![]() Play-of-color is defined as "a pseudo chromatic optical effect resulting in flashes of colored light from certain minerals, as they are turned in white light." The internal structure of precious opal causes it to diffract light, resulting in play-of-color. Precious opal displays play-of-color ( iridescence) common opal does not. There are two broad classes of opal: precious and common. The name opal is believed to be derived from the Sanskrit word upala ( उपल), which means 'jewel', and later the Greek derivative opállios ( ὀπάλλιος), which means 'to see a change in color'. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt. ![]() Due to its amorphous property, it is classified as a mineraloid, unlike crystalline forms of silica, which are considered minerals.
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