Diamond type is a method of scientifically classifying

diamond Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Diamond is tasteless, odourless, strong, brittle solid, colourless in pure form, a poor conductor of e ...

s by the level and type of their chemical impurities. Diamonds are separated into five types: Type IaA, Type IaB, Type Ib, Type IIa, and Type IIb. The impurities measured are at the atomic level within the crystal lattice of

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

atoms and so, unlike inclusions, require an infrared spectrometer to detect. 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 Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...

absorption spectrum.

Types of diamond

Type I

Type I diamonds, the most common class, contain

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

atoms as their main impurity, commonly at a concentration of 0.1%. Type I diamonds absorb in both the

and

ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...

region, from 320 nm. They also have a characteristic

fluorescence Fluorescence is one of two kinds of photoluminescence, the emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow (fluoresce) with colore ...

and visible absorption spectrum (see Optical properties of diamond).

Type Ia

Type Ia diamonds make up about 95% of all natural diamonds. The nitrogen impurities, up to 0.3% (3000 ppm), 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 South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...

, whose deposits are largely Type Ia. Type Ia diamonds often show sharp absorption bands with the main band at 415.5 nm (N3) and weaker lines at 478 nm (N2), 465 nm, 452 nm, 435 nm, and 423 nm (the "Cape lines"), caused by the N2 and N3 nitrogen centers. They also show blue

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 504 nm (H3 center), sometimes accompanied by two additional weak bands at 537 nm and 495 nm (H4 center, a large complex presumably involving 4 substitutional

atoms and 2 lattice vacancies). *Type IaA, where the nitrogen atoms are in pairs; these do not affect the diamond's color. *Type IaB, where the nitrogen atoms are in large even-numbered aggregates; these impart a yellow to brown tint.

Type Ib

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. Type Ib diamonds absorb green light in addition to blue, and have a 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. The visible absorption spectrum is gradual, without sharp absorption bands.

Type II

Type II diamonds have no measurable nitrogen impurities. Type II diamonds absorb in a different region of the infrared, and transmit in the

below 225 nm, unlike Type I diamonds. They also have differing fluorescence characteristics. 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.

Type IIa

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 The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa and is measured in W·m−1·K−1. Heat transfer occurs at a lower rate in materials of low ...

. They are very transparent in ultraviolet, down to 230 nm. 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. 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. Type IIa diamonds constitute a great percentage of Australian production. Many famous large diamonds, like the Cullinan, Koh-i-Noor, Lesedi La Rona, and The Lulo Rose are Type IIa. Synthetic diamonds grown using the CVD process typically also belong to this type.

Type IIb

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 color, though examples with low levels of boron impurities can also be colorless. These diamonds are also p-type

semiconductor A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...

s, unlike other diamond types, due to uncompensated

electron hole In physics, chemistry, and electronic engineering, an electron hole (often simply called a hole) is a quasiparticle denoting the lack of an electron at a position where one could exist in an atom or crystal structure, atomic lattice. Since in ...

s (see Electrical properties of diamond); as little as 1 ppm of boron is enough for this effect. However, a blue-grey color may also occur in Type Ia diamonds and be unrelated to boron. 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 Ionizing (ionising) radiation, including Radioactive decay, nuclear radiation, consists of subatomic particles or electromagnetic waves that have enough energy per individual photon or particle to ionization, ionize atoms or molecules by detaching ...

. 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.

References

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