Opalised fossils or pseudofossils

“Opal is a spectacular gemstone. It is also a dazzling key to Australia’s mysterious past, because buried in the Australian opal fields are fossils of dinosaurs and other strange creatures that lived 110 million years ago, in Early Cretaceous times.”

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Figure 1: Belemnites were cephalopods. The ammonites are another extinct cephalopod. Living cephalopods include octopuses, cuttlefish, squid and nautiloids.

This is the opening sentence on the website National Opal Collection. The Early Cretaceous period is alleged to be from 146 million to 100 million years ago.

Fossils are the remains or impressions of living organisms preserved in sedimentary rocks. When the fossils are opalised they become literally gemstones. Teeth, bones, shells and pinecones have been found fossilized and turned to solid opal. Australia is a ready source of opals.Coober Pedy and Lightning Ridge are well known for their mines. You will find opals for sale in most central tourist areas of Australian cities.

But how does anything become opalised? It’s not magic. Most people think it takes millions of years to occur.

“And surprising as it may seem, the ingredients of opal are commonplace stuff.  Water in the ground carrying dissolved silica (similar to the glass in windows) is said to have seeped through beds of sand and grit, where the silica particles are deposited in cracks.  As the water subsequently evaporated, the silica particles became ‘cemented’ together to form the opal.  Light bending around the silica produces the variety of glowing colours.”3

Australia is the only country where opalised animal fossils are found.4 Continue reading