What is iridescence in mineralogy ?

Iridescence : definition

In mineralogy or gemology, iridescence is a changing chromatic effect that a mineral or a gemstone can take on, and which can have various origins. Some come from the interference between the light rays reflected by oxidation films covering the surface of minerals (hematite, chalcopyrite, fire agate, etc.). Turgite is thus a variety of goethite with intense, very colorful iridescence, with a dominant of green, pink and purple (photo on the right). Fire agate is chalcedony containing thin layers of limonite. Interference can also come from the superposition of thin mineralized layers of aragonite (mother of pearl).

The iridescence of opal have a deeper origin : they result from the diffraction of light by the hydrated silica spheres that compose it. The spheres must have similar diameters (around 0.3 µm) to operate this diffraction, which is not the case with common opals which are effectively devoid of iridescence. These colored flashes are specifically called "play-of-color".

Finally, iridescence can be the result of fine inclusions or even oriented exsolutions which diffract and reflect light rays : this is the origin of the iridescence of labradorite, born from a complex system of plagioclase exsolutions of varying compositions, this specific case is called labradorescence.
Labradorite from Madagascar
Iridescent fire agate from Mexico
Abalone mother of pearl from Indian Ocean