Johannesburg, South Africa—A diamond said to be twice as big as the 530.24-carat Cullinan Diamond—formerly the world's largest—that could be worth up to $30 million has been discovered in northwest South Africa, but the tale of its discovery is still unclear.

The BBC reported that a small South African mining firm made the discovery. A shareholder in the unnamed mine told the news agency that the diamond was unearthed at their operation in the northwest province on Monday.

He said the stone was about 7,000 carats, twice the size of the Cullinan, which is the centerpiece of the British crown jewels.

According to published reports, the stone has been rushed to a bank vault in Johannesburg "until we calm down and decide what we are going to do," a spokesman for the mining company that discovered it reportedly said. A security company is being hired to protect the diamond.

But some industry experts are reportedly skeptical about the unconfirmed claim.

Brett Jolly, a shareholder at the mine, told the BBC he hoped tests today would prove its worth.

In a photograph e-mailed to the BBC, the stone appears to be about the size of a coconut and has a greenish tinge.

A spokesman for De Beers said the northwest province is not known for producing gems, and greenish stones are even rarer.

The firm also said that if the find is genuine, it would be the stone of the century.