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.
This specimen is part of larger collection of significant geological specimens in the Burke Museum that was collected from around the world between 1868-1880. A large percentage of these specimens were collection as part of the Geological Survey of Victoria 1852-1974. The Geological Survey of Victoria was an organisation founded in response to the Victorian gold rush to explore the geological and mineral resources and to record a detailed map of the state. It was headed by British geologist, Alfred Richard Cecil Selwyn (1824-1902), who was responsible for issuing over 60 geological maps during his 17 years as director. These maps were all hand-drawn and coloured and became the benchmark for accuracy for geological mapping. 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.
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.
A hydrated amphora form of silica mineraloid which has a light blue "play-of-colour" in the centre.
Opal is a form of silica so it is made up of silicon and oxygen. Though it shares its chemical formula with quartz, the atoms within it are arranged very differently giving it different physical properties. Opal has what is called an amorphous structure, meaning it is not crystalline and is therefore classified as a mineraloid rather than a mineral. Though not crystalline there is structure within opal. There are four different forms but usually the atoms arrange into spheres which pack around each other. When the spheres are about the same size light diffracts through them and produces many beautiful colours, including blues, reds, greens and yellows. This is called opalescence.