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Diamond type is a method of scientifically classifying diamonds by the level and type of their chemical impurities.

They are separated into four types:
Type Ia, Type Ib, Type IIa, Type IIb.

The impurities that differentiate each diamond into different types are at the atomic level within the crystal lattice of carbon atoms and so, unlike inclusions, require an infrared spectrometer to detect them.

Type I Diamonds

Type Ia diamonds, the most common class, contain nitrogen atoms as their main impurity. They also have a characteristic fluorescence and visible absorption spectrum.

Type Ia diamonds make up about 95% of all natural diamonds. The nitrogen impurities are clustered within the carbon lattice, and are relatively widespread.

The absorption spectrum of the nitrogen clusters can cause the diamond to absorb blue light, making it appear pale yellow or almost colourless. 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.  Type IaA, where the nitrogen atoms are in pairs; these do not affect the diamond's colour.

TypeIb, where the nitrogen atoms are in large even-numbered aggregates; these impart a yellow to brown tint.

Type Ib make up about 0.1% of all natural diamonds. They contain up to 0.05% (500 ppm) of nitrogen, but the impurities are more diffuse, the atoms are dispersed throughout the crystal in isolated sites. More intense or darker yellow or brown colour than Type Ia diamonds. The stones have an intense yellow or occasionally brown tint; the rare canary diamonds belong to this type, which represents only 0.1% of known natural diamonds. Most blue-gray diamonds coming from the Argyle mine of Australia are not of type IIb, but of Ia type; those diamonds contain large concentrations of defects and impurities (especially hydrogen and nitrogen) and the origin of their colour is yet uncertain.[5]
Almost all HPHT synthetic diamonds are of Type Ib.

Type II Diamonds

Type II diamonds have no measurable nitrogen impurities.

Type IIa diamonds make up 1-2% of all natural diamonds (1.8% of gem diamonds). These diamonds are almost or entirely devoid of impurities, and consequently are usually colourless and have the highest thermal conductivity. Occasionally, while Type IIa diamonds are being extruded towards the surface of the Earth, the pressure and tension can cause structural anomalies arising through plastic deformation during the growth of the tetrahedral crystal structure, leading to imperfections. These imperfections can confer a yellow, brown, orange, pink, red, or purple colour to the gem.
Many famous diamonds, like the Cullinan, Koh-i-Noor and Lesedi La rona, are type IIa.
Lab Grown diamonds grown using the CVD process typically belong to this type.

Type IIb diamonds make up about 0.1% of all natural diamonds, making them one of the rarest natural diamonds and very valuable. In addition to having very low levels of nitrogen impurities comparable to Type IIa diamonds, Type IIb diamonds contain significant boron impurities. The absorption spectrum of boron causes these gems to absorb red, orange, and yellow light, lending Type IIb diamonds a light blue or grey colour, though examples with low levels of boron impurities can also be colourless.