Gems & Gemology, Winter 2008, Vol. 44, No. 4
Color Grading “D-to-Z” Diamonds at the GIA Laboratory
John M. King, and Ron H. Geurts, Al M. Gilbertson, and James E. Shigley
Since its introduction in the early 1950s, GIA’s D-to-Z scale has been used to color grade the overwhelming majority of colorless to light yellow gem-quality polished diamonds on which laboratory reports have been issued. While the use of these letter designations for diamond color grades is now virtually universal in the gem and jewelry industry, the use of GIA color grading standards and procedures is not. This article discusses the history and ongoing development of this grading system, and explains how the GIA Laboratory applies it. Important aspects of this system include a specific color grading methodology for judging the absence of color in diamonds, a standard illumination and viewing environment, and the use of color reference diamonds (“master stones”) for the visual comparison of color.