![]() Opal constitutes important parts of many sedimentary accumulations, such as diatomaceous earth. It is found as nodules, stalactitic masses, veinlets, and encrustations in most kinds of rocks. It is widespread and is deposited at low temperatures (up to 400☏/200☌) from silica-bearing, circulating waters. Regularly arranged spheres of a particular size create a diffraction effect called color play. Precious opal is the least crystalline form of the mineral, consisting of a regular arrangement of tiny, transparent, silica spheres. It varies from essentially amorphous to partially crystalline. It is colorless when pure, the vast majority of common opal occurs in opaque, dull yellows and reds.
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