Tags: Chemical Substance, Mineral.
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 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.
