The world's first recorded opalised pearls, relics of creatures in an ancient inland sea dating back 65 million years, have been unearthed by two miners in the South Australian outback.

The "priceless" four-millimetre specimens were found in the Coober Pedy opal fields, an area famed for the colourful gems.

Dr Ben Grguric from the SA Museum, where the pearls have gone on display, said opal miners Dale Price and Tanja Burk were sorting through a spoil heap when they made the discovery.

"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," Dr Grguric.

"It turns out these resembled pearls."

He said opals formed when seas dried up and alkaline soil dissolved the silica in certain rocks, as well as bones and shells - and in this case, pearls.

"A lot of the opal fossils, including bones and shells, were formed during the cretaceous period, which was an era earlier than 65 million years ago and the age of the dinosaurs," Dr Grguric said.

A neutron imaging instrument DINGO produced this image of the larger opalised pearl. ( Supplied: South Australian Museum )

To analyse and avoid damaging the gems, they were sent to be scanned by a neutron imaging instrument at the Australian Nuclear Science and Technology Organisation at Lucas Heights in Sydney.

"We decided to use a technique called neutron tomography, which is like a CT scan using neutrons," Dr Grguric said.

"They established there was a concentric structure which is consistent with pearls as we know them today."

The pearls are still owned by Mr Price and Ms Burk, and only on display for a short time at the SA Museum.

"It's difficult to put a price on them, and from the point of view of a gem they're not particularly valuable," Dr Grguric said.

"But from a scientific view, you'd argue they were priceless."

He said there were a lot of shell fossils in the Coober Pedy region, and those with a sharp eye may come across more opalised pearls in the future.