Opal is composed of silica spheres which are packedtogether. In precious opal the arrangement of the spheres is an orderly three dimensional grid, because of the special pattern of spheres it is the only gemstone that has the unique natural ability to diffract light.

Light passing through the transparent spheres is scattered by the array of voids. White light is diffracted and split into its various colours at different angles.

The colour visible from the opal is dependent on the size of the spheres. Blue and violet colours are created by spheres that are smaller compared to the larger spheres and voids which create red and orange colours.