Classification of natural diamond

Type Ia diamond: Contains nitrogen as an impurity in fairly substantial amounts (of the order of 0.1%), and which appears to have segregated into small aggregates. Also contains platelets, associated with the nitrogen impurity, the exact structure of which is not known. Most natural diamonds are of this type.

Type Ib diamond: Also contains nitrogen as an impurity but in dispersed substitutional form. Almost all synthetic diamonds are of this type.

Type IIa diamond: effectively free of nitrogen impurity. Very rare in nature, these diamonds have enhanced optical and thermal properties.

Type IIb diamond: A very pure type of diamond which has semiconducting properties: generally blue in colour. Extremely rare in nature.