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GIA Master Stones: Only Precision Will Do

May 11, 2009 8:33 AM   By Gems & Gemology
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RAPAPORT... The following article is provided courtesy of Gems & Gemology.

When it comes to diamond color grading, most people have difficulty distinguishing between two or even three full grades. When selecting Gemological Institute of America (GIA) master stones, though, graders must push the limits of visual tolerance, since two masters of the same grade are virtually indistinguishable. This section of the Winter 2008 Gems & Gemology article “Color Grading ‘D-to-Z’ Diamonds at the GIA Laboratory,” by John King, Ron Geurts, Al Gilbertson and James Shigley, details how master stones are selected and used in the GIA diamond-grading process.


Making the grade. As the article explains, only diamonds whose color lies at the upper limits of visual tolerance of a particular grade can serve as a master for that grade. Thus, a G master would be at the border of an F grade; all diamonds with slightly more color would be graded as G, while those with slightly less color would receive an F grade.


All master stones undergo the same grading procedure as commercial diamonds. Multiple graders independently assess them, and their consensus decision is supported by a color measurement device.


While precision color is the first criterion for a GIA master stone, there are other criteria as well. Masters must be well-cut round brilliants in a consistent size range. The common round brilliant shape provides the most consistent color appearance, and 0.25 to 1 carat stones work well for grading the bulk of the stones seen in the marketplace. Graders have found that 1 carat masters can be used to accurately grade very large stones, even those 50 carats and above. Masters cannot have eye-visible inclusions, which distract the observer and often compromise apparent color.


The standard for standards. GIA’s system for grading master stones preceded its commercial grading activities. The institute began to create such sets for American Gem Society retailers in 1941 using the GIA Colorimeter, a device with a standardized, specially filtered light source and a color scale graduated from colorless down to the equivalent of the present-day P. This was the only diamond service offered by GIA for more than a decade.


Today, GIA uses “working master sets” of 10 stones to grade the most commonly submitted diamonds, which fall between D and N. Starting with O, the lab doubles up, with one stone per two-grade range, from O-P down to Y-Z. The GIA Laboratory also, however, uses a face-up Z master at the lowest end of that color range to delineate the border between the D-to-Z scale and the start of the fancy-color scale.


Because dirt can accumulate on diamonds and affect their appearance, GIA “boils” each master set in sulfuric acid every two to four weeks. The authors reported that some master sets submitted to GIA for review had their color altered by as many as four grades because of dirt that had accumulated on their bruted girdles. To avoid abrasions and chips that can also affect color appearance, GIA lab graders use rubber-tipped tweezers to handle all master stones.

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