Opal is a rather common mineral, as it consits of quartz (SiO2) and water, it is also called hydrated silica, SiO2(H2O). Quartz is extremely common, the crust of the earth consists mainly of quartz. Many beaches are full of quartz sand and it is the basis of modern computer chips. And there are numerous minerals and crystals consisting mostly on quartz. Among those varieties are rock crystal, amethyst and agate.
Opal is a submicrocrystalline variety of cristobalite. It is not a crystal in the common sense, as it lacks a crystal structure, it is said to be amorphous. It is iridescent, a certain physical effect splitting white sunlight into the colours of the spectrum and reflecting them into different directions. This makes the opal shimmer in multiple colours and changing its colour when it moves. This special optical effect is also called opalescence.
Opal is a so called semi-precious mineral. Opal is found all over the world, but most deposits are not of the extraordinary quality which is necessary for gemstones. As a matter of fact more than 95% of the worlds opal comes from Australia. In general there are three different areas in Australia famous for opal.
Boulder Opal is mined in a large area between Quilpie in the South and Winton in the north. The boulder and black opal fields nearly meet at the border between Queensland and NSW. Crystal Opal is found at the Coober Pedy, Mintabee, and Andamooka mines. It is a light opal, which includes crystal, grey and white opal. Sometimes it is also called milk opal. Black Opal and semi black opal is found near Lightning Ridge, which is famous for this gem worldwide. White cliffs near the South Australian border was probably the largest opal mine in the world before Coober Pedy was discovered.
The newest development in opal prospecting is a radioactive glow emitted by the opals, which may be detected by special instruments. This technology might revolutionise opal mining in the future. But probably the same research, currently undertaken at the Australian National University in Canberra, will allow to grow opals of extremely high quality artificially. This could even destroy the market for natural opals.