Sat, 24 Feb 2007

Letter to the Editor of the Australian Gemmologist

GIA’s letter to the Editor

This
journal article documents letters to the Editor exchanged between the
GIA cut study team and the cut group who wrote an article in The Australian Gemmologist

The original article from Volume 23, Number 1, January – March 2007 is not reprinted here, but a similar article was posted here on this websites on-line Journal.

Here is the GIA’s letter to the Editor:

Dear editors,

The
recent work by Sivovolenko et al. (“Diamond cut grading: Unintended
consequences and solutions,” First Quarter 2006, pp. 447-454) mentions
our work on Cut Quality grading in round-brilliant-cut diamonds (RBCs).
We found several inaccuracies in this article related to our research
and cut grading system, and therefore felt we should take this
opportunity to clarify some points for the record.

We
agree with Sivovolenko et al. that the current GIA and AGSRBC grading
criteria give different relative results for some sets of proportions,
but we disagree with the reasons they give for these differences. What
is not adequately mentioned by the authors is that there is much
overlap in the cut grades from the two systems, and that many of the
remaining differences in grades may result merely from the fact that
our system uses five grades, while the AGS system uses eleven grades.
We note that the diamond appearances in their figures 3(3) and 3(6) are
brighter, not darker, than the same diamond resting on a gray
background in figure 3(2); this would indicate to us a result counter
to the one provided by the authors. We disagree with the authors’
contention that the environment used in our research led to the
favoring of “steep/deep” diamonds over those with “shallower”
proportions. When diamonds stray too far in proportion space to either
of these areas they are deducted for in our cut grading system. We also
believe, but cannot confirm without further information from the
authors, that the areas being contested are areas at the borders of our
cut grade categories, and would contend that areas at grade borders are
often contested in any evaluation system.

Three
points in the section titled “Some Perceived Problems” warrant comment.
First, as stated in reference iv (again, the Fall 2004 issue of Gems & Gemology),
we knew the system needed more than just brightness and fire metrics
because those two factors alone gave an agreement rate of only 58% ; in
contrast, the finished GIA cut grading system for RBCs showed 92%
agreement between human observations of overall appearance and system
assignment of grades – the same agreement rate the observers
showed with each other. We would also like to emphasize that much of
our observation testing was devoted to “overall appearance” and not
just to brightness and fire as stated.

Second,
the Cut Group may feel that 38 million proportion combinations is
“excessive and un-workable for rough planning,” but both Sarin and OGI
have incorporated the GIA cut grading system with this number of data
points, and diamond manufacturers have been using these tools
successfully for eight months.

Third,
the authors’ point about the difficulty of using the same approach for
fancy shapes as we used for rounds is a valid one. We will need to
examine the feasibility of using our cut system for RBCs as we enter
our next stage of research on fancy shapes. However, we also believe
that an accurate and comprehensive system for grading the cut quality
of round brilliants should not be jettisoned merely because its
wholesale application to fancy shapes is uncertain. Future research
(including many more observation tests) will show whether our current
system is applicable to fancy shapes, or whether particular shapes
might need their own adapted cut grading system.

In
contrast to the statement that GIA or AGS “have yet to find an adequate
metric for scintillation,” footnote 3 on page 454 of the article shows
that the lack of a specific scintillation (sparkle) metric was based on
the comments we gathered during human observations when we asked why
one diamond had a superior overall appearance to another. People spoke
a great deal about “dark-zone distribution” but very little about
relative sparkliness. Also note in this same footnote we state that
during our research we found that a diamond’s relative sparkle was
accounted for in our brightness and fire metrics.

The
authors state that “green spectral flashes are rarely seen in a
diamond,” and assert that a very small aperture and a small light
source at a great distance are required to photograph such a flash. As
described in reference xi (Fall 2001 issue of Gems & Gemology),
although our model conditions use a point light source and a
hemispheric detector, both infinitely far from the virtual diamond, our
photographs of fire used real diamonds, an actual hemisphere 40.6 cm
(16 inches) in diameter, a hole for the light 0.95 mm in diameter with
a fiber-optic light 20 cm (17.5 inches) behind the hole, a 50 mm lens
and f/# of 16, yielding a camera aperture of 2.8mm, which is not “very
small”. This reference also showed the similarity between our modeled
results and photographs despite the differences in conditions. Both
photographs and human observations of hundreds of round brilliant
diamonds revealed a variety of expressions of fire. Most samples
displayed all spectral colors, including green. We agree completely
with the authors’ recommendation that readers try to reproduce this for
themselves: take a round brilliant, rock it back and forth under
spot-like illumination and look at the fire. See whether any of the
flashes are green – our experience indicates that some will be.

There
are also incorrect statements and implied assertions that do not
address our work directly. For example, the earliest diamond reports
containing proportions were Diamond Certificates issued by the AGS
starting in 1936, and the earliest effort to grade cut from proportions
was made by GIA in 1938, not in the mid 1950s as stated by the authors.
Also, to the best of our knowledge, ASET (both program and viewing
device) was developed by AGS and Dr. Jose Sasian; the article implies
that GH was involved in its development.

In
addition, we noticed technical mistakes in this article, such as almost
identical captions for figures 5 and 6, and a nonsensical illustration
for figure 8 with a caption that appears to belong to figure 6.

We
wish Sivovolenko et al. all the best in their research intentions, but
hope that any resulting articles are vetted conscientiously.

Sincerely,

The GIA Diamond Cut Group*

* The GIA Diamond Cut Group consists of:
Troy Blodgett
Ronald H. Geurts
Al M. Gilbertson
Barak Green
T. Scott Hemphill
John M. King
Mary L. Johnson
Thomas M. Moses
Ilene M. Reinitz
James E. Shigley

 

The Cut Group Letter to the Editor
Here
is the Cut Group letter to the Editor in reply. (We have named
ourselves the Cut Group for several years. The GIA had previously
referred to themselves as the Cut Study Team).

Dear Editor,

Thank
you for giving us the opportunity of responding to the letter of
critique sent to the Editor from the GIA Diamond Cut Team concerning
our paper in the April-June 2006 issue of The Australian Gemmologist..
This response from our Cut Group* represents a rare opportunity to
engage in open dialogue with the GIA Diamond Cut Grading research team,
and we also whould like thank them for this opportunity.

With
regard to differences between the GIA’s and American Gem Society’s
(AGS) grading systems; we have choosen only to highlight examples of
extreme grade differences between the two systems to avoid the very
argument outlined in the GIA Cut Grading research team’s letter “that
there is much overlap in the cut grades from the two systems”. We have
taken the GIA team up on this challenge by comparing the overlap
between the AGS’s 11 grades and GIA’s 5 grades for the common
‘Excellent’ table size of 57%. Comparing that overlap is difficult
because GIA arranged its pavilion axis opposite to the generally
accepted chart format that AGS (and other researchers) have
traditionally used. However, Peter Yantzer, the Director of the AGS
Lab, kindly provided us with the AGS ‘candidate’ charts in the GIA’s
format (as displayed at www.diamondcut.gia.edu/charts/
). A comparison of the 57% table size charts showed that GIA
‘Excellent’ proportions (outlined in black on figure 2) had only 26
good AGS0 and AGS1 matches, and 31 bad matches. Included in these bad
matches were several very bad matches—specifically 7 AGS 5s and 4 AGS
6s that would be graded Excellent by GIA (see table 1 and figure 1 for
details). We consider this to be a poor match and, in addition,
confusing for both consumers and the industry. The situation is worse
for table sizes below 52%, where GIA has no ‘Excellent’ proportion
sets. For example, at the 51% table size AGS has 17 AGS0 and 22 AGS1
‘candidate’ proportion sets that GIA would grade as Very Good, Good or
Fair.

Table 1 The good and bad matches between different grades of GIA and AGS ‘candidate’ grades.
(The authors deliberately excluded AGS 2 because in a 5 v 11 grade comparison no significant conclusions can be drawn.)

Good matches Bad matches
GIA Ex & AGS0 13 GIA VG & AGS1 12
GIA Ex & AGS 3 3
GIA EX & AGS1 13 GIA Ex & AGS 4 5
GIA Ex & AGS 5 7
GIA Ex & AGS 6 4
26 good matches 31 bad & very bad matches

Fig.
1. The AGS ‘candidate’ proportions by pavilion and crown angles for
table size 57%. The black overlain shape contains the GIA ‘Excellent’
proportions used to produce the data in table 1.

 

Page 3

We
would like to clarify the comments made concerning the diamond in
figure 3 of our paper, with a 57% table size and comparatively steep
crown (35.3° ) and deep pavilion (41.4° ). This diamond was given a
grade of ‘Excellent’ cut by GIA (you can check its position on the
chart in figure 2), but it did display very unattractive leakage just
inside the table when viewed over skin or on a dark background. This
leakage was not apparent when the stone was resting on the light
coloured (18% gray) tray used in GIA’s observation-based survey.

The
GIA team had noted that diamonds appear different when observed (and
also photographed) on backgrounds of differing shade. But, after
personal communication with a key member of the GIA team, at the GIA
Symposium August 2006, we do not believe they understand fully why.

We
will show that the way our eyes adapt to a brightly lit and light
colored background reduces our ability to make fine distinctions
between diamonds. GIA’s Common Viewing Environment (CVE) that was used
in their survey environment had a background and stone trays of a shade
of 18% gray, with twin diffused fluorescent tubes mounted less than 33
cm (13 inches) above the diamonds. This causes an observers eyes to
‘overexpose’, creating a bad environment for discerning differences
between diamonds of differing brightness. The authors of this response
spent a little time with the commercially available Diamond Dock®
version of the CVE at the GIA Symposium in August 2006, This confirmed
that the viewing environment is quite similar to a diamond dealer’s
twin fluorescent desk lamp and viewing position. Yet, when asked to
evaluate the brightness of a diamond, buyers and dealers usually do not
use the desk lamp for this purpose. Instead, they typically examine
diamonds under the edge of the desk, with their body turned away, using
ceiling lighting near a window or even in a hallway under spot
lighting.

At the Symposium, the Cut Group authors
conducted a brief experiment with GIA’s Diamond Dock® CVE and took the
two photographs shown in figure 2. We apologise for the quality of
these images; but we were stopped by a GIA Director who protested that
we were conducting experiments “to use against GIA”. Note that our
eyes, and a camera with auto exposure, adapt to the background
brightness in a similar manner. What we ‘saw’ was indeed similar to the
photographs shown in figure 2.

With the twin fluorescent lamps
turned on (Fig. 2, LHS, when looking at these sets of coloured cubic
zirconia , which were used for the poster presentation of Sergey
Sivovolenko and Yuri Shelementiev, the princess and oval cut stones
appeared brighter than the ideal cut rounds. With the lights turned off
and only with ambient room illumination (Fig. 2, RHS), the relative
brightness of these various cuts gave the opposite results, the results
one would usually expect to see, for the round stones appeared
brighter. It is indeed unfortunate that as the stones were adhered to
black cards, as this somewhat lessened the overall effect of the bright
background illumination that we were seeking to demonstrate.

Fig.
2. These coloured cubic zirconia of different cuts were photographed
inside GIA’s common viewing environment Diamond Dock®. The photograph
on the left was taken with the fluorescent grading lights turned on.
For the photograph on the right the lights were turned off and only the
ambient room light was illuminating the stones. Note that with the
lights on the oval and princess cuts appear brighter than the well cut
rounds. The results are the complete opposite when the brightness
grading lamps are turned off, with the rounds appearing brighter. The
reason for these differences is that the camera’s auto exposure adapts
to the background in a similar way as our eyes.

 

Page 4

The
GIA authors’ contention that they have struck a balance between shallow
and deep diamonds does not concur with the widely accepted Tolkowsky
standard. Holding the pavilion angle at 40.8°, the GIA ‘Excellent’
grade’s average crown angles for table sizes between 52% to 62% are
around 1.5° steeper than Tolkowsky’s 34.5° crown angle. Alternatively,
holding the crown angle constant, GIA’s Excellent proportions have an
average pavilion angle that is 0.3° deeper than that recommended by
Tolkowsky. Clearly the GIA cut grading system favours deeper diamonds
than the generally accepted optimum proportions.

The GIA
team wrote in their letter: “we state that during our research we found
that a diamond’s relative sparkle was accounted for in our brightness
and fire metrics”. Unfortunately, to date, GIA have never
reported any metric that measures a “diamond’s relative sparkle” or
scintillation. As their brightness and fire metrics had only a 58%
match to the observation results, and since the GIA have never
developed a sparkle or scintillation metric, how can they claim a 92%
match between the metrics and observation results? This seems to be a
circular argument? It would appear that GIA’s first 10 years of virtual
diamond modeling study have been of little or no value. This is
unfortunate. For GIA does not, as they agree in their letter, have the
tools to develop cut grading systems for fancy shaped diamonds.

Our
reference to 38 million proportion combinations as being excessive was
related to fancy shaped diamonds. Such a data base would be thousands
of times larger and more complex than that for the round stone. Also it
probably would be unworkable for rough diamond planning.

The
round data base is relatively simple, and the vast majority of the 38
million data points are of no use to manufacturers who are only
interested in the boundaries of Excellent and Very Good cuts. After GIA
released its cut grading charts in February 2006, our information is
that sales of the expensive Facetware® in Sarin and Ogi rough diamond
planners have all but ceased.

 

Page 5

We
did not claim that green is never seen in polished diamonds. What we
said was “rarely”. We too hope that all readers will do some simple
observations in normal lighting too to discover for themselves how rare
green flashes are. It would appear that the GIA team has confused what
the eye can see when a sparkle of light from a diamond shines onto a
surface, such as a dome, with what the eye would see if the dome was
removed and an observer looked from this position with their naked eye.
Most flashes we see with our eyes appear white because the spectral
colours would have merged together. To help explain why green is rare
we have paraphrased a good explanation of ‘fire’ by Anton Vasiliev that
was published 12 years ago in a work still available in full at www.gemology.ru/cut/english/rainbow/5.htm )

Consider
the observer, which GIA did not in their published studies. For this
observer with a pupil diameter of 3 mm, a viewing distance of 25 cm
creates an angular size of 0.7° to allow for the possibility of light
entering the eye. For an observer to see a ‘pure’ spectral colour, a
gem must have a high ‘fan’ of angular dispersion. But because green is
in the central region of the spectrum, there is a high chance that
green will be mixed with other colours, and appear whiter (see figure
3, left side for an example). For this reason, the greater the distance
between the gem and the observer, the more pure the ‘fire’ one can see.

Now consider the type of illumination , which GIA also did not
do in their published ‘fire’ studies. To create a full spectrum, a
prism should be illuminated with a light that has a very small angular
size—such as that from the sun, a fiber optic source shining through a
pin hole, or a small lamp at a very great distance. In contrast, when a
light is larger and closer, as in most indoor situations, the light
beams fan out and cause the spectral colors to mix, degrading the
purity of the observed color. Hence, good green fire requires
illumination of the gem from a light source of small angular size. The
source’s angular size affects the color purity in a similar way as that
of the the size of the pupil in the observer’s eye..

Fig. 3 These images show OctoNus Effective Total Angular Size (ETAS) DiamCalc models for various viewing conditions. (Click on the pictures to open larger images)

The image on the left
is based on the information provided in the GIA letter to the Editor
and shows the full spectrum of flashes that could be seen on the
surface of a dome. On the lower right of the left side image a small
circle shows that part of the flash that could enter a 4 mm pupil if
the dome was removed and an observer looked from this position with
their naked eye. Under these circumstances the flash would appear white
because the spectral colours would merge together. If the observer
moved a little up and to the left they would see a reddish flash, and
if they moved down slightly to the right, the flash would appear blue
as in the example shown here www.gemology.ru/cut/english/grading1/5.htm. The image on the right
shows the probability of the type of flash an observer might see from
25 cm if the source of light was 10 mm in diameter and was positioned 2
metres away from the diamond. In reality (with the dome removed) the
probability of seeing green is very small.

As we have shown
in figure 3, the illumination and observer are just as important as the
diamond itself. We believe the GIA used inadequate illumination models
for their virtual studies, and to the best of our knowledge, they have
not studied or published any work concerning observer’s eyes or their
vision systems.

The diamond on the left of figure 4 has been
photographed in office lighting and shows predominantly blue coloured
flashes, with some red and yellow. As we have explained green
flashes are harder to see if the light source is large in size, and relatively close
to the diamond. The diamond on the right shows some green; for it was
photographed in partly shaded direct sunlight, effectively parallel
rays of light. In addition, both pupil and camera apertures’ are small
because we are outdoors in very bright daylight. Our narrow pupil
aperture allows us to see some green flashes without the merging of
yellow and green or blue and green. An additional explanation can be
found here at
www.gemology.ru/cut/english/grading1/4.htm

Fig.
4. The 15 ct diamond on the left has been photographed in office
lighting where green will rarely be seen because the lights in the
office have a large angular size. The 9 ct diamond on the right shows
some green; for it was photographed under a broad leafed tree in partly
shaded direct sunlight, effectively parallel rays of light. In very
bright daylight both pupil and camera apertures’ are small which allows
us to see more green flashes without the merging of yellow and green or
blue and green. Both photographs were taken with the same Canon Ixus
camera, set to auto exposure mode, with the diamonds positioned on
author GH’s fingers.

 

Page 6

After
the close of the GIA Symposium in San Diego all four authors were able
to experiment with (but not photograph in) the GIA Diamond Dock® as we
all attended a GIA Cut Grading Course.

We noted different
instructors assessed ‘brightness’ using differing lighting/observer
angles with the light box (and other light sources, such as the small
fluorescent lights on microscopes. When asked about the correct
lighting/observer angle, the instructors could offer no consistent or
quantifyable recommendation.

In the original Diamond Dock®
Users Manual, a recommendation is made that the observer views diamonds
at an angle of 45° to the vertical plane, which we believe was the same
angle observers used for the survey testing of 70,000+ observations
that formed the basis of GIA’s new Cut Grading System. Anecdotal
information from volunteers who participated in GIA’s observation
testing seems to confirm that observational tests were performed in a
seated position that would have subtended an angle of 40° to 45°
between the stones and the centre of the lights (depending on observer
height).

When the incident light from the fluorescent tubes
comes from this angle, slightly shallower diamonds have less brightness
in Diamond Dock®, whereas Tolkowsky proportioned and deeper diamonds
appear brighter. We were able to demonstrate how important this viewing
position is to two GIA instructors, who by their own choice, viewed
diamonds from a much higher view point than 45°—one where the incident
light would have been subtending an angle of approximately 20° to 30°
relative to the vertical and the light source. We had with us two
diamonds of similar colour, clarity and diameter. One diamond of
1.19ct, with proportions very near that of Tolkowsky’s, had been graded
Excellent by GIA Gem Trade Lab. The other shallow 1.16ct stone graded
Very Good (a border line Good when using Facetware™). From the higher
viewing angle, both instructors preferred the shallow stone for
brightness (and were split one each for ‘fire’ using the instrument’s
LED lighting only.

Figure 5 shows ETAS data based on images
produced from three dimensional models of these two stones made with
DiamCalc® along with the actual steep deep diamond used in our original
article. This demonstration can be used to explain why the shallower
stone appears brighter when the incident light comes from a higher
angle; for the shallower diamond draws and returns more light from
directly above, while as round diamonds get deeper they draw more light
from lower angles closer to the horizon. Readers are invited to use
similar modeling software at this website: www.cutstudy.com/cut/english/grading1/sphere1.htm

Shallow 1.16ct GIA VG ‘Ideal’ 1.19ct GIA Ex Deep GIA 1.28ct Ex
Potential to see the Diamond Dock lights in a diamond from a 45° viewing angle
Potential to see the Diamond Dock lights in a diamond from a 23° viewing angle
Fig.
5. Effective Total Angular Size (ETAS) images modeled in DiamCalc based
on 3D scans from three actual diamonds. The flares of light show the
potential and direction that light in the environment around each
diamond could come from to cause a sparkle in the face up position. The
lines mark 10° increments on a virtual dome, with the zenith in the
centre facing the observer. The diamond is shown as a small blue dot in
the centre. The 6 arcs in the upper rows represent the position of the
diffuser over two fluorescent tubes in Diamond Dock.

The
steeper lighting-viewing angle makes most round diamonds look brighter,
which probably explains why Diamond Dock® users seem to naturally
prefer to view diamonds from higher angles than the original designers
intended. We believe GIA would have arrived at shallower Excellent
proportion parameter sets had the observers in their observational
tests been directed to use a higher viewing angle, but then these
proportions may have been too shallow.. However, as we have already
shown, the light background and very close proximity and brightness
lights in the CVE lighting box its narrow source of angular size of
lighting and lack of consistency in viewing angles make this method of
cut grading useless. Basing a cut grading system on observation and
survey results from this method is, we believe, flawed. In particular
this lighting environment would have even less validity for brightness
grading of fancy shaped diamonds, many of which tend to gather very
little light from the illuminated area in the CVE.

 

Summary

In
summary we consider that the round brilliant cut grading system
established by the GIA Diamond Cut Team is below the standard expected
from The World’s Foremost Authority in Gemology™. In
our article and this exchange of letters we have outlined a number of
shortcomings: when the same diamond can be graded with very different
cut grades by the AGS and the GIA, two of the worlds most respected
diamond cut grading labs, then neither the Internet savvy diamond
buying public, nor the trade can feel confident. Observers using the
common lighting environments had only a 58% with light return and fire
metrics from GIA’s previous virtual studies. The implied use of an
unpublished metric for scintillation, and the inadequacy of light
return and fire metrics bodes poorly for the future GIA’s grading of
other shapes of diamond cuts means that round diamonds will continue to
dominate the market (one of the main concerns noted in our original
article). The issue of ‘green fire’ shows an apparent lack of
understanding of the GIA Diamond Cut Team of common illumination
sources and of human eye sight. Finally we have traced some major
failings in their research work to the Diamond Dock(R) or Common
Viewing Environment that was used to assess diamond brightness; the
background is too light a shade of gray and the fluorescent lights are
too bright or too close, the observers angular orientation relative to
the diamond and the light source was not regulated and the illumination
area is too narrow an angular source to be useful for grading diamonds
with a range of different proportions. The GIA Gem Trade Laboratory
began commercially issuing grading reports with cut grades for all D-Z
coloured round brilliant cut diamonds in January 2006. The grading
reports being issued by the GIA and advice given by its associated
Facetware(tm) have already influenced the pricing of tens of billion
dollars worth of diamonds in the past year. We urge the Gemological
Institute of America to consider our arguments concerning its previous
research and withdraw its diamond cut grading system immediately.

Kind regards, The Cut Group

* Members of the Cut Group include Sergey Sivovolenko, Garry Holloway, Yuri Shelementiev and Janak Mistry

 

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