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Flawed Cut Grading Systems
Demand for diamonds with top cut grades endorsed by internationally respected institutions has increased dramatically in the past decade since Sarin Technologies innovation of digital proportion-measuring instruments. This technology enabled labs like of the American Gem Society (AGS) and Japanese labs to offer diamond reports with cut grades. This led to a lucrative new market of ‘cutting for paper grades’.
However all existing cut grading systems are features - oriented, rather than results or benefits - based.
Grading by Features
All laboratories that grade cut take the worst scoring feature and assign that score as a cut grade. This ignores the complex interrelationships of facets as light reflects and refracts on its passage through a diamond. Each lab has selected tolerances of varying strictness, but all base the crown and pavilion angles on a range of tolerances applied to Tolkowsky’s design.
The problem is that expressing tolerances as unrelated maximum and minimum values defines orthogonal rectangular zones on a graph. The upper left and lower right of these zones are good combinations for crown and pavilion angles, while the upper right and lower left are poor performing combinations. Standards need to take account of the combined effects of all proportions.
Tolkowsky did not intend his parameters to be used with tolerances chosen for convenience. The misuse of his design and the attraction for diamond cutters to maximize yield has resulted in many diamonds being cut within the NGZ. The table shows the maximum crown and pavilion angles and table size for some cut grading systems in use today. Scores for the worst performing proportion combinations using HCA show poor results for diamonds that are sanctioned by the most popular institutions. Those standards that score well are very narrow and cutters who achieve these grades often suffer large cutting weight losses, hence few diamonds submitted to these institutions.
Labs (especially in Europe) with broad standards, or those like the GIA, with no a cut grades at all, have tended to be receive the ‘rejects’. The AGS even offers a pre-grading service for partly polished diamonds so the final product can gain its top grade and thus fetch a considerable premium.
Possibly the worst affected in terms of loosing market share has been the respected GIA Gem Trade Laboratory. However the GIA is the only lab doing the right thing by not offer a cut grading service. The GIA have correctly concluded existing standards are flawed and they have been working on establishing a better system. They would rather live with the current criticsm, than face the embarrassment of retracting or changing its system as other labs will be bound to do.
The absence of a cut standard the GIA means they often receives diamonds for certification1 that are outside the proportion standards of labs that specialize in cut certification. Contrary to popular belief I have found just as many GIA-graded diamonds achieve high HCA scores and visually outperform diamonds certified in the top grade of cut specialist labs like the AGS. The reason for this is that many of the diamonds submitted to cut specializing labs fall within or close to NGZ’s.
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Table 10. HCA scores and comments for the worst performing proportions in the top grades of a number of grading systems in use around the world today.
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Grading
system |
Cut adviser score |
Top grade maximum table size |
Top grade maximum crown angle |
Top grade maximum pavilion angle |
Cut adviser comments
e, vg, g, f, p |
| AGA |
1.0 |
58.0 |
34.7 |
40.8 |
Excellent |
| AGJ |
3.2 |
58.0 |
35.0 |
41.3 |
Very Good |
| AGS* |
3.9 |
57.5 |
35.8 |
41.2 |
Very Good |
| AGT |
1.2 |
60.0 |
35.0 |
40.7 |
Excellent |
| CGL |
3.2 |
58.0 |
35.0 |
41.3 |
Very Good |
| EGL |
1.2 |
57.0 |
35.0 |
41.0 |
Excellent |
| HRD |
5.9 |
66.0 |
37.7 |
42.2 |
Good |
| IGI |
4.4 |
6.00 |
36.0 |
41.3 |
Good |
| Russian |
5.3 |
62.0 |
35.0 |
41.5 |
Good |
Programming HCA into Sarin rough diamond analysers will enable cutters to avoid cutting poor proportion combinations when planning the cutting a rough diamond.
Recent advances by Sarin to enable uploading Sarin data for buyer convenience will allow the use of HCA to filter out NGZ diamonds in business-to-business and consumer purchases. We will enable direct import of data from Sarin to advanced HCA. This will give ease of use and allow us to process more data.
Buyers will also be able to utilize the Internet to identify diamonds that meet predetermined HCA requirements, just as it is possible to utilize web search functions for the other 3 C’s on business-to-business and business-to-consumer websites.
1 The GIA Brilliance study (Hempill, et al., 1998) examined proportions of 67,621
diamonds submitted to the GIA GTL and found less than 3% were within a commonly accepted range
for a top grade.
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