Research on the causes of colour in gemstones is one of the most fascinating aspects of Gemmology. It combines areas of optics and chemistry with the latest findings on the structure and forces of atoms.
Coloured stones are divided into three categories:
Stones, in which the colouring element is an integral part of the mineral, are called idiochromatic. These gemstones therefore only appear in one colour and include stones such as turquoise, peridot and rhodonite.
Most gemstones would yield colourless crystals with a pure chemical composition. They obtain their colour from a small addition of colour-imparting elements; they are therefore called allochromatic.
So where does colour come from? Colour often comes from the random presence of other colouring elements in the crystal lattice. These extraneous atoms of titanium, chromium, iron, nickel, vanadium, manganese, cobalt, and copper provoke the amazing phenomenon of colour. Sapphire, ruby, emerald, tourmaline and many others all belong to the allochromatic category of colour. Even more astonishing is that depending on the host structure of the stone; the same chemical element can produce different colours, for example chromium gives ruby its element of purple, yet in emerald is gives an inimitable green.
Another means of colouration can be found in the fascinating charm of the colour of opal, which lies in its extraordinary play of colour due to the scattering of light off of its crystal structure.