Opal is a hydrated
amorphous form of
silica (SiO
2·
nH
2O); its
water content may range from 3 to 21 y 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 the national gemstone of
Australia.
