Diamonds occur in all spectrums of colours. White diamond range from colourless – the purest, rarest and most valuable – to varying degrees of yellow. They are graded on a colour scale from D (colourless) to Z (heavily tinted).
The diamond colour evaluation of colourless (white) diamonds is based on the absence of colour. A chemically pure and structurally perfect diamond has no hue, and therefore a higher value. GIA's D-to-Z diamond colour-grading system measures the degree of colourlessness by comparing a diamond under controlled lighting and precise viewing conditions to diamonds of established colour value.
The GIA’s diamond D to Z colour grading scale is the industry's most widely accepted grading system. The scale begins with the letter D, representing colourless, and continues, with increasing presence of colour, to the letter Z.
Although many of these colour distinctions are so subtle that they are invisible to the untrained eye; however, these distinctions make a very big difference in diamond quality and price.
Natural fancy colour diamonds are graded differently, as they lie outside of the white diamond colour range and are extremely rare and valuable.