Iridescence is the play of color, or a series of colors, produced by interference or diffraction (or both), either when light is reflected from thin films (inclusions), twinning planes or from the unique structure of precious opal.
There are several types of iridescence that have their own particular causes:
- Adularescence (or Schiller)
LabradorescenceLabradorescence is the effect seen in Labradorite (a Feldspar) and Spectrolite (a Labradorite found in Finland). It is caused by interference on the boundaries of lamellar twin planes, which are usual in Feldspars. Many Labradorites are carved to exploit this unique type of sheen.
|Unpolished moonstone. Photo: Crystalarium|
|Rainbow pyrite, a variety of mineral pyrite. The iridescence is caused by tarnishing by oxidation and molybdenum traces in the mineral. Photo: Ryan lay|
|Stunning Australian opal! Photo: Able Ground|
OpalescenceThe causes of play of color in Opal were long uncertain until the invention of the electron microscope. This enabled scientists to see the unique structure of Opal at high magnification, and to discover that Opal is made up of small spheres of silica.
In precious Opal, larger silica spheres of about 350 µm (micro meters) in diameter give off red flashes with changes in viewing angles. The smaller spheres result in green, blue or purple flashes which cannot increase in wavelength to give a red flash. Therefore, the sizes of the 'gaps', or 'voids', determine which color is seen.
|Ammolite is a rare iridescent gemstone formed from the fossilized shell of Ammonite. Photo: Lou (@themineralcollective)|
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