How is Opal Formed?
The Formation of Opal
Australia produces more than 90% of the world’s precious opals, with the remaining portion found in other regions like Mexico, Brazil, Indonesia, the Czech Republic, Ethiopia, and the USA. Opals from Mexico (called “Mexican Opals”) have more water and are transparent or highly translucent. Opal formed in various ways depending on their global location. Note that this explanation focuses on the scientific process, contrasting with the mythical dreamtime narratives of Aboriginal Australians.
Hydrophane Opals
Many opals mined in Ethiopia, especially from the Welo deposits, are hydrophane. The term “Hydrophane” describes a porous opal with the ability to absorb water. A change in color or transparency often accompanies the water absorption. This reaction occurs due to volcanic rock deposits and lava within the volcanic region of Ethiopia, differing significantly from the formation process of Australian Opals.
Australian Opal
Australian Opal forms through a chemical reaction involving 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 colors 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, etc.), 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.
Aboriginal Stories
Many stories exist about how opals are formed in different Aboriginal tribes around Australia. Some believe they came from a giant rainbow serpent who spread water throughout Australia to form the opals on 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.
Regional Variations
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.The reason for the formation of different opal types in various regions remains unknown. Presumed differences in rock density within the Earth, particularly in Australia, are thought to be the reason for this.
Understanding Opal Categories
There are two different categories of opals: Precious Opal and Potch or Common Opal. Precious Opal is the sort after colors 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. Opal cutters and polishers often discard this colorless stone or use it as backings in doublets or triplets, maintaining it later when cutting the opals.