Feature Gems & Gemology, Summer 2005, Vol. 41, No. 2

Characterization and Grading of Natural-Color Yellow Diamonds

John M. King, James E. Shigley, Thomas H. Gelb, Scott S. Guhin, と Matthew Hall, and Wuyi Wang

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To better understand the yellow diamonds currently in the marketplace, as well as identify possible changes in their trends seen over a five-year period, researchers at the GIA Gem Laboratory analyzed gemological data collected on more than 24,000 natural-color yellow diamonds examined in the calendar years 1998 and 2003. These data included color grade, type of cut, clarity grade, weight, ultraviolet fluorescence, and UV-visible and infrared spectra. Among natural-color colored diamonds, those with a yellow hue are some of the most abundant; even so, they are much less common than the colorless to light yellow diamonds associated with GIA’s D-to-Z color grading scale. Since the yellow color is a continuation of the gradation of color associated with the D-to-Z scale, there can be misconceptions about the color grading, which involves different procedures from those used for D-to-Z grading. The grading and appearance aspects, as well as other characteristics of yellow diamonds, are discussed to clarify these differences. The authors have also identified five subgroups of type I yellow diamonds, which (with some overlap) are characterized by representative spectra and color appearances.