Opals in Australia are formed by silica bearing water stuck inside any type of rock. The silica gel in the rock precipitates at low temperatures forming layers or nodules of opal in fissures, veins and cavities of volcanic or sedimentary rocks. Opal is a form of silica, chemically similar to quartz but containing three to twenty-one percent water within its mineral structure. Opals are gemstones that contain six to ten percent of water.
There are mainly three types of opal. Opal jewellery is made using precious opals which are identified by change of colour within a particular stone as it is rotated or tilted. The fire opal is a transparent opal that has a colour range from yellow to bright orange to dark red. Common opal, also known as potch is usually a colourless or white stone (may look grey, brown, yellow or red because of impurities). This type of stone which has no commercial value accounts for eighty to ninety percent of all opals. Most of the world’s precious opals come from Coober Pedy and Andamooka fields in southern Australia.
The precious opals are known for their play of colours which is caused by diffraction. Silicates in the soil dissolve in water to form silicate rich solution, which on entering a cavity deposits as tiny small balls. The layers of precipitated silica spheres form a jelly like water mass, producing diffraction grating when spheres are even in size and well ordered. A rainbow of sparkling light can be seen within the stone because of diffraction.
The play of colours is mainly due to uniformity of tiny balls (one-tenth of a micron) in the stone. A common opal is formed if the balls are random in shape and arrangement. The colours caused by regularly packed spheres depend on sphere size and space between them. If you move the stone, light hits the stone in different angles creating play of colours.
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