Gemologists place diamonds in categories called type Ia, Ib, IIa, and IIb. Each category has its own characteristics and color
variations. The classification system is based on the presence (or absence) and type of impurity element. Dividing diamonds into types helps explain the science of diamond color.
All type I diamonds contain nitrogen. Nitrogen is fairly plentiful in type Ia diamonds, and its atoms are arranged in pairs or clusters. About 95% of cuttable-size natural diamonds are near-colorless to yellow type Ia diamonds.
Type Ib diamonds are rare, representing only 1% of all nitrogen-colored diamonds. They have fewer nitrogen atoms than Ia diamonds, and the atoms are scattered and isolated.
Because of the arrangement of the nitrogen atoms, type Ib diamonds usually have a more saturated yellow color.
Very rare type II diamonds contain very little or no nitrogen. Most are colorless, but crystal distortion can make them brown or gray.
The majority of the world’s most famous colorless diamonds are type IIa, including the Cullinan, which weighed 3,106 ct in its rough form and produced seven major stones, including the 530.4 ct Cullinan I.
Type IIb diamonds contain traces of boron, which turns them blue and makes them electrically conductive. One of the most famous type IIb stones is the 45.52 ct. Hope diamond.
Other diamond colors can also fall into one or more types.
Image: Diamonds are classified into types based on the presence or absence of different impurity elements. Diamond types help explain the science of diamond color. Photo © De Beers.
Courtesy of GIA (Gemological Institute of America) © 2012.