Diamond color grading of gem quality diamonds is actually based on the absence of color. The chemical makeup of a perfect diamond has no hue. The Gemological Institute of America (GIA) color grading system uses a letter scale to measure the degree of colorlessness by comparing a diamond to master diamonds under controlled lighting thus establishing a color value.
GIA’s D to Z Color Grading Scale is universally accepted for grading color. The scale begins with D (represents colorless) and continues with intensifying hues to the letter Z. Most color distinctions are so very subtle that they are invisible to the untrained eye. Nonetheless, these color distinctions make all the difference in diamond quality and price.