Opal is probably the most famous of South Australia’s gems. Having been declared both Australia’s National and South Australia’s State gemstone.
The opal formed as the Great Artesian Basin dried; the sediments weathered from sandstone and shale to kaolin clay. Soluble silica percolated down into the ground, eventually reaching impermeable clay layers, where the silica spheres hardened into opal seams.
Most opal is believed to have settled between 15 and 20 million years ago. Of course, there have been distinct differences in the timing and emplacement of the various opal fields.
The Andamooka field, discovered in 1930, is currently in operation and produces crystal opal, white opal, some black opal, and ‘Painted Lady’ (coating opal along surfaces). Mining occurs predominantly on one horizon, locally known as “the level”.
At the Coober Pedy field, discovered in 1915, most opal is mined in a sandstone layer in sub-horizontal to sub-vertical veins. Due to its location in the Artesian Basin, it is not uncommon to find fossilised shells or opal that has infilled spaces left by shells. A few examples of opal-fossilised plesiosaurs and ichthyosaurs have been found.
The main type of opal found is light opaque to translucent opal, as well as some crystal.
At the Mintabie field, which was discovered in 1929, the town has closed, and the mine has been decommissioned, although mining is still possible with appropriate permits. As Mintabie is on APY Lands (Anangu Pitjantjatjara Yankunytjatjara), access permits are also required.
Opal types include black opal, transparent crystal, and white opal, which often exhibit distinct darker and lighter bands.
Although not known for its diamonds, South Australia has produced more than 100 macro and micro diamonds over the years, as well as indicator minerals. Both kimberlite and lamprophyre occurrences are found within the state. Three locations in particular have been considered highly prospective for diamond exploration.
Historically, diamonds were found by gold panners between 1859 and 1900 at Echunga. Interestingly, only one diamond has been found since.
Diamonds were also found in the mid-1980s in the Springfield Basin, along with garnets, indicating that the source is close.
Eurelia is another area where diamonds have been found. Microdiamonds were found associated with kimberlite dykes. To date, these are the only diamonds recovered from source rocks. Exploration continues in South Australia to try to locate the source of the diamonds.
Nephrite jade deposits were discovered in Cowell on the Eyre Peninsula in 1965. This is one of the biggest nephrite jade deposits in the world, and to date, more than 100 outcrops have been discovered in the area.
There are three main varieties found there: green nephrite, black nephrite and premium black nephrite. They are all of fine grain and take on a high polish.
Chrysoprase was mined at Mt Davies, the north-west corner of South Australia. Historically, mining took place in the early 1960’s and has occurred intermittently since. The extent of the deposit is unknown; however, the area has now been backfilled and rehabilitated.
Kangaroo Island, situated 13kilometres from the mainland of South Australia, is technically part of the Adelaide geosyncline, separated by a body of water known as the Backstairs Passage.
Even though the island is geologically similar to the mainland, there are stark differences that are yet to be fully understood. For example, Kangaroo Island has an abundance of tourmaline that is not commonly found on the mainland. Furthermore, Trilobite fossils have also been found in abundance on Kangaroo Island, but not in the same sequence on the mainland. One theory posed is that metamorphism has destroyed the fossils and tourmaline on the mainland.
Finally, while beryl, corundum, garnet and zircon deposits have also been discovered in South Australia, to date these have mainly been mined for industrial uses.
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