A 232-carat white diamond worth as much as $20 million has been discovered at a South African mine famed for its big-gem finds, owners said Tuesday.

Petra Diamonds said the “exceptional” jewel was found at Cullinan, east of Pretoria, which has supplied stones for Britain’s crown jewels and where the world’s largest diamond was unearthed more than a century ago.

“The stone is... of exceptional size and clarity, and is a magnificent example of the large, high quality diamonds for which the mine is known,” Petra Diamonds said in a statement. In 1905, the world’s biggest diamond — the 3,106-carat “Cullinan” — was discovered there, and later cut into two pieces to be included in the British crown jewels.

The last white diamond sold by the company was a 507.3 carat which went for $35.3 million dollars in 2010, the highest price ever paid for a rough diamond, spokesperson Cathy Malins said.

Malins said analysts had forecast that the newly found “rare and unique” gem, which will be auctioned before the end of this year, could sell for anything between $10 million and $20 million dollars.

The London-listed company operates six diamond mines on the continent - five in South Africa and one in Tanzania.

In addition to its history of yielding diamonds bigger than 10 carats, Cullinan is also a source of blue diamonds — some of the rarest jewels in nature. In June, a 122.52-carat blue diamond was unearthed at the same South African mine.