Opals are valuable gemstones known for their lustre and famous for the remarkable brilliance of its colors. They are formed of hydrated silica.Australia is a major source of the world's opals, and they are found in several fields, particularly at Lightning Ridge in New South Wales, Coober Pedy in South Australia and Quilpie in Queensland.When opals were first mined at the end of the nineteenth century, their value was not recognised and they even had a reputation for causing bad luck.
Where are Opals Found?
Opals are mined in the Czech Republic, Honduras, Mexico and Nevada but chiefly in Australia.Most of the world's opal production is mined in Australia, with Mexico the nearest competitor.Australia is the world's major producer of opals. It supplies more than 90 percent of these gemstones to the world market. The main centers of production are Lightning Ridge in NSW and Coober Pedy and Andamooka in South Australia.Less important centers are White Cliffs in far NW or NSW and WInton, Quilpie and Cunnamulla districts in Queensland and near Kalgoorlie in Western Australia.Opals are Australia's most valuable gemstone exports. The current Australian production figures for uncut gems vary between A$100 million and A$200 million. A large proportion of Australia's rough opal is exported to Hong Kong, Japan, the United States and Germany.Even though they are located in the desolate arid interior of the continent, the opal mining areas have become popular tourist attractions where visitors can try their luck at fossicking or purchase stones from the local miners.
A Closer Look
One of the chief characteristics of the opal is the brilliant play of colors which may be seen in superior stones.
An opal is made up of minute grains, or uniform spheres of amorphous silicia, the colors of which vary from grain to grain. Thus when an opal is turned the colours continually change.
Some opals with very small grains will give 'pin-fire' while others may give a single band of color. The colors range through the full spectrum and are found in an inifinite variety of patterns. The most valuable stones are those which contain at least four colors which are not lost as the stone is turned.
Opal occurs in fissures and cavities of any rock type and is mined at shallow depths with very little mechanisation. The seams are usually thin but rich fields have been discovered, causing 'rushes' similar to gold rushes.
There is no fixed price for opals and they are usually sold in parcels containing a mixture of qualities.
Most opals are described as 'white' or 'black' depending on their background shade, even though they may 'flash' brilliant colors, ranging from fiery red to blue or green.
Opals with a blue background are found at Amberooka in South Australia. These stones are semi-transparent and the color flashes are green.
White opal is mined at Coober Pedy and Andamooka, South Australia.
In New Zealand, geyserite, a less valuable stone related to opal is found in thermal regions.
The most valuable form of opal is black opal with a dark red fire, found only in the Lightning Ridge area.
The History of Opal Mining in Australia
Opals are thought to have been found in Australia as early as 1849 when a German geologist Johann Menge made a discovery a Angaston, South Australia, although the strike was not officially recorded. However the first official find was made in 1872 when boulder opal was discovered at Listowel Downs in central Queensland.
In 1889, four prospectors, traveling in semi-desert country about 1100 km inland from Sydney, came across lose opals or 'floaters' lying on the ground. This was the beginning of the famous White Cliffs opal field, 104km NW of Wilcannia. Australia's first commercial field and the largest opal field in the world, producing light-colored opals of extraordinary beauty.
The first Lightning Ridge field was discovered in 1891 but not developed until 1903. The second strike was made in 1904.
The highly prized black opal was first discovered at Lightning Ridge, New South Wales, in 1887 but its worth was not immediately recognized. The gems taken from this field are now considered to be the finest in the world and along with stones form Coober Pedy and Andamooka, South Australia, continue to dominate world markets. Other opal-producing fields are located at Mintabie, South Australia, and Quilpie, Cunnamulla, Yowah, Eromanga and Duck Creek in Queensland.
Opal Mining Techniques
Opal mining or gouging is mainly carried out by individuals using shaft-mining methods, but there is a limited use in some places of open-cut methods with heavy machinery such as bulldozers. Although seam opal is sometimes found at Lightning Ridge, the gemstone is almost always found in the form of 'nobbies' or stones of varying size which occur in pockets or patches. Each individual miner usually works a lease measuring up to 50 meters by 50 meters. He sinks a vertical shaft through a hard white rock called shincracker (because it spliters easily, often hitting the miner on the shins).Some of thse shafts extend 25 meters down before they reach opal-bearing clay. On most opal fields, the whole area is honeycombed with shafts. As far as the eye can see, there are mounds of earth and rocks, looking like mole hills bleached white by the sun. Opal and potch are found as seams or veins, not unlike gold-bearing seams, in sandstone or conglomerate rocks, or as rounded lumps (nobbies) in clay. A practiced eye is needed to identify valuable stones in the raw state. Washing with water or nipping with pliers is usually necessary to bring their color to light.Once unearthed, opals are cut and polished into the desired shapes, usually dome-shaped cabochons. Others are backed with a slice of potch - valueless opal-bearing matrix - and are called doublets. Triplets have another layer of transparent rock crystal added to prevent the surface from scratching. Because opals contain water, some crack or craze after being removed from the ground and exposed to dry air.
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