Opal, a national precious stone for Australia. it is hydrated silica (SiO2.nH2O) and formed of minute particles of silica in tight spherical aggregates. Opal fits into the category of amorphous mineraloids, unlike the others mineral gems, its structure is not truly crystalline or cryptocrystalline.
Opal is made from small silica spheres arranged in a regular pattern. Opal silica contains water within the mineral structure. Precious Opal generally contains about 6-10% water. The vast numbers of colors in Opals are caused by the regular array of silica spheres diffracting white light, and breaking the light into the colors of the light spectrum. The actual range of color in a particular Opal is determined by the diameter and spacing of its silica spheres. The colors of an Opal can also change depending upon the angle of light incidence, so when an Opal is rotated, its color can change or disappear. This feature makes Opals extremely interesting and unique.
Opal is a true precious stone, not a semi-precious one, as many believe. There are, however, ‘common’, or non-precious forms. Common opal is found throughout the world, but precious opal is found overwhelmingly in Australia, with only 5% being found elsewhere, mainly in Mexico. One form of common opal is potch, of which there are many varieties and colours. Transparent amber potch is called fire opal. When it’s very transparent and bright yellow in colour, it is called a sun opal. Black potch is often used as the backing in opal doublets and triplets.
Over 90-95% of the world’s supplies of Opals come from Australia. The remaining 5-10% is from countries included Mexico, Nevada in USA, Brazil, Peru, Turkey, Hungary, Nicaragua, Ethiopia and Guatemala.
Black Opals come from mainly from Lightning Ridge (New South Wales - Australia)
Boulder Opal comes mainly from Quilpie (Queensland, Australia)
Crystal & White/Milk Opal comes mainly from Coober Pedy & Mintabie (South Australia)