Most diamonds of gem quality used in jewelry vary in shade from completely colorless down to a visible yellow or brown tint.
The rarest and most expensive are diamonds in the colorless range graded D,E and F on a scale that descends to Z. Diamonds with more color than Z, or in other shades such as orange, pink, blue, etc. are classified as "Fancy Colored Diamonds" and are graded on the IGI Colored Diamond Report.
To determine the correct color, all submitted diamonds are compared to an internationally accepted master set of stones, the colors of which range from D, or colorless (the most sought after) to Z, the most yellow/brown - aside from "fancy" yellow or brown.
IGI assigns a color grade for diamonds in the D-Z range with the diamond face-down and viewed through the pavilion. This is because size, shape, cut quality and the presence of fluorescence can influence visible face-up color. In fact, lighting, mounting choice and even the clothes one wears have an impact on color, so IGI uses the most neutral environment possible to ensure accurate and consistent results.