Petra Diamonds - Cullinan 29,6Ct Blue Diamond.The company CEO Johan Dippenaar holding the Diamong recovered in January 2014.Photo Supplied

London - An acorn-sized blue diamond, one of the rarest and most coveted in the world with a possible price tag of tens of millions of dollars, has been discovered at a South African mine by Petra Diamonds.

The producer said the “exceptional” 29.6 carat diamond was unearthed at the Cullinan mine near Pretoria.

The mine, owned by the firm since 2008, was also where the Cullinan Diamond was found in 1905. This diamond was described as the largest rough gem diamond ever recovered and weighed 3 106 carats.

The mine’s other notable discoveries include a 25.5 carat Cullinan blue diamond, which was found last year and sold for $16.9 million (R183.4m). A diamond found in 2008, known as the Star of Josephine, was sold for $9.49m.

Chief executive Johan Dippenaar said the latest discovery could outstrip recent finds.

“By some margin… this is probably the most significant [blue] stone we’ve ever recovered,” he said.

“The stones in the last year or so are selling well above $2m per carat. That’s not my quote, that’s updates in the market,” he said ahead of the first-half trading statement.

London-listed Petra Diamonds is due to release figures on production and sales for the six months to December tomorrow, but these will not take into account this month’s discovery of the 29.6 carat gem.

Analyst Cailey Barker at brokers Numis thought the diamond could fetch between $15m and $20m at auction.

Diamonds from the Cullinan mine and the Williamson facilities in Tanzania, both owned by Petra, have been displayed at Buckingham Palace and are regarded as among the world’s rarest and most valuable.

Dippenaar said the company would decide what to do with the diamond in the next week. - Reuters