A general overview of opal....
There are many countries in the world that have produced opal, not all of it is stable. There are three main forms of opal; Precious opal, which has play of colour and fiery flashes. Hydrophane opal which is formed under volcanic conditions, and common opal or potch, which is opal with no play of colour.
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White Cliffs crystal opal
Lightning Ridge black opals
Mexican contra luz opal
Andamooka crystal opal
Yowah nut
Opalton pipe opal
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Australia was the biggest producer of opal for a long time, recently opal has been found in large quantities in Ethiopia. Ethiopian or Welo opal is hydrophane which is volcanic in formation, this means that it is porous and therefore absorbs liquids then dries out. It is very prone to cracking for this reason and is also dyed and called black opal. Australia produces one of the most stable forms of opal. Opal was first found in Hungary in the 1500s, the mines there have long been exhausted. White Cliffs, NSW was the first opal field to be mined commercially, a German syndicate exported it to Europe where it was made into jewellery by the likes of Lalique. In those days the market was only interested in crystal opal, (transparent, translucent opal) and when the black opal was first found in Lightning Ridge it was thought to be rubbish and thrown away. The same for QLD matrix which is hugely popular these days. Black opal is the most expensive, sought after opal at the moment and many people do not realise that opal on black potch is not black opal unless the body tone of the opal layer itself is dark. Most of Australia's opal comes from our former inland sea sediments with the exceptions of Tintenbar opal which was mined near Byron Bay and is volcanic and therefore hydrophane. It can be unbelievably beautiful, as can the Welo opal, but it is no longer being mined as the area is now suburbia. Western Australia produces dendritic opal and some orange body toned opal some people refer to as fire opal, it has no play of colour and is usually faceted. True fire opal comes from Mexico, orange body tone and very beautiful. It is also volcanic in formation but is usually very stable. Mexico also has water opal (completely transparent and colourless in body tone with fire and play of colour, and contra luz opal which looks clear with light in front of it and has incredible play of rainbow colour when the light is behind it. Mexico's host rock (the rock the opal formed in) is rhyolite and lovely little pockets of colour form within it from which cabochons are cut. Honduras produces opal in basalt and some of it very much resembles boulder opal. It is stable and has its own unique beauty, often forming in bands of colours. Brazil has one tiny area that produces top quality stable crystal opal, volcanic in formation and never mined commercially as the nodules of opal are randomly scattered here there and anywhere in the dirt so it is found by the locals which is great. Peru produces common opal in pinks, green and blues. Indonesia has a lot of wood replacement opal, usually very dark in body tone and not very stable. Oregon in the USA produces opalised wood (they call it conk) and some pretty opal but it is very unstable and cannot be used in jewellery. Even within the Australian artesian basin the opal has very different formation from field to field and often from mine to mine on the same field. Black opal has been found on all fields but is rare, it is still rare in Lightning Ridge but more common there than anywhere else, due to the presence of freshwater inlets. Lightning Ridge produces all forms of precious opal - black, crystal, milky and they have a larger variety of opalised fossils than the other fields. The host rock is clay. The potch is grey, dark blue or black. They have seam opal, lots of fossils and 'nobbies". White Cliffs produces milky and crystal precious opal and marine fossils. The host rock is clay or sandstone. The potch is white. Coober Pedy also produces crystal and milky opal and marine fossils. Potch is white or cream. Andamooka produces some of the finest crystal opal as well as a type of sandstone matrix (where the opal is in the host rock) that is 'cooked' to turn the host rock black and show the opal more clearly, usually called fairy opal. Mintabe produces black opal and stunning crystal, the host rock is white sandstone. Yowah produces 'nuts' ironstone concretions that contain veins of opal, as well as pipe opal (former branches) which is often stunning crystal. The ironstone is often ochre, red brown or dark brown. Koroit is very similar to Yowah but the ironstone tends to be harder and darker, it is sometimes almost purple, black, red, and dark brown. Both Yowah and Koroit also produce conglomerate - ironstone nuts embedded in sandstone. The potch from both these field is white, yellows, ochres, oranges and reds, by far the largest variety in potch colours. Opalton produces pipe opal and ironstone concretions which are usually softer than Yowah and Koroit and the veins of opal are more spread out so they are usually split rather than faced as a setting stone. They call their boulders 'spuds' or 'pancakes'. The potch is often orange. Most of the other QLD fields produce pipe opal and veins of opal in sandstone which are usually split. Opalised wood is quite common in QLD but they don't have as many fossils for some reason as yet unknown to me. |
THE FORMATION OF OPAL IN CENTRAL AUSTRALIA
Exactly how opal was formed is still something of a mystery, however, the latest research, as presented at the 8th National Opal Symposium in Winton 2016, is as follows...
Sometime between 135 and 95 million years ago, during the Cretaceous age of dinosaurs, a large section of Central Australia was covered by an inland sea. The Great Artesian Basin was shallow, muddy, cold and still. There was no oxygen at the bottom. Layers of sediment built up, including bones, shells and plants as they fell to the bottom.
Eventually the sea dried up and through a process of erosion and uplift (where there is a vertical elevation of the earth’s surface), the buried sediments rich in pyrite and iron were progressively brought to the surface where they oxidised on contact with air between 95 and 60 million years ago.
This oxidation led to the production of large amounts of sulphuric acid which broke down the feldspar and volcanic ash into clay, and clay into large volumes of silica, and iron was released to form goethite, which is what gives the host rock its rusty orange and red colours.
Hydraulic fracturing (where rock is fractured by pressurised liquid) was able to concentrate the amorphous (without a clear shape or form) silica gel into veins in the rocks. Voids were also formed from the decay of bone, shell and wood. The seams and cavities were filled with the silica saturated fluids, which were then sealed by secondary minerals and the carbonate of fossils were slowly replaced by silica gel.
The amorphous silica gel then evolved in a closed environment where the pH became alkaline, which is necessary for the formation of homogenous silica spheres that lead to the formation of precious opal.
The silica spheres in precious opal have an orderly array (they are all the same size – between 200 and 400nm in diameter- and lined up and stacked in neat rows).
It is the spaces between the spheres, which are usually filled with water, that diffract light and give opal it’s play of colour.
Potch, or common opal, has a disorganised array - silica spheres are all different sizes so they are unable to line up neatly and therefore do not diffract light.
Most opal in the rough has areas where there is bright play of colour amidst potch, sometimes just a thin bar of colour, usually on the bottom or in the middle. This happens when some silica spheres of the same size separate out of the mix of sizes and line up. The silica gel takes time to harden which allows for a process of sedimentation to occur and the more alkaline the PH, the more likely precious opal is to form.
Black opal (where the body tone of the opal itself is dark) is formed through the same process but the solution of silica also contains other minerals, finely grained particles and microorganisms, which cause the black colour (like muddy water where the mud settles out). Hence you can see the black colour mostly on the bottom. Black opal also contains black minerals like pyrite and trace elements such as iron, sulphur, zinc, titanium, cobalt, copper, arsenic, nickel, lead and small amounts of carbon.
What is interesting to note is that Central Australia is like no-where else on Earth and it is a lot like Mars. Opal was recently found in a piece of meteorite from Mars.
Professor Patrice Rey, School of Geosciences, University of Sydney Dr Paul Thomas, University of Technology, Sydney
Dr Alexander Fink, LaTrobe University, Melbourne
for sharing their research and commenting on this summary.
Silica sphere photos courtesy of Dr Alexander Fink, opal photos by Linda George
I would like to say Thank You to...
Most opal in the rough has areas where there is bright play of colour amidst potch, sometimes just a thin bar of colour, usually on the bottom or in the middle. This happens when some silica spheres of the same size separate out of the mix of sizes and line up. The silica gel takes time to harden which allows for a process of sedimentation to occur and the more alkaline the PH, the more likely precious opal is to form.
Black opal (where the body tone of the opal itself is dark) is formed through the same process but the solution of silica also contains other minerals, finely grained particles and microorganisms, which cause the black colour (like muddy water where the mud settles out). Hence you can see the black colour mostly on the bottom. Black opal also contains black minerals like pyrite and trace elements such as iron, sulphur, zinc, titanium, cobalt, copper, arsenic, nickel, lead and small amounts of carbon.
What is interesting to note is that Central Australia is like no-where else on Earth and it is a lot like Mars. Opal was recently found in a piece of meteorite from Mars.
Professor Patrice Rey, School of Geosciences, University of Sydney Dr Paul Thomas, University of Technology, Sydney
Dr Alexander Fink, LaTrobe University, Melbourne
for sharing their research and commenting on this summary.
Silica sphere photos courtesy of Dr Alexander Fink, opal photos by Linda George
I would like to say Thank You to...