Australia possesses the largest deposits of precious opal in the world. This is due largely to the unique presence of the Great Artesian Basin which covers almost 1/5 of the total Australian continent and takes in parts of 4 Australian States: Queensland, NSW, South Australia and the Northern Territory. These aquifers allowed and still allow life in the desert today. This Great Artesian Basin was a natural 'dip' or 'basin' in which the ocean rose or fell as a result of huge geological events forming a sea and then emptying out to expose land again. As a result large sediments were left behind - clay and silt deposits that hardened to impermeable stone. Land crossed by innumerable rivers carried sand and gravel which later formed sandstone.
On-going deep weathering produced silica which deposited itself as opal about 18 million years ago in seams, voids left behind by dissolving fossils or simply cracks and crevices in the drying muds or sandstones.
Opal is defined chemically as a hydrated form of silica with water content between 1% and 21% Precious opal usually contains 6% to 10% water.
Opal colours range across the total spectrum from deep red to violet. Precious opal may be transparent, translucent or opaque with patches or flashes of colour which appear, change and disappear as the stone is moved.
Categories of precious opal have arisen due to the varying nature of the opal's host rock or mother-stone which also generally identifies its geographical location within the Great Artesian Basin.