One of Australia's oldest pearl farms is celebrating a rare moment this week after opening up a pearl oyster shell to discover 10 pearls inside.

Third generation pearl farmer James Brown and his family have been pearling at Cygnet Bay in the West Kimberley for more than 60 years.

Mr Brown said with natural pearls only forming in one of every 5,000–10,000 Pinctada Maxima pearl oyster shells, it was potentially a once-in-a-lifetime discovery.

"I've operated on hundreds of thousands of shells myself and I've never seen anything like this," he said.

"The pearls we found range in size from four to five millimetres to two or three times that. So they're actually big pearls as well which is the other quirky thing about this.

"We've got these 10, quite lustrous, very saleable pearls out of this strange pearl oyster."

How did this happen?

Mr Brown said it was difficult to explain such a rare find, but the placement of the pearls inside the oyster shell gave some clues.

"We've never seen a cluster like that before. If they're a natural pearl they're normally in at the mantle areas which is more on the extremity of the pearl," he said.

Mr Brown says it the find it unusual as Keshi pearls rarely form in a cluster underneath an oyster's reproductive system. ( Supplied: Cygnet Bay Pearls )

"So the fact that most of them were clustered together in that gonad region probably does point to the fact that it was an artefact of an attempt to culture a pearl."

While Mr Brown suspected the 10 pearls were cultured, he said that it was remarkable that such a large number had been produced in a single oyster shell.

"You've also got to remember when we do culture pearls in this species we only get one at a time, and it takes two years, and you might get only 70 to 80 per cent success," he said.

"But in this one, for some reason, instead of giving one cultured pearl it's crafted 10 — probably what are seedless or otherwise known as Keshi pearls — in this really tight cluster."

What are they worth?

Mr Brown said there was a small market for natural pearls around the world and they were extremely rare and valuable.

A closer look at the 10 natural Keshi — or seedless — pearls that were discovered in a single pearl oyster. ( Supplied: Cygnet Bay Pearls )

"A natural pearl can be as much as 10 times more valuable for the same size, shape and quality if you can prove it's natural," he said.

"But you need a certificate from a laboratory stating its authenticity as a natural pearl.

"We photograph them and keep them aside in a safe [but] if we were to sell them we would as Keshi, or seedless, and they're sold generally by the gram."

Mr Brown said they had not decided whether they would sell the pearls or make them into jewellery, but for now would put them on display at the farm.

Cygnet Bay Pearls has a history of holding onto their most valuable pearls.

They still hold the record for the world's most valuable cultured pearl measuring 22.24 mm and weighing 156 grams.

It is believed that the Brown family have been offered up to $1 million for the gem, but it has never been sold.

The priceless pearl remains on show at the their showrooms in Broome.