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 diamond under controlled lighting and precise viewing conditions to master stones (genuine diamonds; pre-graded by GIA for their diamond color grade).
The Gemological Institute of America (GIA) created the diamond color grading system; from D thru Z. This the world’s most widely accepted grading system. The scale begins with the letter D, representing colorless, and continues, with increasing presence of color, all the way to the letter Z.
There are twenty three grades in all; therefore 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 universalized the D-to-Z Color Grading Scale, a variety of other systems were used loosely, from A, B, and C (used without clear definition), to Arabic (0, 1, 2, 3) and Roman (I, II, III) numbers, to descriptive terms like "gem blue" or "blue white," which are notorious for misinterpretation. So the creators of the GIA Color Scale wanted to start fresh, without any association with earlier systems. Thus the GIA scale starts at the letter D. Very few people still cling to other grading systems, and no other system has the universal acceptance of the GIA scale.
Are Zs considered fancy-color?
No. Naturally colored diamonds outside the normal color range are called fancy-color diamonds. The FTC provides no guidelines for the use of the term “fancy-color” in the US, but there is general agreement in the international trade that fancy-color diamonds are either yellow or brown diamonds that have more color than a Z master stone or they exhibit a color other than yellow or brown. The rarest natural color diamonds are red and pink. Natural blue and green diamonds are also very rare. Today there are diamond color treatments; you can have a genuine diamond that is not a natural color diamond.
Color is just one of the 4Cs; learning all of the 4Cs makes you a better educated diamond shopper.