Opals

Opalised Fossil of a Cockle Shell, South Australian Museum

Opals are known for their brilliance of colour. The colour range of opals is large and includes, green, blue, magenta, rose, pink, slate, olive, brown, black, white and they can even be clear. Many colour combinations can take place in an opal. White and green coloured opals are the most common and the rarest colour combination is red against black.

The opal was declared as the gemstone emblem of South Australia on 15th August 1985. Almost a decade later, the Governor-General, the Hon Bill Hayden elevated the opal to being Australia’s national gemstone. In 2008, the black opal was declared as New South Wales’ gemstone emblem. Aboriginal people call the opal the fire of the desert and it is a powerful symbol from the arid centre of Australia.

There is a Dreamtime story that tells of a Creator that placed the colours of the rainbow into a stone to create opal. There is another Dreamtime story that tells of Birring Ooloo and Cunnum-Bielle, the two wives of the Supreme Spirit, Bhiamie, of the Dreamtime being stalked by the crocodile, Gurria. As the two women bathed in a spring, Gurria swallowed them both whole.

This angered Bhiame who tracked Gurria to a hiding spot in a lake, and speared him at a water crossing. As Gurria laid dying, he rolled around forming two holes, one became Cochorance Lake and the other, Angledool Lake. His death was said to have been marked by a shower of rain and the appearance of a rainbow. The colours of the rainbow were trapped in the scales of Gurria and these formed into opals. The areas of Cochorance and Angledool at Lightning Ridge have yielded some of the best examples of gem quality opal.

Opal isn’t quite a mineral. It is mineraloid. It does not have the ordered crystalline structure that a mineral does.

Opalised Fossils of Ichthyosaurus vertebrate, South Australian Museum

Opal does have some order in its crystal structure but it doesn’t continue throughout all of it. It starts and stops throughout the the entire substance. The water content in an opal by weight can be anywhere from 3%-21% though on average, the content is between 6%-10%. Opal with higher water content tend to crack during cutting and polishing, and in some cases during hot dry weather. The range of colours associated with the opal is due to its internal structure being able to diffract light.

So, how is opal formed? There must be some clever chemistry involved. Well, there is but no one knows exactly how opals are formed. Geologists and geochemists are yet determine the exact process of opal formation but there are three models of how it could happen.

Deep Weathering Model

During the Tertiary period, (geological time between 65 million and 2.6 million years ago), rocks that now contain opal were being worn away. During this time, small amounts of silica were being leached from the layers of sandstone by water. The water containing silica passed through layers of sandstone until it was trapped by impermeable claystone where under particular conditions, opal precipitated out of solution.

Syntectonic (Pecover) Model

Pressurised hot water containing silica from deep within the earth rose towards the surface along faults and channels in rock. This mineral rich water reached within tertiary rocks like sandstone and claystone travelling into fractures and faults. As the water travelled through the cracks, it became less pressurised and began to deposit opal leaving opal veins behind. Some of this hot water has escaped to the surface forming hot springs.

Microbe Model

The opal deposits found in claystone are often found in conjunction with fossils. These fossils are not visible to the naked eye but underneath the microscope, fossils of bacteria have been found within the opal. They have been found in high numbers. It is thought that the feeding and waste processes of the microbes created an environment that allowed opals to form.

Apart from being found naturally, opals are also created in the laboratory since the first synthesis in 1974 by Pierre Gilson. Synthesised opals are much more ordered and this can be seen by the patches of colour appearing in regular intervals like a patchwork quilt or in a chicken wire pattern. Another way to tell the difference between synthetic opal and real opal is that synthetic opals do not fluoresce under UV light.

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