The world's only opalised pearls, 65 million years in the making

The world's first opalised pearls have been found in the Australian desert, dating back 65 million years, when it was covered by an inland sea.

The opal fields of Coober Pedy, in the deserts of South Australia, are unique. No other region of the world is so rich in the precious stones. Once an inland ocean, Coober Pedy is the source of 90 percent of the world's opals.

Now, it's also the discovery place of a one-of-a-kind (well, two-of-a-kind) pair of pearls. Just 4 millimetres in diameter, turned over the course of 65 million years into shimmering opals, these are the first known opalised pearls ever discovered.

The pearls were discovered a decade ago, by Dale Price and Tanja Burk of Coober Pedy's TADA Opals, while sorting through a spoil heap from an area known for its opalised shells. Price and Burk, thinking they might be pearls, took them to be assessed. At that time, they were dissuaded from pursuing it further.

The tiny pearls, found in an opal mining spoil heap.

South Australian Museum

When the pearls came to the South Australian Museum at the end of 2015 for the Opals exhibition, head of earth sciences Ben Grguric thought Price and Burk might be onto something.

"The miners pick out anything that glows with ultraviolet light, because even a small chip of opal might be worth something if it's high quality with a high range of colours," Grguric said. "It turns out these resembled pearls."

Because no opalised pearls had ever been discovered before, further analysis was required. This is where the Australian Nuclear Science and Technology Organisation got involved. The facility conducted full 3D neutron tomography scans of the pearls and their interior structure.

What the ANSTO team found was that the interior structure was concentric, consistent with how pearls are formed from layers of calcium carbonate and nacre.

The pearls would have formed when the inland sea dried up. This initially increased the acidity levels in the shallow top layer beneath the sandstone, which released silica from the sandstone into the clay beneath, where bones and shells and pearls lay buried. Further weathering lowered the acidity levels again, which allowed the silica, soaking into objects such as bones and pearls, to harden into opals.

"It's extraordinary that in the vast, moon-like landscape of what was an ancient inland sea these tiny 4mm specimens have been found," Grguric said.

"What's even more extraordinary is that opals rarely survive because the organic material in them oxidises. These are the only example of opalised pearls known in the world, we believe, which suggests that these were fossilised quickly and secluded away so they couldn't oxidise. They may even reveal something about the origin of pearls, which is still a mystery."

He noted that the actual value of the pearls would probably be impossible to calculate.

"It's difficult to put a price on them, and from the point of view of a gem they're not particularly valuable. But from a scientific view, you'd argue they were priceless," he said.

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