Iridescence is the play of colour, or a series of colours, produced by interference or diffraction (or both), either when light is reflected from thin films (inclusions), twinning planes or from the unique crystal structure of opal.
There are several types of iridescence:
This is the effect seen in a mineral called Labradorite and Spectrolite (a Labradorite found in Finland). It is caused by interference on the boundaries of twinning planes. Many Labradorites are carved to exploit this unique type of sheen.
This type of sheen is exhibited in a rock called Moonstone caused by reflection on Moonstone's twinning planes. The effect is a blue colour floating just below the surface of the stone. Adularescence is also known as Schiller.
Named after Aventurine Feldspar, which is also known as Sunstone. This type of iridescence is due to the play of colours caused by reflection on tiny, thin inclusions of goethite and hematite (iron minerals). This gives the stone a golden or reddish-brown colour and, often, specular reflections.
Opalescence refers to play of colour displayed by opal. However, there are three notable types of opal (precious, common, and fire).
This play of colour is a result of refraction (precious and fire) or reflection (common) due to the layering, spacing, and size of the myriad microscopic silicon dioxide spheres and included water (or air) in the opal.
When the size and spacing of the silica spheres are relatively small, refracted blue-green colours are prevalent; when relatively larger, refracted yellow-orange-red colours are seen; and when larger yet, reflection yields a milky-hazy sheen.