About Australian Opals
Silicate minerals in the stone add to its weight giving it a specific gravity of between 1.98 and 2.2 of pure water (precious opal is between 2.1-2.2).
Opals scratch hardness the Mohs scale ranges from 6.0 to 6.5 which is between the hardness of a moonstone and quartz and its refractive index varies from 1.44-1.46
Opal is a closely packed array of billions of spherical particles stacked in a three dimensional grating that has the unique ability to diffract white light into beautiful colours of the rainbow without any impurities. The colour is created when light is split by the voids between the spheres.
The diameters of the spheres therefore determine the maximum size of the wavelength or colour that can be developed. Visible colour spheres must be no smaller than 1500 angstroms for violet, indigo and blue and no larger than 3500 angstroms for orange or red.
Therefore a stone that can display red can display all shorter visible wave lengths, orange yellow green and blue.
Opal in Australia was formed about 60 million years ago when the deserts of central Australia were a great inland sea with silica rich deposits situated around its shoreline. Over time climatic change caused the sea to recede and disappear becoming the great artesian basin. During this time a solution subsequently was deposited in open cavities and cracks in sedimentary rock, not volcanic, around the Cretaceous period or the Age of the Dinosaurs, (explaining the occasional discoveries of prehistoric opalised skeletons, wood and shells).
Opal is found in three states in Australia:
New South Wales, Queensland, and South Australia:
The New South Wales opal fields are situated in flat, arid inland areas, some of the mines are White Cliffs is situated 295km north east of Broken Hill and is the oldest commercial opal field where mining started in 1889 when George Hooley and Alf Richardson discovered opal while kangaroo shooting. The town did not grow until the 1890 drought broke in 1893 when the population grew to 800 and production was in full swing growing to a population of 5000. Mining at White Cliffs continued to be a top producer for 20 years and much of the opal that came out was opalised wood, shells plants, and animals including the famous pseudomorphs (pineapple).?
Lightning Ridge situated 770km North West of Sydney first discovered opal in the late 1880s but its commercial value was not recognized until the great find of 1907. Lightning ridge is currently the major contributor of black opal in Australia. Many of its fields have effectively been mined out such as Three Mile, Thorleys Six Mile, Nobbys Deep Four Mile, Six Mile, Nebea Hill, Shallow Belars. Nevertheless there are many closely located fields.
Coocoran lake opal fields is about 20km west of lightning ridge and was first mined in the depression years, it was the site of a major rush in the 1990s and produced a large amount of highly valuable black nobbies.
Sheepyard field near Glengarry found in 1985, produces seam opal both black and white
Grawin is 42km south west of Lightning Ridge and most the opal is seam opal, light predominantly green in colour.
Glengarry is 4 km north east of Grawin, again a seam opal field
Carters Rush is 5 km north east of Grawin first developed in 1974 and again seam opal
Mehi is an old field discovered in 1924 and is still mined up until today
Queensland is famous for the beautiful boulder opal
Opal was first discovered in 1869 on Listowel Downs in Western Queensland prior to that the world had only known of the white opal from Europe. It took many years for it to be successfully marketed
The Yowah opal fields were discovered in the 1880s being named after the Yowah Creek. Opal from this area is highly renowned because of the famous Yowah opal nut.
Koroit is another opal field 85 km from Cunnamulla discovered by Lawrence Rostron in 1897. Koroit is still one of the best producing opal fields in Queensland famous for its matrix
Black Gate is a small opal field 90km from Eulo discovered in 1894 when it was known as Dynevor Downs field
Duck Creek and Sheep Station Creek were small opal fields discovered between 1889 and 1891. Duck creek was renowned at that time for its fine crystal opal but mining was short lived because of the lack of water
Quilpie is a town which is a central point for the Queensland opal industry, there are many mines situated around it
Eromanga is a town 104km west of Quilpie, the most inland town in Australia. Opal was discovered in the area in the 1870s and this was the main selling point for opal at the time it was the only opal town prior to White Cliffs
Opalton is a field 112km south of Winton discovered in 1886 by George Cragg renowned for the large volume and quality of opal produced.
Mayneside is another area where there are many mines; it is located 45km south of Opalton. It produced quality pipe opal some of which was black.
Kynuna discovered in 1894 by Peter Karaff was a small mine which mining stopped many years ago
South Australia was the first reported state where John Menge, found opal in the Adelaide hills in 1840. However it was not until 1914 when a young boy discovered Coober Pedy whilst searching for water that the opal mining industry began in South Australia
Coober Pedy is the largest opal mine in Australia , situated 1000km north of Adelaide , the name comes from the meaning White man in a hole it was not until the discovery of the Eight Mile mine in 1946 that Coober Pedy made its mark as the opal capital of the world. People who lived in the area adapted to the hot dry conditions by making their homes underground in what is called a dugout. Opal from Coober Pedy is of a light nature.
Andamooka was discovered in 1930 by Sam Brooks and Roy Shephard it is situated near Woomera. The opal from this area is of a light nature but many consider this opal to be the best light crystal opal available, although very little is mined in this area these days.
Mintabie is another mine situated 30km west of Marla on aboriginal land it is not known when mining first began however, mining was noted in the 1920s, the mine was small in comparison to Coober Pedy but it produced very high quality white as well as black opal in the 1970s and 1980s
Lambina is 58km east of Marla mining was rumored to begin in the 1920s and early 2000 big finds were again made the opal found was light opal with generally more blue/green.