Gemstones mined in Australia include opal, sapphire, diamond, chrysoprase, garnet and zircon.
Sapphire is a variety of the mineral corundum, which consists of aluminium oxide. The presence of small amounts of oxides of iron, titanium and chromium can impart a wide range of colours to the mineral. Sapphire occurs in alluvial deposits. Mining of sapphires varies from simple hand mining of surface and shallow wash through digging shafts in deeper ground to large-scale open cuts operations involving heavy machinery.
Opal is a hydrated form of silica in which water is chemically bonded. There are two main forms of opal: precious opal and common opal. Precious opal exhibits the characteristic play of spectral colours and is comparatively rare. Common opal shows no play of colours. It may be colourless or coloured and can be transparent to opaque. Opal is mined from underground shafts but the larger mines are open-cut operations. Ironstone boulders are carefully removed from the ground for processing.
Garnet comprises a group of complex aluminium silicate minerals, some varieties are used as a gemstone. Garnet is being increasingly used as a high quality abrasive for sandpaper, in sand blasting, on non-skid surfaces and as a filtration medium. Mining involves drilling and blasting. The material is reduced in size using two-stage crushing and rolling to produce 3 mm size material. The material is then dried. Magnetic separation gives a 99 percent pure product.