A South African mining company has claimed to have discovered the world’s biggest diamond, the BBC reports.

A shareholder in the unnamed mine told the BBC the stone had been unearthed at their operation in the north-west province on Monday afternoon.

He reportedly said the giant gem was about 7,000 carats – which would be twice the size of the Cullinan Diamond, center-piece of the British crown jewels.

But BBC reports that industry experts have expressed skepticism about the unconfirmed claim

Brett Jolly, a shareholder at the mine, told the BBC the stone had been taken to a bank vault in Johannesburg, BBC reports. Jolly said he hoped tests on Tuesday would prove its worth.

The BBC received a photograph of the stone and said it “appears to be about the size of a coconut, and has a greenish tinge.”

JCK gem editor Gary Roskin is among the unbelievers. “My bet is it is a nicely carved piece of topaz,” he said.

A spokesman for De Beers told the BBC the north-west province was not known for producing gems and greenish stones were even rarer, adding that if the find were genuine it would be the stone of the century.

JCK