Almost all diamonds contain impurity atoms, mainly nitrogen and boron, but these cannot be seen even at the highest magnification. The effect of these impurities is to change the properties of the stone, mostly by giving it a light colour. For instance, the nitrogen tends to collect into small clusters of two three atoms and the interactions between the carbon and nitrogen atoms create new energy levels in the crystal. An optical centre is formed that absorbs light of wavelengths longer than 230 nm. Natural diamonds may have several such optical centres of different configurations. When a single nitrogen atom substitutes for a carbon atom, an N3 optical ventre is formed and the ultraviolet absorption edge comes close to the visible region. The 'A' optical centre atoms gives substantial infrared absorption in the region 10,000 to 7000 nm. An arrangement of four nitrogen atoms around a missing carbon atom gives a B centre with the result that the ultraviolet absorption is lowered though the infrared spectra of A and B centres are similar. The addition of a nearby vacancy in the carbon lattice results in the so-called H3 and H4 centres.
Natural diamonds contain these optical centres in randomly variable concentration and gemmologists label the type of diamond according to the absorption spectrum. Most diamonds have N3, A and B centres and these are the Type 1 sort. Type 1a diamonds contain primarily A and B centres, but if N3 centres are predominant, it becomes a Type 1b. The Type 1a is further divided into Type 1aA and Type 1aB according as to whether there are greater A or B centres. Type 2 diamonds have little or no nitrogen in them but may contain boron makes them 2b semi-conducting stones. In nature, it is possible to find stones which are of more than one type. Syntheric diamonds are usually Type 1b and may have up to ten per cent of iron and nickel as contamination from the process. Ultraviolet, X-rays and electrons excite luminescence in diamond that is usually blue thought often yellow and green colours are seen.