More than 90% of the world’s precious opals come from Australia, and the rest can be found in other parts of the world such as Mexico, Brazil, Indonesia, Czech Republic, Ethiopia and USA. Opals from Mexico (called “Mexican Opal”) has more water and is transparent or extremely translucent. Opal is formed in diﬀerent ways; depending on where they are found around the world. Please note that this explains the ‘scientiﬁc’ way of how opals are formed, apposed to the magical dreamtime stories of how the native Australians (Aboriginals) believe that opals are formed.
Hydrophane Opals. Many of the opals mined in Ethiopia, especially from the Welo deposits, are hydrophane opals. “Hydrophane” is a name used for a porous opal that has the ability to absorb water. A change in color or transparency often accompanies the water absorption. This reaction is formed from the volcanic rock deposits and Lava within the Volcanic region of Ethiopia. It is very diﬀerent to the way that Australian Opals are formed.
Australian Opal is formed from a chemical reaction of silicon dioxide and water. As water runs down through the earth, it picks up silica from sandstone, and carries this silica-rich solution into cracks and voids, caused by natural faults or decomposing fossils. As the water evaporates, it leaves behind a silica deposit. These deposits have formed a shiny rock appearance with different colours and patterns that people refer to as Opals. They are found in ironstone rocks, sandstone rocks and sometimes in fossils of pre-historic animals such as dinosaurs, bugs, wood (tree roots ect) and even in shells. It takes about 5 to 6 million years to make a 1 cm opal to mature. Opals were formed in Australia from a giant inland lake that once covered what is now the Australian desert.
There are many stories in different Aboriginal tribes around Australia of how opals are formed. Some believe they came from a giant rainbow serpent who spread water through-out Australia to form the opals in the Earth. Other tribes believe a giant rainbow hit the Earth in the beginning of time, and formed the opals as a gift to the men of Australia. There are many different stories from the Aboriginal people, and one could spend a lifetime discovering them.
The different opal types come from different regions in Australia. The opal ﬁelds of Australia are in the harsh outback where only the hardened miner can make a living. Black Opal comes from Lightning Ridge in New South Wales. Light or White Opal is mined principally from the South Australian ﬁelds of Mintabie, Coober Pedy and Andamooka. Boulder Opal comes from the Queensland mining regions including Winton, Koroit and Yowah. It is not known why the diﬀerent types of opals have formed in the different regions. It is presumed that it is because the different type of rock density in the Earth that is Australia.
There are two different categories of opals; Precious Opal and Potch or Common Opal. Precious Opal is the sort after colours and patterns that you can see in Opals. These make up about 15% of all opals discovered in Australia. The other 85% of opal discovered is Potch or Common Opal. This is colourless stone and is often either discarded or used as backings in doublets or triplets making, or kept as a backing when the opals are cut by the opal cutters and polishers.