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Approximately 95% of all the world's precious opals are found and mined in Australia
The remaining 5% is shared to a greater or lesser extent by some 19 state territories. In most of these states, however, precious opals are only found and only in nine states are they mined.
World map of precious opal deposits
Much has been said and written about Australia's precious opal deposits. A brief summary or repetition of this information will therefore suffice. Australian precious opals differ from precious opals found in the rest of the world primarily in their origin. Whereas in all cases in the rest of the world they were the result of volcanic activity and hydrothermal processes, in Australia opals were mostly the product of weathering.
All of Australia's opal deposits are located in the eastern part of what is known as the Great Artesian Basin, which covers almost one-third of the continent's surface. The rocks found here were formed during the Jurassic and Cretaceous periods, between about 200 and 70 million years ago. During this period, much of Australia was covered by a shallow continental sea, on the bottom of which sandy and clayey sediments were slowly deposited. Around the Tertiary period, approximately 70 to 35 million years ago, a major geological change (the continent emerging from the sea) occurred, the sea receded and the sedimentary basin area became a desert.
It is natural that in different parts of the Great Artesian Basin there were different conditions for the retention of silica-saturated solutions in the subsurface and for the progress of opal mineralisation, which significantly influenced the type and quality of the opal formed.
In principle, the main types of Australian precious opals can be divided into:
Black and Dark Opal - the colour of the basic mass is black, semi-black, dark grey to light grey. The colouration is due to the presence of carbonaceous substances in the molecular structure. It is characterised by a multitude of beautifully opalescent rich colours highlighted by a black or dark background. It is always accompanied by a common opal, the so-called potash (of the same colour as the base material). black opal is found in layers of clay.
Crystal opal - characterised by soft pastel colours dispersed in a milky base. The base colour is white, grey-white to yellowish. It is always accompanied by a general opal, the so-called potche (the same colour as the base). White opal is found in layers of clay.
Boulder opal - stands out with incredibly rich to wild colours, exciting play of colours and an unparalleled array of colour patterns. It occurs in irregular, angular and undulating veins in ferruginous sandstone, eventually forming impregnations.
The first occurrences and discoveries of precious opals in Australia date back to the late 19th century, while mining did not begin until the 20th century. The production of mined Australian precious opals has rapidly and significantly influenced the world market for this commodity virtually to the present day. Australian precious opals have earned an extraordinary status and have become a sort of internationally recognised symbol of quality and appreciation in this field.
Text: RNDr. Jan Mraz, CSc.
foto a video: Petr Winkelhofer, www.drinkteatravel.com, www.mineshaft.com.au