The evaluation of diamond color 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 of established color value. A diamond of color D has no discernible color saturation. The weaker the color saturation the rarer the diamond and accordingly higher the price.
Many of these diamond color distinctions are so subtle that they are invisible to the untrained eye. The difference between two colour grades is almost impossible to spot, especially when set in jewellery.
Before GIA universalized the D-to-Z Color Grading Scale, there was no clear standard to define what diamond color is. 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 clarity and universal acceptance of the GIA scale.
No. Colored diamonds outside the normal color range are called fancy-color diamonds. There are no binding guidelines for the use of the term “fancy-color” in the US, but there is general agreement in the international trade about what diamond color range is customary for fancy-color diamonds. These 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.