#Opal is a hydrated amorphous form of silica (sio2·nh2o). When opal is formed, silica gel fills crevices in rock.
As water evaporates, the silica is deposited in the form of tiny spheres.
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.
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