Diamonds occur in a variety of colors — steel gray, white, blue, yellow, orange, red, green, pink to purple, brown and black, virtually in every color of the rainbow. Colored diamonds contain interstitial impurities or structural defects that cause the coloration, whilst pure diamonds are perfectly transparent and colorless. Diamonds are scientifically classed into two main types and several subtypes, according to the nature of impurities present and how these impurities affect light absorption:
Type I diamonds have nitrogen atoms as the main impurity, commonly at a concentration of 0.1%. If the nitrogen atoms are in pairs they do not affect the diamond's color; these are Type IaA. If the nitrogen atoms are in large even-numbered aggregates they impart a yellow to brown tint (Type IaB). About 98% of gem diamonds are type Ia, and most of these are a mixture of IaA and IaB material: these diamonds belong to the Cape series, named after the diamond-rich region formerly known as Cape Province in South Africa, whose deposits are largely Type Ia. If the nitrogen atoms are dispersed throughout the crystal in isolated sites (not paired or grouped), they give the stone an intense yellow or occasionally brown tint (Type Ib); the rare canary diamonds belong to this type, which represents only 0.1% of known natural diamonds. Synthetic diamond containing nitrogen is Type Ib. Type I diamonds absorb in both the infrared and ultraviolet. They also have a characteristic fluorescence and visible absorption spectrum (see Optical properties of diamond).
Type II diamonds have no measurable nitrogen impurities. Type II diamonds absorb in a different region of the infrared, unlike Type I diamonds. They also have differing fluorescence characteristics, but no discernible visible absorption spectrum. Type IIa diamonds can be colored pink, red, or brown due to structural anomalies arising through plastic deformation during crystal growth—these diamonds are rare (1.8% of gem diamonds), but constitute a large percentage of Australian production. Type IIb diamonds, which account for 0.1% of gem diamonds, are usually light blue due to scattered boron within the crystal matrix; these diamonds are also semiconductors, unlike other diamond types (see electrical properties of diamonds). However, a blue-grey color may also occur in Type Ia diamonds and be unrelated to boron. Also not restricted to type are green diamonds, whose color is derived from exposure to varying quantities of radiation.
* excerpt from Wikipedia entry