The color evaluation of gem-quality diamonds is based on the absence of color. Like a drop of pure water, a chemically pure and structurally perfect diamond has no hue and consequently, a higher value.
GIA's diamond grading scale is the industry's most widely accepted grading system. The scale begins with the letter D, representing a colorless diamond, and continues, with increasing presence of color, to the letter Z. The diamond’s color is established by comparing a diamond under controlled lighting and viewing conditions against a set of master stones.
Many of these color distinctions are so subtle that they are invisible to the untrained eye. These distinctions, however, make a very big difference in diamond pricing.