From the Ballarat School of Mines.Four geological specimensrocks, geology, opal potch, lightening ridge

In Australia, precious opal is found in Cretaceous age sandstones and mudstones. These sedimentary rocks were deeply weathered and this weathering released silica into the groundwater.Australia is the only part of the world where opalised animal and plant fossils have been found. Opal artefacts several thousands of years old have been discovered in East Africa. As early as 250 BC the Romans prized opals, thought to have come from mines in Eastern Europe, the ancient world's main source of opals. There are many aboriginal dreamtime stories that feature opal. Australian opals discovered during the late 1800's found little favour with European markets but their commercial value increased in the 1900's and in 1932 Australia took over as the major producer of opals in the world and remains the largest producer to this day. Opal is found around the world (Brazil, Mexico, Honduras and the western US) however Australia produces 95% of the world's precious opal and it is our official national gemstone. Opal was first mined commercially at Listowel Downs in Queensland in 1875 and later at White Cliffs in NSW. Today, Coober Pedy (SA) is the main producer of white opal, though in recent years this field has expanded and all types of opals are found. Other centres in SA include Andamooka and Mintabe. Lightning Ridge (NSW) is renowned for black opal and formerly White Cliffs was a large producer of high quality opal. Boulder opals (opals in concretionary ironstone) are mined in Queensland from numerous localities in a zone extending from the Eulo and Cunnamulla district in the south and northwest for a distance of over 700 km to Kynuna in the north. The towns of Quilpie, Yowah and Winton are the main opal mining and wholesale centres. Opals are considered gemstones and have been used in jewellery for thousands of years.Throughout much of history, opals were actually believed to be good luck. The Romans thought that opals were one of the luckiest gemstones and a symbol of hope. In the Middle Ages, opals were believed to be bestowed with all the positive properties of coloured gemstones due to its rainbow-like play of colour. Finally, there is a superstition that you should not wear an opal unless it is your birthstone otherwise misfortune will befall you. This, of course, is far-fetched, but the notion could have been promoted in the late 19th and early 20th Centuries by diamond traders who were trying to increase sales of diamonds and deter people from buying opals. Possibly related to this is the thought that you should set opal jewellery with diamonds as their powers of good fortune will override any negativity held by the opal. The great majority of opal does not show play of colour and is called common opal or potch however this is not the case with a precious opal. Opal is a precious gemstone, like rubies, emeralds or diamonds. Opal is rare, and it is expensive to prospect and mine for.Silica is one of the most common minerals on the planet, but precious opal is very rare – far more rare than diamonds. Precious opal is rare because the natural processes that create it rarely occur.Most (at least 95%) of the opal found by miners is common opal without gem colour. In Australia we call it potch. It can be white, grey, black or amber coloured. Even when a miner finds gem-coloured opal, most of it can’t be cut into gemstones because it’s too thin, or sandy. This specimen is part of a larger collection of geological and mineral specimens collected from around Australia (and some parts of the world) and donated to the Burke Museum between 1868-1880. A large percentage of these specimens were collected in Victoria as part of the Geological Survey of Victoria that begun in 1852 (in response to the Gold Rush) to study and map the geology of Victoria. Collecting geological specimens was an important part of mapping and understanding the scientific makeup of the earth. Many of these specimens were sent to research and collecting organisations across Australia, including the Burke Museum, to educate and encourage further study.It is not known where this opal originated, except that it is probably from Victoria, as it has been recorded from many localities in the State. Common Opal is formed from silica-rich water circulating through rocks near the Earth’s surface. It consists of minute spheres of silica arranged in different ways. In common opal, the spheres are of different sizes and randomly arranged, unlike in precious opal where the spheres are of similar size and uniformly arranged in three dimensions. These differences account for common opal generally being translucent to opaque and without the play of colours, or opalescence, displayed by precious opal. Common opal is found in many localities and different geological environments throughout Australia and the world. Precious opal requires special conditions to form and is much less common. Australia produces most to the world’s precious opal. burke museum, beechworth, indigo shire, beechworth museum, geological, geological specimen, precious opal, opal, brazil, mexico, honduras, queensland, coober pedy, gemstones, jewellery, play-of-colour, light blue