 |  |
|
September 1998
Diamonds:Gemology Ground ZeroA big, fat zero is a good thing at the American Gem Society Lab When the American Gem Society Laboratory decided to assign a cut grade for round diamonds, it was unprecedented. Most labs avoid a cut grade because of differing opinions on what constitutes good cut. It's a matter of beauty and, after all, beauty is subjective. But AGS wanted to try anyway because of growing interest in cut as a diamond quality factor. The decision has proved popular with clients. AGS reports a 200% increase in the number of stones submitted to the lab in Las Vegas, NV, since last year. In addition, AGS "0" graded stones (the highest cut grade) command a premium at Rapaport diamond tenders on the Internet. What the lab did was set parameters based on the American Ideal Cut. The AGS "0" cut grade is not a strict definition by Tolkowsky's exact Ideal Cut proportions. Instead, the lab evaluates seven factors that go into defining good cut and rates each of them on a scale from "0" to "10." Then it assigns an overall cut grade of "0" to "10." "The scale means some '0' grades may be a bit better than others in the same range," says Peter Yantzer, lab director. "But one thing is certain: a '0' grade is an utterly beautifully crafted diamond. "The grade also fulfills the AGS tenets of empowering retailers with more information about the diamonds they sell and protecting consumers." Yantzer says he's always ready to discuss the finer points of a grade (before it's assigned) and will send a fax, if requested, telling the client what the lab is seeing. (The AGS Lab offers different tiers of diamond-quality statements. A Diamond Quality Document is a full-blown diamond analysis with all seven factors shown. It also offers less-detailed Diamond Quality Reports or Diamond Quality Consultations, depending on the client's needs.) All in all, it takes a lot to be a "0" at the AGS Lab. Five of the seven factors deal with proportion and can be measured within seconds with the Sarin Corp. DiaMension machine. The last two quality factors are polish and symmetry. Here's a closer look at each. 1. Average crown angle: 33.7° to 35.8°. Anything in this range receives a "0" rating for this portion of the examination; variances above or below receive ratings from "1" to "10." 2. Average pavilion depth: 42.2% to 43.8%. Anything in this range receives a "0" rating for this portion of the examination; variances above or below receive ratings from "1" to "10." 3. Average table diameter percentage: 52.4% to 57.5%. Anything in this range receives a "0" rating for this portion of the examination; variances above or below receive "1" to "10." 4. Average girdle thickness: Thin, medium, slightly thick. Girdles rated as thin, medium or slightly thick receive "0" for this portion of the examination. Very thin girdles receive a "1," extremely thin ones a "7" and thick to extremely thick ones from "3" to "10." 5. Culet size: pointed, very small, small and medium. These receive a "0" rating for this portion of the examination. AGS has defined a new standard: pointed culets are now acceptable to qualify as "0" grades. Culets that are slightly large, large, very large and extremely large merit grades from "1" to "10," depending on size. 6. & 7. Polish & symmetry: With the mathematical factors decided, the lab turns to the more subjective side of evaluating cut. Symmetry evaluations look at off-center culets or tables, out-of-line or wavy girdles, or characteristics with less of an impact, such as facets that are misshapen or don't point properly, as well as "naturals" (unpolished surfaces of the original diamond crystal often visible on a short length of the girdle). Polishing evaluations, meanwhile, examine characteristics such as abrasions, bearded girdles, pits and polish lines. AGS used to use charts that didn't allow any symmetry or polish flaws to be visible under 10X magnification. "But we realized that diamond cutting is an art and not a science," says Yantzer. Consequently, polish and symmetry are now rated by description, with a range from "0" to "10." Here's how the numbers break down:
Yantzer says polish and symmetry will only "drive" a grade if either is worse than the proportions. To merit a "0" grade or a "1" final grade, diamonds must exhibit superb polish and symmetry. Once all seven factors are evaluated, the lab assigns its ultimate cut grade as follows:
– by Robert Weldon, G.G.
|
