1 and 2) has the following proportions:
• Table: 57%
• Crown angle: 34.0°
• Pavilion angle: 40.7°
• Girdle thickness: 2.5%
• Star length: 50%
• Lower half-length: 80%
• Additional features: excellent symmetry and no culet,
painting or digging out.
Calculated values derived from these proportions are
total depth, 60.0 percent; crown height, 14.5 percent and
pavilion depth, 43.0 percent.
These reference proportions define a Weight Ratio of
1.00,the lowest value for which this parameter is evaluated.
Round brilliants receiving Excellent proportion grades
most commonly have Weight Ratio values between 1.02
and 1.05.Diamonds with higherWeight Ratio values have
extra thickness in the crown,the pavilion or,most typically,
the girdle region.In some cases,extra thickness in all three
regions results in a higherWeight Ratio value and therefore
a lower cut grade.
Opposite: Cutter at William Goldberg Diamonds, New York.
Photo courtesy Eric Welch/GIA.
Figure 1. This profile diagram shows the relative proportions of the
theoretical round brilliant reference diamond.
Figure 2. This grayscale image was made with DiamCalc (version 3.2),
using the Photoreal function with office lighting and a 350 mm
viewer distance.
EXAMPLE CALCULATIONS
Figure 3. This proportional profile diagram shows the heavier round
brilliant used in Example 1.
Figure 4. This grayscale image, made with DiamCalc using the same
conditions listed in Figure 2, shows the effects of the higher crown and
deeper pavilion.
Figure 5. The profile of the reference stone, shown in green, is overlaid
on the proportional profile of the first example stone, shown in white.
The example round brilliant is clearly thicker in the crown, girdle and
pavilion than the reference stone.
1.A diamond with an average diameter of 10.00 mm
(Figures 3 and 4) is submitted to the GIA Laboratory
for grading services.The following rounded proportions
are obtained for this diamond:
• Table: 53%
• Crown angle: 35.0°
• Pavilion angle: 41.2°
• Girdle thickness: 4%
• Star length: 50%
• Lower half-length: 80%
• Additional feature: no culet
Calculated values derived from these proportions
are total depth,64.2 percent;crown height,16.5 percent;
pavilion depth,43.8 percent and weight,3.943 carats.
The weight for the theoretical reference diamond
is calculated using the same diameter as the diamond
to be graded — 10.00 mm.The resulting weight is
3.592 carats.To determine the Weight Ratio, the
derived weight of the submitted diamond is divided
by the calculated weight of the reference diamond:
Weight Ratio = 3.943 carats divided by 3.592
carats = 1.098
ThisWeight Ratio value of 1.098 falls into theVery
Good grade range. In this case, the total depth value,
calculated from the rounded proportions,is also large
enough to limit the grade.This combination — round
brilliants too heavy and too deep for an Excellent cut
grade — is fairly common (Figure 5).
2. A round brilliant with an average diameter of
4.70 mm is submitted for grading (Figures 6 and 7).
The following rounded proportions are obtained
for this diamond:
• Table: 55%
• Crown angle: 34.0°
• Pavilion angle: 40.8°
• Girdle thickness: 4.5%
• Star length: 55%
• Lower half-length: 80%
• Additional feature: no culet
Calculated values derived from these proportions are
total depth, 62.8 percent; crown height, 15.2 percent;
pavilion depth, 43.2 percent and weight, 0.405 carat.
The weight for the theoretical reference diamond is
calculated using the same diameter as the diamond to be
graded — 4.70 mm.The resulting weight is 0.373 carat.
To determine the Weight Ratio, the derived weight
of the submitted diamond is divided by the calculated
weight of the reference diamond:
Weight Ratio = 0.405 carat divided by 0.373
carat = 1.086
Unlike the first example, this diamond carries
almost all of its extra weight in the girdle region
(Figure 8).The total depth for this proportion combination is within the Excellent grade range,but the
Weight Ratio exceeds the threshold for the highest
grade and limits this proportion combination to a
grade of Very Good.
Figure 6. The proportional profile diagram of this round brilliant diamond
shows that it carries most of its extra weight in the girdle region.
USING WEIGHT RATIO
Weight Ratio values are not provided on GIA
Diamond Grading and Diamond Dossier reports
because they are not used as a separate parameter for
determining the overall cut grade.As the GIA Diamond
Cut Grading System evolved, the benefit of linking
all the grading aspects related to proportions to each
combination of rounded proportions in the system
became apparent.Associations between the appearance
and design aspects of the system and the six proportion parameters not only simplified the system, but
allowed for the development of a predictive tool, the
GIA Facetware® program.Rather than determine each
diamond’s individual scores for brightness, fire, various scintillation components,weight ratio,total depth,
durability, etc., and the resulting cut grade, the
Facetware program contains the results of precomputed
scores for each rounded proportion combination in
the system.This means that the estimated grade given
by the Facetware program,whether online or imbedded
in a measuring system, already includes the cumulative effect of all the grading system components related
to the reported proportions.The same six proportion
Figure 7. This grayscale image from DiamCalc, again using the conditions
listed in Figure 2, shows that additional girdle thickness does not produce
much variation in the face-up appearance, compared to the image of
the reference diamond.
Figure 8. The profile of the reference stone, shown in green, is overlaid
on the proportional profile of the second example stone, shown in
white. Here, the crown of the example round brilliant is a little
thicker than the reference stone, and the girdle is considerably thicker.
The pavilions show very little difference.
(A) EX - VG
(B) VG - G
Figure 9. These four profile diagrams show the volume increase
for each successive pair of examples. (A) The Very Good example
is slightly larger in the crown, pavilion and girdle than the
Excellent example, which substantially increases the weight ratio.
(B) The Good example is slightly larger than the Very Good in all
three sections. (C) The Fair example is significantly larger than
the Good example. (D) The Poor example is larger still, with an
extremely thick girdle.
(C) G - F
parameters that determine the estimated cut grade — in
the Facetware program or in GIA reports — also determine
whether Weight Ratio limits the grade, as in the example calculations above, and the five examples in the
chart below.
Chart 1 shows five sets of proportions, one set for each
grade range — Excellent,Very Good,Good,Fair and Poor
— along with the corresponding calculated total depth
percentages and Weight Ratio values.These examples are
provided to illustrate common proportions for which
Weight Ratio is a significant component of the cut grade.
Overlapping profile views of these five examples (Figure 9)
show how the volume of the diamond increases across each
grade boundary.
An estimated Weight Ratio value can be useful for
manufacturers to consider as they pursue maximum yield
and a particular cut grade.A simple formula can be used to
calculate a reference weight:
Reference weight = Average diameter multiplied
by Average diameter multiplied by Average diameter
multiplied by 0.003592
To finish estimating the Weight Ratio, divide the actual
weight of the diamond by this reference weight.The graded
diamond’s actual weight may differ slightly from the derived
weight because of rounded proportions, symmetry factors
Chart 1
Grade
Table%
Crown°
Pavilion°
Girdle %
Star %
Lower half %
Total depth %
Weight Ratio
(D) F - P
Excellent Very Good Good
57
35
41.0
3.0
50
80
61.3
1.04
56
36
41.4
4.5
50
80
64.3
1.12
56
37
41.6
5.5
50
80
66.2
1.18
Fair
Poor
53
38
42.0
7.0
50
80
70.2
1.27
52
39
42.6
9.0
50
80
74.2
1.42
Five examples, one from each grade in the GIA Diamond Cut
Grading System, show how the Weight Ratio increases as the cut
grade decreases.
and painting or digging out.However,an estimatedWeight
Ratio is easy to calculate and can provide a helpful warning
of diamonds that are pushing grade boundaries, especially
those weighing 0.50 carat or more.
When the estimated Weight Ratio is between 1.08 and
1.09,the diamond’s rounded proportions are quite close to
the border between Excellent andVery Good cut grades.
For such cases, it would be prudent to use the Facetware
program to check both the specific proportions of the diamond and the surrounding proportions.If a change of one
unit to table percentage,crown angle,pavilion angle or average girdle thickness changes the grade, the diamond may
be too close to that grade border for comfort. For most
cases in which the Weight Ratio is 1.09, the diamond is
likely to receive aVery Good cut grade. Similarly,Weight
Ratios between 1.16 and 1.17 occur for proportions close
to the border between cut grades of Very Good and Good.
Note that the relationship between diameter and weight
varies with the proportion combination.Although diameter is an important component of the overall volume, a
round brilliant is not a sphere;other proportions also make
important contributions.The conversion factor above works
for these reference proportions, but not for other proportion combinations,even similar ones,as shown in Chart 2.
Some diamonds with a highWeight Ratio also exceed
the total depth limits of the grading system. For more
information on total depth and other parameters,
see “Estimating a Cut Grade Using the GIA Diamond
Cut Grading System,” available for download at
www.gia.edu/diamondcut/08_tools_for_the_trade.html.
As shown in the examples, a total depth percentage is
calculated from the rounded proportions that are used to
determine the estimated cut grade.This rounded total depth
percentage may be slightly larger or smaller than the actual
measured and reported total depth percentage.An upgrade
to the online Facetware program in the autumn of 2009
eliminated total depth percentage as an input parameter in
order to encourage the use of the same proportion parameters used for GIA reports. Eliminating this parameter
increased the accuracy of the online estimated cut grades.
The program displays total depth calculated from the
rounded proportions as output, for comparison to the
reported total depth percentage.
Chart 2
reference example example example example
set
1
2
3
4
Table%
Crown°
Pavilion°
Girdle %
Star %
Lower Half %
57
34.0
40.7
2.5
50
80
57
35.0
41.2
4.0
50
80
55
35.0
41.2
4.0
50
80
53
36.0
41.2
5.0
50
80
53
36.0
41.2
5.5
50
80
Weight Ratio
Cut Grade
1.00
EX
1.08
EX
1.09
VG
1.15
VG
1.17
GD
diameter
mm
weight
ct
weight
ct
weight
ct
weight weight
ct
ct
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
15.0
20.0
0.154
0.230
0.327
0.449
0.598
0.776
0.986
1.232
1.515
1.839
2.206
2.618
3.079
3.592
12.122
28.733
0.166
0.248
0.354
0.485
0.646
0.839
1.066
1.332
1.638
1.988
2.384
2.830
3.329
3.882
13.103
31.060
0.168 0.177 0.180
0.250 0.264 0.269
0.357 0.377 0.383
0.489 0.517 0.525
0.651 0.688 0.699
0.845 0.893 0.908
1.075 1.135 1.154
1.342 1.417 1.441
1.651 1.743 1.772
2.004 2.116 2.151
2.403 2.538 2.580
2.853 3.013 3.063
3.355 3.543 3.602
3.913 4.132 4.201
13.207 13.947 14.180
31.307 33.060 33.612
The reference proportions are compared to four examples with
successively larger Weight Ratio values. These examples lie near
grade boundaries within the GIA Diamond Cut Grading System, and
show just a few of the ways that a round brilliant can be overweight.