Precious opal is composed of minute uniform spheres of silica which are arranged together in an orderly three dimensional grid. The spaces between these spheres contain silica in solution. In common opal the spheres are of varying sizes and are not uniformly stacked.

Light passes through the transparent spheres directly, but when it reaches the silica in solution it is deflected at angles. These diffracted beams of light may show all the colours of the spectrum, or particular colours may predominate.

The colour emitted from opal is dependent on the size of the spheres which determine the wavelengths.

For instance, blue colours are evident where the spheres are smaller, and at the other end of the spectrum orange and red will be evident where the spheres are larger. The intensity and brilliance of the colours are a result of the degree of uniformity of size and regularity of the grid.