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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 or mother stone which also generally identifies it's geographical origin:
| 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 from opal rock having a relatively thick colour-band. These are the most valuable. However thinner colour-bands of opal may be cut into: |  |
Doublets are made by glueing thin slices of opal to a dark backing stone thereby rescueing the beautiful colour-play which would normally be lost. Doublets are generally less valuable than solid opals.
Triplets are made by adding a clear protective cap of quartz or glass to a doublet. As the slice of precious opal may be even thinner, triplets are generally less valuable again than doublet opals. |
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About Opals 
