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Opal is a hydrated amorphous form of silica, its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Because of its amorphous character, it is classed as a mineraloid, unlike crystalline forms of silica, which are classed as minerals. 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. Opal is considered a precious opal when it displays "play-of-color", [N 1] whereupon light entering the stone is first refracted. As light waves enter the opal and bounce around inside the opal's microstructure, some waves exit in a rainbow spectrum of colors. Precious opal ranges from clear through white, gray, red, orange, yellow, green blue, magenta to rose pink.