Opalised Shells: Nature’s Rare Fossil Gems from Coober Pedy

Opalised Partial Shell

When most people think of fossils, they imagine bones turned to stone, but in certain parts of the world fossils take on a much more colourful and dazzling form. These are opalised shells, ancient marine remains that have transformed into shimmering gems over millions of years.

One of the only places on Earth where opalised shells are regularly found is in Coober Pedy, a remote outback town in South Australia. Known as the opal capital of the world, Coober Pedy is a hotspot for some of the rarest and most stunning opal discoveries on the planet.

So how exactly do shells become opalised instead of traditionally fossilised?

Opalised Clam Shell

It all comes down to geology, chemistry and a lot of time. Around 100 million years ago during the Cretaceous period, much of central Australia was underwater. As sea creatures like molluscs, snails and bivalves died, their shells settled into the seabed and were gradually buried by layers of sediment. In a typical fossilisation process, minerals like silica or calcite slowly seep into buried remains, gradually replacing the organic material and turning it to stone. This usually forms what we call permineralised fossils, which preserve the shape of the original organism in rock, but without any gem-like qualities.

Opalised Belemnite

However in rare places like Coober Pedy, the conditions are different. The ground here was once part of an ancient inland sea, and over time it became rich in a watery silica solution, essentially a kind of natural gel. Instead of simply turning to stone, this silica formed into precious opal, a gemstone known for its brilliant flashes of colour. When a shell or bone decayed underground, this opal could fill the void left behind, preserving the shape of the original object in spectacular, colourful detail. This process is called opalisation and it’s far rarer than regular fossilisation.

To put it into perspective just how rare these are, only 1 in 1,000 organisms fossilise, for a fossil to opalise this statistic is reduced to 1 in 100,000!

Opalised Partial Gastropod Shell

The result is a fossil unlike any other, both scientifically valuable and visually breath-taking. Opalised shells can display vivid colours including electric blues, greens, fiery reds and glowing purples. Opalised shells are a testament to how nature can turn something ordinary into something extraordinary.