2005

GIA Issues Details on Cut Grade

January 2005

The GIA slowly is leaking out news about its planned diamond cut grade. The new cut grade is scheluded to start appearing on reports next year. We discussed the issue of whether GIA should issue a cut grade last month (we’re still not convinced.) But let’s take a look at the research that led to the new grade, a lot of which is extremely interesting.

It was almost a century ago (1919) that a Belgian mathematician, Marcel Tolkowsky, wrote his PhD thesis on diamond design and invented what we now call the “Ideal Cut” in the process. The GIA’s effort is an attempt to figure out once and for all what constitutes a well-cut stones. Its findings are based on ten years of research, including 70,000 observations of nearly 2,300 diamonds by more than 300 individuals.

It starts by noting that there are three factors that contribute to a beauty of a diamond:

* brightness, or “brilliance” — the appearance, or extent, of internal and external reflections of “white” light;

* fire—the appearance, or extent, of light dispersed into spectral colors;

*scintillation—the appearance, or extent, of spots of light that flash as the diamond, observer or light source moves (sparkle), and the relative size arrangement, and contrast of bright and dark areas that result from internal and external reflection seen while that diamond is still or moving (pattern.)

No Ideal Cut

One thing that GIA has said all along is that there is nothing to back up the term “Ideal cut,” and that other stones can be equally beautiful. In fact, it specifically notes that: “personal and international market preferences should be accounted for. Diamonds with different appearances can be found within each cut grade, so individuals need to look at the diamond itself, not just its grade, to choose the one they like best.”

Because of this, the new grade is more complicated than the traditional method of cut grading, where one just judged if a diamond fell within certain parameters. In other words, judging a diamond’s cut is more than just knowing the numbers for tables and depth percentage, they say. “Proportions need to be considered in an interrelated manner,” says an article in GIA’s Gems and Gemology. “The combination of proportions is more important than any individual proportion value.”

Also important is the design of the stone (as evidenced by its weight ratio and durability) and its craftsmanship (as shown by its polish and symmetry.)

This can be seen in the five different categories of diamonds that are shown in the G and G article. In the fifth (lowest) category, you have stones with tables that range from 54 to 61. Some of them might be rated higher by traditional measures, but their extremely thick girdles and steep pavilion angles place them in the bottom categories of the GIA.

A steep pavilion angle can get a stone placed in the fourth category, and a steep crown angle gets into the third category. Darkness and higher crown height means a stone gets no higher than the second category.

In the first category, which is the best, the tables range all the way from 54% to 61%. It is one of the striking things that getting a top grade means not just having the right of set of numbers and angles, but on not having things that could possibly distract from the stone’s beauty a thick girdle or darkness in the pavilions.