GIA’s diamond colour grading system measures the degree of colourness of a diamond, starting with D as colourless and continuing to Z representing light yellow or brown.
Diamonds have a carbon crystal structure. When chemically pure and structurally perfect, a diamond has no hue, like a drop of pure water. However, when impurities are incorporated in the carbon structure, the diamond will carry a coloured tint. Nitrogen is a commonly found impurity and it gives diamonds a yellow colour. The more colourless a diamond, the higher the value.
The distinctions between colour grades are so subtle that they are often invisible to the untrained eye but can make a big difference in diamond quality and price.