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, Photo Cody Opal
Mexican contra luz opal
Mexican fire opal. Photo Opal Auctions.
Peruvian pink opal, unpolished.
Matrix in basalt from Honduras. Photo Anthony Dabdoub.
White Cliffs crystal opal.
Painted lady from Andamooka which has been painted on.
Yowah nut
Koroit matrix. Photo Ed Vaceitis.
Opalton pipe opal
Opalised wood from Yowah.
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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, referred to as cantera opals. 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. 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. 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 (once wood branches) 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. |