This description provides an overview of Australia's opal industry. It explains that Australia is the world's largest producer of opal, a precious gemstone found in a variety of colors and shapes. It also discusses the history of opal mining in Australia, the different types of opal found
Opal is a hydrated amorphous form of silica (SiO2·nH2O); its water content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to the amorphous (chemical) physical structure, it is classified as a mineraloid, unlike crystalline forms of silica, which are considered 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.