Iridescence is a display of colours which is seen when white light reflects from structures on or in gemstones. In opal, prismatic colours are seen because spectral colours are separated. In labradorite and other gemstones, there are blended to produce interference colours. Iridescence, or play of colour, may be seen in soap bubbles or insects such as butterflies and beetles, as well as in gemstones.

Iridescence can be caused by the diffraction of light from regular structures, or by reflection from thin film on or in materials. The thin films may be liquids gases or solids. Crystals, liquid-filled inclusions, fractures and cleavages may all cause the effect.
Donec Massa Integer

Light striking a thin film is partly reflected from the upper surface but part refracts into the film and is reflected from the lower surface. The light is divided into two parts which re-combine at the upper surface of the film. They are said to be ‘in phase’ when thin crests and troughs coincide; these light waves become brighter. Some are out of phase; wave crests combine with wave troughs so that the waves are eliminated.