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Opal is a hydrated amorphous form of silica. Because of its amorphous character, it is classed as a mineraloid, unlike the other 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. The internal structure of precious opal makes it diffract light; depending on the conditions in which it formed, it can take on many colors. Opal is the birthstone for October. Image by Géry Parent.