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About Opals

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. 

Opal Categories

Black Opal is precious opal on a black mother-stone or potch. Origin confined to Lightning Ridge in New South Wales, Australia.

White Opal is precious opal on a milky white or translucent mother-stone or potch. Originating in the South Australian fields of Coober Pedy, Mintabie and Andamooka as well as White Cliffs in New South wales.

Boulder Opal is precious opal in veins and cavities in dark ironstone or sandstone which are found in the form of boulders. Originating in South-Western and Western Queensland, Quilpie, Yowah, Opalton and Winton.


The Opals we have been describing above are all solid opals. This means that they have been cut to include their host-rock as a natural backing ...... in other words 'as they have been formed in nature'.


However, sometimes these colour-bands of opal are thicker and they can be cut into several thinner colour-bands which are then glued, for stability, to a dark backing stone thereby rescueing the beautiful colour-play which would normally be lost. These are referred to as either Opal Doublets consisting of a thin colour-band of opal glued to a dark backing stone (double layered) or Opal Triplets consisting of a clear protective cap of quartz or glass on top of the colour-band of opal and its dark backing stone (triple-layered).