COLOUR IS THE MOST OBVIOUS visual feature of a gem, but in fact it is just one of many optical properties, all of which are dependent upon light. The individual crystalline structure of a gemstone, interacts with light in a unique way, and determines the optical properties of each gem species. Effects produced by light passing through a gem are described here; those produced by the reflection of light.

The colour of a gem depends largely on the way it absorbs light. White light is made up of the colours of the rainbow (spectral colours), and when it strikes a gem some spectral colours are "preferentially absorb­ed". Those that are not absorbed, pass through or are reflected back, giving the gem its colour. Each gem in fact has a unique colour "fingerprint" (known as its absorptionspectrum), but this is only visible when viewed with a spectroscope (see p.38). To the naked eye, many gems look the same colour.

Splitting of light through a Prism splitting white light into its spectral colours is called dispersion, and gives gems their internal fire.

Allochromatic ("other-coloured") gems are coloured by trace elements or other impurities that are not an essential part of their chemical composition. Corundum, for example, is colourless when pure, but impurities in it (usually a metal oxide) create the red stones we know as rubies, blue, green, and yellow sapphires, and orange-pink padparadscha. Allochromatic gems are often susceptible to colour enhancement or change.

A crystal that consists of different-coloured parts is called partly-coloured. It may be made up of two colours (bicoloured), three tricoloured), or more. The colour may be distributed unevenly within the crystal, or in ones associated with growth. The many different varieties of tourmaline probably show the best examples of partly-colouring, exhibiting as many as 15 different colours  shades within a single crystal.