Most gem-quality diamonds are evaluated based on the absence of color. A chemically pure, perfect diamond is completely clear, like a drop of water. Consequently, these diamonds have a higher value for their purity. The “D-to-Z diamond color-grading system”, as established by GIA, measures the degree of color by comparing a stone to masterstones of established value in the color scale. “D” on the scale represents the highest quality, while “Z” represents the lowest diamond color quality. This comparison is done under precise viewing conditions to ensure accuracy.
Comparison is done with such precise conditions because many diamond color distinctions are so subtle that they are nearly invisible to the untrained eye. However, these small distinctions can make a big difference in diamond quality and price.