Australia produces over 95% of the world's supply of gem quality opal.
In Australia, the opal producing areas are Lightning Ridge and White Cliffs in New South Wales, Coober Pedy, Andamooka and Mintabie in South Australia, and various locations in Queensland, which produce boulder opal.
Lightning Ridge is the world's foremost producer of the rare, gem quality, black opal.
Opal is unique amongst all gemstones and no two opals are alike, each having its own definite play of colours and pattern and by rotating the stone, different patterns and colours emerge.
In general, the value of opal is determined by the rarity of the colours and pattern, the brightness of the stone, the thickness of the colour bar (which usually occurs naturally on a potch or common opal back) the lack of inclusions in the stone and the underlying body colour of the opal.
The underlying body colour designates whether the opal is classified as black, dark or light opal. The most valuable underlying body colour of opal is black ranging through dark to light, being the least valuable. The majority of opal is opaque but it also occurs in a translucent form and although technically incorrect, is commonly called crystal.
Opal is a hydrated form of silica (SiO2.nH2O) and precious opal usually contains 6-10% water. Other properties include: Hardness - 5.5-7; Specific gravity - 2.1-2.2 (precious opal); Lustre - vitreous to sub-vitreous; Refractive index: 1.44-1.46; Streak - white; Fracture - conchoidal; Diaphaneity - transparent to opaque; Cleavage - absent.
Most opals in Australia were formed in deeply weathered sedimentary rocks, in what are now arid areas, during the Cretaceous Age around the southern and western sections of the inland sea.
Common opal or potch, ranging from black through to light and honey, and displaying no other colours, occurs when the molecules form into irregular spheres.
Precious opal occurs when the molecules form in regular spheres of the same size in regular rows. It occurs in all the colours of the rainbow with red being the rarest down through the spectrum to violet, the most common colour.
Most opal is cabochon cut and polished into oval shapes, although over the past ten years more and more is left in a free-form shape or carved. This maximises the amount of precious opal and lends itself to individual jewellery designs.
In addition to solid, naturally occurring stones, precious opal is also sold as doublets, where slices of opal are glued onto a potch back and triplets, where slithers of opal are glued onto a potch back and a clear quartz cap is glued on top.
Opal was declared Australia's National Gemstone on 28 July 1993.
Black opal was declared a State Emblem of NSW on Friday 31 October 2008.