Type I diamonds have considerable nitrogen impurities (at a concentration of up to 1%, commonly at a concentration of 0.1%), while type II diamonds have no measurable nitrogen impurities (very few if any nitrogen impurities). Each of these two main types are subcategorized to “a” and “b” types.
Type Ia diamonds, making up about 98% of all natural diamonds, have their nitrogen atoms in pairs or larger aggregates giving them an appearance from almost colorless to pale yellow. Type Ib diamonds like canary diamonds, representing only about 0.1% of known natural diamonds, have their nitrogen atoms in isolated sites (not paired or grouped) giving them an intense yellow or occasionally orange or brown tint.
Type IIa diamonds, making up 1-2% of all natural diamonds, are almost pure carbon, devoid of impurities, and they are colorless unless imperfections and plastic deformations in their crystal structures give them colors such as yellow, brown, orange, pink, red and purple. Many famous large diamonds like the Cullinan, Koh-i-Noor and Lesedi La Rona are type IIa. Type IIb diamonds, representing only about of 0.1% of natural diamonds, contain significant boron impurities giving them a light blue or grey color.
Also one should consider that type I-II categorization is based on impurities mainly nitrogen and not based on colors or imperfections in crystal structure beside the fact that imperfections and deformations in crystal structure or some impurities like hydrogen can have considerable effect on diamond color. For example, there exist blue-gray diamonds of type Ia and not type IIb.