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Boulder Opal Treasure Box

About Opals

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How is opal formed?

Opal is thought to have formed approximately one hundred million years ago, at the end of the Jurassic and beginning of the Cretaceous periods.

The Cretaceous period was known for its high sea levels and consequently the Australian continent was covered, in part, by a shallow inland sea (see map).

How is opal formed - Map showing the impact of high sea levels on the Australian continent during the Cretaceous Period 130-120 million years ago

Map showing the impact of high sea levels on the Australian continent during the Cretaceous Period 130-120 million years ago

In Australia, opal formed in clay beds, sandstone and mudstones rocks from this period.

Opal formation is thought to have occurred when these extremely weathered rocks, during the process of weathering, produced silica which was released into the ground water.

Small faults and joints in the rocks formed pathways for the silica rich ground water to penetrate and settle in.

The silica filled rocks were then trapped between impermeable barriers of sandstone and the underlying rocks in the geological strata and the silica-rich ground water slowly hardened into a gel forming opal in veins and lenses. (Senior, B).

That’s one theory anyway.

There are many theories on how opal is formed but none is conclusive.

Over time, the water levels receded and the land dried out. What had once been a sea now became a semi-desert.

how is opal formed - opal country

Opal Country

 

Remnants from this period include opalized animal and plant fossils and the Great Artesian Basin, the largest underground water system in the world, located on the map in Queensland, NSW and South Australia. It is in this geographical location that opal is found.

How is opal formed - Map showing the current location of the Great Artesian Basin

Map showing the current location of the Great Artesian Basin

While opal is found in several locations around the world (Mexico, Brazil, Honduras and  Ethiopia) Australia is the only part of the world where opalized animal and plant fossils have been found (Senior, B)

How is opal formed - Eric

[Image] ‘Eric’ the pliosaur was a short necked, carnivorous reptile that lived in the inland seas of South Australia in the Cretaceous period 120-100 million years ago. The pliosaur bones, as well as the tiny fish bones found inside its stomach became opalised during the follisation process. Learn more about Eric here: https://australianmuseum.net.au/blog/museullaneous/a-national-treasure/

References:
Aracic, S. Rediscover Opals in Australia. Underwood, Qld: Kingswood Press Pty Ltd, 1999. Print.
Senior, B. Opal. Australian Government Geoscience Australia. http://www.ga.gov.au/education/classroom-resources/minerals-energy/australian-mineral-facts/opal

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