How was opal formed? Scientists have a variety of theories.

Here’s the facts, as far as Australia’s opals were formed:

The Great Artesian Basin is a stretch of Australia that covers about half of the eastern half of Australia, mostly in Queensland, but with the famous Coober Pedy and Andamooka fields in South Australia, and Lightning ridge in the north of New South Wales.

This geographical feature was formed during the Jurassic and Cretaceous periods, betwee 200 million to 70 million years ago. For much of this time, this part of what would become Australia was a shallow, inland sea. As with any sea, sandy sediments, and sandy clay sediments, settled out of the water.

Then, 37 to 70 million years ago (the Tertiary period), the earth’s climate changed. (Perhaps caused by a bit of non-manmade global warming.) The sea receded, and what was left behind was millions of years of history. The sediments were now over a hundred feet thick, layers of gray shale, shaley mudstone, limestone, and sandstone.

Most scientists theorize that most of Australia’s opal was formed after the Cretaceous sediments began to weather, which freed the silica trapped within. This silica then would dissove during rainstorms, trickle through faults in the subsoil, and become trapped within wood, shell, bone (thus “opalizing” these items), or would simply become trapped in cavities in the subsoil. The water would evaporate, the silica would remain, and become concentrated to such an extent that they would form microscopic spheres.