Gemstone occurrences and mines in Australia
Australia supplies around 95% of the world’s precious opal from large deposits in New South Wales, South Australia and Queensland (around the edges of the Great Artesian Basin).
Opals in Australia
In Australia, precious opal is found in Cretaceous age (145 to 66 million years ago) sandstones and mudstones that have been weathered. This weathering released silica into groundwater, where it flowed through cracks in the rocks and slowly hardened into sub-microscopic spheres. These spheres produce the amazing colours of opal, as the different sphere diameters create various sizes of voids between the spheres approximately equal to the wavelengths of light.
The red colour in opal is rare as red light’s wavelength is longer than blue or green, so red would only be created if the opal contained larger silica spheres.
Why isn't all opal coloured?
Common opal (potch) is generally similar to precious opal in that it consists of silica spheres, however in the common variety the spheres are not of a regular size and the ability to diffract light is lost.
Opalised mussel shell (Eyrena sp.)Geoscience Australia
The original minerals making up these fossils have been dissolved away by groundwater and precious opal has been formed in their place.
Opalised cockle shell (Cyrenopsis sp.)Geoscience Australia
Opalised belemnite rostrumGeoscience Australia
Belemnitida (or belemnites) are an extinct order of cephalopods which existed during the Mesozoic era (252-66 million years ago). They were similar in appearance to squids.
Opal in jewellery
Opal frequently occurs as thin seams of material within a host or matrix rock. Although beautifully coloured, some deposits are so thin and fragile that they are useless for jewellery. A doublet or triplet is created by cementing one of these thin layers to a strong black backing.
Doublets consist of two layers adhered together with glue:
- a black backing which is made of either black industrial glass, black potch (colourless opal), hard plastic, brown ironstone or sometimes vitrolite.
- a thin slice of opal. The thickness of the opal in a doublet can vary, however it is generally thicker than the opal found in a triplet. The edges of the slice of opal are generally rounded off (if there is enough opal) to give the stone a cabochon (domed top).
Triplet Triplets consist of three layers: - a black backing, - a paper-thin slice of opal in the middle, and - a clear glass, quartz, or plastic capping.
The slice of opal in a triplet is usually extremely thin (paper-thin) so the clear capping serves to give the stone a nice cabochon on top. The clear capping may also magnify the colour of the opal and can also help to protect the opal.
Triplets are normally cheaper than doublets because they contain less real opal. Because the top of the stone can be made from synthetic material, triplets can also be a lot more resistant to wear than solid or doublet opal stones. Opal by its nature is a fragile gemstone.
The National Mineral and Commonwealth Paleontological Collection, Geoscience Australia and The National Museum of Australia Mineral Collections (specimens).
Chris Fitzgerald (photographer).
Theo Chiotis (cartographer).
Geoscience Australia, 2014. Exploring Minerals and Crystals: Teacher Notes and Student Activities. Record 2014/55. Geoscience Australia: Canberra. http://www.ga.gov.au/metadata-gateway/metadata/record/gcat_79032 (content).
Opals Downunder, http://www.opalsdownunder.com.au/learn-about-opals/introductory/opal-doublets-and-triplets
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