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Geological Significance


The Australian Opal has a history from about a million years ago. When vast inland seas covered parts of Australia, sediments were deposited along the shorelines (Gem by Gem). As the mass amounts of water flooded back they flushed water containing silica and the remains of plants and animals into the resulting cavities and niches in sedimentary rocks (Gem by Gem). Slowly, the casing preserved the shells and turned into shale-rock. All precious opals in South Australia occur naturally in rocks affected by weathering during the Tertiary Period, approximately 15 to 30 million years ago (Origin Energy). However, opals are said to have been here for more than 60 million years when dinosaurs roamed the earth (Jewelry Mall, 2007). Opals were found near the earth's surface where ancient geothermal springs once flowed (Jewelry Mall, 2007). As the minerals slowly bubbled up through the cracks of the earths surface, over a long period of time, it eventually lined the walls, veins and underground cavities in the bedrock (Jewelry Mall, 2007).



Chemical Formula: SiO2 - nH2O

Streak: white

Hardness: 4.5 - 6.5

Transparency: transparent to opaque

Luster: usually vitreous, but can also be waxy, pearly or even resinous

Cleavage: none

Fracture: conchoidal

Tenacity: brittle 



-The Opal, being amorphous  is not really a mineral but a mineraloid meaning that it does not have a crystal structure (Hershel, 1997).


-The Pearl Opal also known as Tabasheer; in its organic form can be found in some species of bamboo (Hershel, 1997).


-Its structure is not truly crystalline (Jewelry Mall, 2007).


-Opals are simply gel from silica- hydrated silicon dioxide and usually between three and twenty percent water- however, most opals are usually found between three and ten percent (Wikipedia, 2007). Geologists can then determine the temperature of the host rock when the opal was formed (Amethyst, 2006).


-The mineraloid gel in opals is deposited at relatively low temperatures and may occur in the fissures of almost any kind of rock, but they are most commonly found with limonite, sandstone, rhyolite and basalt (Wikipedia, 2007).










     The Opal does not play a significant geological role, however, the silica found in the Opal does have some significance.

-Silica is one of the world's most abundant minerals found in the Earths crust (Mineral Zone, 2005).  It provides an important constituent for almost all rock-forming minerals (Mineral Zone, 2005).

-Manufacturers fuse High-class silica sand or clear quartz crystals to make ophthalmic glasses and lenses (Mineral Zone, 2005).

-There are many different kinds of siliceous rocks, such as mica schists, sandstone, quartz and quartzite that are used in the manufacture of silica bricks (Mineral Zone, 2005).  Quartz and Quartzite are also used to make ferro-silicon for the manufacture of silicon steel (Mineral Zone, 2005).


Geological Significance    Where Opal's Are Found    Way Cool Facts   Works Cited