Next after their profusion of crystal forms, the most striking characteristic of minerals is without doubt the great variety of their colours. In only a few cases is the colour of the mineral the actual colour of the substance of which it is composed; minerals of this kind always have the same characteristic colour by which they are recognised. Examples are: yellow—native sulphur, red—cinnabar, green—malachite , blue—azurite , and lead-grey— galena. These minerals have such distinctive colours that the terms sulphur-yellow, cinnabar-red and malachite-green are now widely used. Also yellow diamonds have beautiful color.
Considerably more minerals, however, owe their colour, not to the substance of which they are primarily made, but to impurities, which are often present in only minute amounts. Foreign ions in a crystal lattice can produce certain colours, as can minute included scales, grains or fibres of other minerals . Atomic radiation, too, can account for certain colourings. Minerals whose colour is due to the admixture of minute foreign particles are termed allo-chromatic, and their colour may vary from crystal to crystal. The colour of most minerals is, therefore, not a diagnostic feature. It is even possible to find single crystals whose colour changes towards the apices, while clusters of differently coloured crystals all belonging to the same mineral are frequently encountered.
Some minerals can occur in a surprisingly large number of colours or shades. Fluorspar crystals, for instance, may be colourless, white, grey, wine-yellow, honey-coloured, brownish-yellow, pink, greenish, grass-green, blue-green, blue, violet, deep bluish-purple, or almost black. Quartz and chalcedony, too, occur in a large variety of colours.
The colour of certain allochromatic minerals may be changed by the application of heat, atomic radiation or ultra-violet light. Some coloured minerals, such as certain varieties of topaz and fluorspar, bleach slightly when exposed to sunshine.
