The diamond color evaluation of most gem-quality diamonds is based on the absence of color. A chemically pure and structurally perfect diamond has no hue, like a drop of pure water, and consequently, a higher value.
GIA’s D-to-Z diamond color-grading system measures the degree of colorlessness by comparing a stone under controlled lighting and precise viewing conditions to masterstones stones of established color value. GIA’s diamond D-to-Z color-grading scale is the industry’s most widely accepted grading system.
The scale begins with the letter D, representing colorless, and continues, with increasing presence of color, to the letter Z. Many of these color distinctions are so subtle that they are invisible to the untrained eye; however, these distinctions make a very big difference in diamond quality and price.
WHY DOES THE GIA COLOR GRADING SYSTEM START AT D?
Before GIA developed the D-Z Color Grading Scale, a variety of other systems were loosely applied. These included letters of the alphabet (A, B and C, with multiple A’s for the best stones), Arabic (0, 1, 2, 3) and Roman (I, II, III) numerals,and descriptions such as “gem blue” or “blue white.”
The result of all these grading systems was inconsistency and inaccuracy. Because the creators of the GIA Color Scale wanted to start fresh, without any association with earlier systems, they chose to start with the letter D – a letter grade normally not associated with top quality.