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The phenomenon of the soft rainbow-like play of prismatic colours in certain gemstones sometimes caused by the interference of light reflected from air filled cracks which create layers of different refraction, e.g. as in rainbow quartz (iris quartz), inclusions of a thin layer or a film of lamellar nature that has a different refractive index than the mass of the stone, or the minute spheres within precious opal.
Iridescence is to be distinguished from ordinary absorption colours by its shifting character, the colours changing with the angle of viewing or the variation of the angle of incidence of the illumination.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson