Dueling Cut Grades

By Gary Roskin, G.G., FGA -- JCK-Jewelers Circular Keystone, 1/1/2007

The American Gem Society and the Gemological Institute of America are in a showdown over which organization has the best system for grading diamond cut. The latest shootout took place in August at the annual midyear conference of the National Association of Jewelry Appraisers.

In the opening session of a daylong program called “Diamond Dazzle or Diamond Daze,” AGS director Peter Yantzer provided evidence that his organization's system is the most exacting, while GIA's Patrick Ball and Al Gilbertson showed why theirs is the easiest to use.

Yantzer discussed the science of ray tracing and demonstrated how images from the angular spectrum evaluation tool (ASET) relate to the AGS cut grade. Ball and Gilbertson, using knowledge from ray-tracing computer modeling as well as 70,000 human paired- comparison observations, said their grading scale allows for easy calculations based on Sarin measurements and visual observations.

The grading scales of both systems are close. While the AGS 11-grade scale is mathematically more discerning, GIA's five-point scale corresponds to what their research says is actually visible to human observation.

Diamond appraiser Michael Cowing examined both systems and provided charts showing where they agree. Cowing also noted that top AGS and GIA grades agree with the 1919 Tolkowsky proportions of 40.75-degree pavilion angles and 34.5-degree crown angles. GIA's top grade also agrees with the 41-degree pavilion angles and 35-degree crown angles of 1860s diamond cutter Henry Morse. Cowing noted that these proportions fall within the range of what he calls the “sweet spot” for cutting.

Cowing placed all of the potential combinations of GIA Excellent cut grades and AGS 0 and 1 cut grades on a graph and found that both systems have a sweet spot of approximately 56 percent for tables. That agrees with the conventional wisdom of the last century and a half.

For diamonds with a 56 percent table, the GIA sweet spot for crown and pavilion angles is 41.2 degrees for pavilion main angles and 34 degrees for crown main (bezel facet) angles. For AGS, it's 41.1 degrees for pavilion main angles and 33.75 degrees for crown main angles.

During the 20th century, pavilion mains got skinnier and showed more lower halves (lower girdle facets), increasing scintillation. However, decreasing the size of the mains meant less obvious fire.

Cowing believes the best balance between the area of the main reflections and the area of the halves is obtained with a 77 percent to 78 percent length of the halves. Perhaps more than coincidentally, the range of lower girdle lengths for possible GIA Excellents is 70 percent to 85 percent, with a sweet spot of 77.5 percent.