Scientific Classification of Diamond
In this article, I give some details about the classification scheme that scientists apply when studying diamond. Note that this is quite different to the way in which a jeweller might classify a diamond.
While gemstone diamonds are typically classified according to ‘the Four C's’ — cut, clarity, colour, and carat (weight) — all diamond samples can be classified according to their content and type of nitrogen impurities.
‘Type I’ diamonds contain significant amounts of nitrogen as an impurity; the great majority of natural diamonds are of this type. Diamonds of ‘Type II’ contain so little nitrogen that it is not readily detectable by ultraviolet or infrared absorption measurements. These two types of diamond are defined and further sub-divided as follows:
- Type I: Significant concentrations of nitrogen impurities.
- Type Ia: Nitrogen is present as aggregates of substitutional atoms and can account for up to 0.3% of the material (3000 ppm).
- Type IaA: Nitrogen atoms exist mostly in nearest-neighbour substitutional pairs (the ‘A’ aggregate) and the material does not tend to show any fluorescence.
- Type IaB: Nitrogen is present in groups of four substitutional atoms that surround a vacancy (the ‘B’ aggregate) and samples can exhibit moderate to strong blue fluorescence.
- Type IaAB: In addition to A and B centres, groups of three nitrogen substitutionals around a vacancy (‘N3’ centres) are present. The B centres lead to possible blue fluorescence, while high concentrations of N3 centres can give rise to strong absorption of visible blue light, which leads to yellow colouration of the diamond.
- Type Ib: Nitrogen exists as isolated substitutional impurities. Absorption of light increases toward the blue end of the spectrum, which results in a deep yellow colour for the material. Type Ib diamonds show nitrogen concentrations up to around 500 ppm, although the yellow colouration is supposedly noticeable at a nitrogen concentration of ∼ 50 ppm.
- Type Ia: Nitrogen is present as aggregates of substitutional atoms and can account for up to 0.3% of the material (3000 ppm).
- Type II: Very low or undetectable amounts of nitrogen.
- Type IIa: These are those that do not show significant electrical conductivity, and are usually colourless. These have no clear nitrogen-related features in the infrared absorption spectrum in the range 400–1332 cm-1 (the single-phonon region).
- Type IIb: Substitutional boron acceptors are uncompensated due to the lack of nitrogen donors, and the material therefore displays a significant p-type semiconductivity. The boron impurities also cause absorption of light toward the red end of the visible spectrum, and therefore lend a slight blue colour to the diamond.