Opal fits into the category of amorphous mineraloids because, unlike the others mineral gems, its structure is not truly crystalline. It consists of water and hydrated silicon dioxide. The water as an ingredient usually presents around three to ten percent but it can be as high as twenty percent. Stones with such high water percentage are very fragile and prone to cracking and dehydration and therefore their value is very low. Precious opal gemstones contain 4-6% of water. Opal is a type of gel or semi-liquid material that is formed at comparatively low temperature. They can be found in the fissures or gaps of any type of rocks. Most commonly opal is found in the fissures of basalt, limonite, rhyolite and sandstone. Boulder opal occurs in ironstone.
Opal gem is found in various shades, such as magenta, white, green, gray, black, red, blue, yellow and brown. Among all the shades, black and red are rarely found. Also, they are the most expensive and sought after opals. On the contrary, blue, green and white are found much more frequently. Although nature of opal is amorphous (it lacks the crystalline structure) valuable, good quality opal has a well structured format.
In the past centuries, Slovakia was the only place where mining of expensive opal was taking place. Presently valuable, high quality opal is mined in Australia, Mexico, US and a few other places. Ordinary opal without much shade and not of any significant value is available in many nations across the World.
Opal is Australia’s national gemstone. The country mines over 95% of the total world’s opal production out of which boulder opal represents up to 2%, black opal 8% and unadulterated hydrated silica or crystal opal around 30%.
Different shades and kinds of opal are found in various parts of the World. For instance, black opal is found in abundance in New South Wales in Australia. In Brazil, opal is mined from the lava ash settlement or deposits. Queensland (Australia) is the resource of boulder opal. Fire opal is available in the Mexican region and in some USA’s states like Nevada for instance. Other nations of the world where mining of opal resources takes place are Brazil, Peru, Turkey, Nicaragua, Hungary, Ethiopia and Guatemala.
Black opals are by far the most valuable and sought after opals of them all. They are extremely rare. The “Lightning Ridge” area in New South Wales, Australia, produces the highest quality black opals in the World. Therefore the gems mined from this region are always in high demand from gemstones’ collectors and investors from around the Globe.
The other area where black opals can be found is “Mintabie” in the state of South Australia. Color of these opals is not as intense and bright as in those from Lightning Ridge area but they are still of very high quality and value.
Some of the most spectacular black fire opals come from the “Royal Peacock” mines in Virgin Valley, Northwest Nevada, USA. In the past 25-30 years more than 250,000 carats of precious fire opals have been extracted from this location.
Apart from Australia as a country, its state of South Australia also has opal as its official gemstone. Even the Australia’s national women’s basketball team is referred to as “The Opals”. Mining of opal is a lucrative business and is an important employment provider in the above mentioned countries. Opal mining industry contributes a great deal of money to the countries’ economies.
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