Question: What types of diamonds are there?

Diamond type

Diamond type is a method of scientifically classifying diamonds by the level and type of their chemical impurities. Diamonds are separated into four types: TypeIa, TypeIb, TypeIIa, and TypeIIb. The impurities measured are at the atomic level within the crystal lattice of carbon atoms and so, unlike inclusions, require an infrared spectrometer to detect.[1]
Different diamond types react in different ways to diamond enhancement techniques. Different types can coexist within a single stone; natural diamonds are often mixes of Type Ia and Ib, which can be determined by their infrared absorption spectrum.[2]
Types of Diamond

TypeI diamonds, the most common class, contain nitrogen atoms as their main impurity, commonly at a concentration of 0.1%. TypeI diamonds absorb in both the infrared and ultraviolet region, from 320nm. They also have a characteristic fluorescence and visible absorption spectrum (see Optical properties of diamond).
TypeIa diamonds make up about 98% of all natural diamonds. The nitrogen impurities, up to 0.3% (3000ppm), 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 colorless. Most Ia diamonds 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. Type Ia diamonds often show sharp absorption bands with the main band at 415.5nm (N3) and weaker lines at 478nm (N2), 465nm, 452nm, 435nm, and 423nm (the "Cape lines"), caused by the N2 and N3 nitrogen centers. They also show blue fluorescence to long-wave ultraviolet radiation due to the N3 nitrogen centers (the N3 centers do not impair visible color, but are always accompanied by the N2 centers which do). Brown, green, or yellow diamonds show a band in the green at 504nm (H3 center), sometimes accompanied by two additional weak bands at 537nm and 495nm (H4 center, a large complex presumably involving 4 substitutional nitrogen atoms and 2 lattice vacancies).[3]
TypeIaA, where the nitrogen atoms are in pairs; these do not affect the diamond's color.
TypeIaB, where the nitrogen atoms are in large even-numbered aggregates; these impart a yellow to brown tint.
TypeIb make up about 0.1% of all natural diamonds. They contain up to 0.05% (500ppm) of nitrogen, but the impurities are more diffuse, the atoms are dispersed throughout the crystal in isolated sites. TypeIb diamonds absorb green light in addition to blue, and have a more intense or darker yellow or brown colour than TypeIa 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. The visible absorption spectrum is gradual, without sharp absorption bands.[4] 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 color is yet uncertain.[5] Almost all HPHT synthetic diamonds are of TypeIb.[6]
TypeII diamonds have no measurable nitrogen impurities. Type II diamonds absorb in a different region of the infrared, and transmit in the ultraviolet below 225nm, unlike Type I diamonds. They also have differing fluorescence characteristics, but no discernible visible absorption spectrum. The crystals as found tend to be large and irregular in shape. Type II diamonds were formed under extremely high pressure for longer time periods.
TypeIIa 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. They are very transparent in ultraviolet, down to 230nm. Occasionally, while TypeIIa 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. Type IIa diamonds can have their structural deformations "repaired" via a high-pressure high-temperature (HPHT) process, removing much or all of the diamond's color.[7] TypeIIa diamonds constitute a great percentage of Australian production. Many famous large diamonds, like the Cullinan, Koh-i-Noor, and Lesedi La Rona, are TypeIIa. Synthetic diamonds grown using the CVD process typically also belong to this type.
TypeIIb 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 TypeIIa diamonds, TypeIIb diamonds contain significant boron impurities. The absorption spectrum of boron causes these gems to absorb red, orange, and yellow light, lending Type IIbdiamonds a light blue or grey color, though examples with low levels of boron impurities can also be colorless.[1] These diamonds are also p-type semiconductors, unlike other diamond types, due to uncompensated electron holes (see Electrical properties of diamond); as little as 1ppm of boron is enough for this effect. However, a blue-grey color may also occur in Type Ia diamonds and be unrelated to boron.[5] Type IIb diamonds show distinctive infrared absorption spectrum and show gradually increasing absorption towards the red side of visible spectrum.
Not restricted to type are green diamonds, whose color is derived from exposure to varying quantities of ionizing radiation.[1]
See also
Diamond color