The mystery gem

Get six geologists in a room to discuss the origins of opal and you’ll get six different answers. Unlike many other minerals, relatively little is known about the processes involved in the formation of opal or exactly how old opal is.

The opal genesis mystery, while part of the romance of the industry, is holding back the exploration of new sites as old opal fields become worked out.

Opal is a form of silica, chemically similar to quartz but containing water within the mineral structure. Precious opal contains between six and 10 percent water and is made up of small silica spheres arranged in a regular pattern.

It is this ordered array of silica spheres and voids that diffracts white light, breaking it up into the colours of the spectrum. The colour range is determined by the diameter and spacing of the sphere – small (150-200nm) produce opal of blue colour only, while larger spheres produce red colour. Red fire is the most rare colour, followed by green/orange, green/blue and blue. However, regardless of colour, opal value is mostly determined by brilliance and clarity of an open proportioned pattern.

The major genesis theories

There are three major opal formation models currently in circulation that are backed with a scientific basis.

The ‘deep weathering theory’ believes that during the Tertiary period the rocks, which now contain opal, were subject to significant weathering. Over time small amounts of silica tended to be leached from the sandstone layers by water that then passed through the rock until it became trapped by underlying layers of relatively impermeable claystone. Under the right chemical and physical conditions opal was precipitated from the water in porous areas or voids.

There is a tendency for opal to be found close to faults in the rock layers and near blows (disturbed ground). If this model is correct then these faults and blows would have provided paths for water containing silica to flow along.

The Syntectonic (Pecover) theory says opal was deposited by heated water under pressure that originated from underground. The water tended to flow preferentially along faults and blows, depositing opal.

The Microbe model is based on the fact that opal bearing claystones commonly contain substantial amounts of fine, fossilized organic matter. Various types of microbe fossils, primarily aerobic bacteria, have also been identified as fossils within opal. Although the microbe fossils are not visible to the naked eye, microscopic studies have shown them to be quite abundant. Under this model the ongoing feeding and waste production processes of the microbes, while they were alive, created favourable physical and chemical conditions for the formation of opal.

The ‘open-system’ theory

Jack Townsend is a retired geologist with the Aussie Department of Mines (now PIRSA) and co-author of The Opal Handbook, published by PIRSA.

After a lifetime of studying the gem, Townsend is convinced opal is about 18 million years old, or between 15-20 million years old.

“The age of formation of Australian precious opal estimates range from Late Cretaceous through to mid Tertiary and attempts at age dating opal in the laboratory have always been very much younger than deduced geological age dating.”

The prevailing concept assumes a wet climate during the Tertiary following a long period of deep weathering from Late Cretaceous to Early Tertiary, he says.

“The source of silica bearing water, the commencement of opalisation and the patchy nature of precious opal has given rise to many theories on the growth and deposition of opal.”

There also a theory among some Australian universities that the opal process is ongoing, he says. That’s its an ‘open system’ where trace elements are continually lost and gained.

“Triggers for the initiation of opal growth theories include silica concentration resulting from a falling water table on a semi - permeable clay layer (aquitard), microbes or similar chemical mechanism which changes the pH and radionuclides that kick start the process.”

The slow settling of silica spheres previously envisaged, but now backed by scientific work, has shown that the greater the viscosity, the larger the spheres and the slower the spheres settle in a gel – results in full spectral colours and, therefore, better quality precious opal, says Townsend.