An overview of Australian opals

We publish courtesy of Helium

Author: Gail Kavanagh


Most gems come out of the ground giving little hint of their scintillating beauty until they are cut and polished – but not the Australian opal. Even as it emerges from the surrounding rock, it glitters like stained glass with rich vibrant colors.

This rainbow effect is caused by  minute spheres of silicon gel which has been trapped in the fissures and cracks of the ironstone in which it is found.

Australia gives the world 95 per cent of its opals, which come in several different types, the rarest being the black opal from Lightning Ridge, which forms in an iron oxide rich material known as ‘potch’. Potch itself is almost worthless, but the opal from it is extremely precious.

Black opals are rare because the potch only yields about five per cent of the colored opal, but they are the most beautiful of all, the dark material in which they form giving the colors a radiance that surpasses all other gemstones.

Boulder opals are similarly rare, and form in ironstone in western Queensland. Like black opals, they benefit from the dark backing provided by the ironstone, but only one carat of fine opal can be extracted from around 5 tonnes of ironstone. The opal forms in small crevices in the ironstone.

Considerably more common is the crystal opal, which forms from pure hydrated silica at Coober Pedy in South Australia. This tongue in cheek name means ‘white man in a hole’, which describes exactly how opal is mined. Up to 3 per cent of all opals mined are crystal opals. These are very attractive opals, transparent because of the silica, but not as dense and vibrant as black and boulder opals.

White opal is the type you will most often see on sale. Found at Coober Pedy, the white opal contains magnesium oxide, so forms on a white base. They are less vibrant than black or boulder opals, but they do have the range of colors, albeit in a paler form.

To enhance the colors of pale opal, and give it the intensity of black opal, without increasing the cost, opal cutters place a thin layer of potch on the back of a crystal opal. This is called a doublet opal.

To further enhance the colors, opal cutters add a third layer, a crystal quartz cap which magnifies the colors. This is called a triplet opal.

There are also manufactured opals called Gilson opals, which are made in laboratories to imitate the natural process of raw opals.

Opals are rich in folklore, in Australia and around the world. Long believed to be a healing stone, the opal was praised by Pliny, although in Roman times they were extremely rare and usually came from India.

In Australian Aboriginal folklore, opals are part of many Dreamtime legends, including one in which the creator, a great opal, wept for the sorrows of mankind, and his tears fell to earth. This is similar to Greek legend in which the tears of Zeus became opals.

Steeped in legend, wreathed in rainbow glory, the opal maintains its fascination in the hearts and minds of all who see it.

 


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