Petra Diamonds Ltd said it sold a 29.6 carat blue diamond mined at its Cullinan mine in South Africa for $25.6 million (R280 million) to US luxury jeweller Cora International NY.

Petra Diamonds said the acorn-sized “exceptional” diamond, whose discovery it announced last month, was sold to Cora through South Africa-based Golden Yellow Diamonds.

“This sale result affirms this stone as one of the most important blue diamonds ever recovered,” Petra Diamonds chief executive Johan Dippenaar said in a statement.

Analyst Cailey Barker at brokers Numis had estimated in January that the diamond could fetch between $15 million and $20 million at auction.

Petra Diamonds said last month that the sale of the blue diamond would boost second-half results.

The company's revenue rose 19 percent to $185.5 million in the six months ended December 31.

Cora has cut and sold some of the biggest diamonds in the world, according to the website of the Fifth Avenue, New York City-headquartered company.

The company has also cut the “extraordinary” 110 carat fancy yellow pear-shaped diamond - The Sundrop - exhibited at the London Museum of Natural History and valued at $12 million.

Petra Diamonds' blue diamond was recovered from the same mine as the Cullinan Diamond - the largest rough gem diamond ever recovered.

The 1905 Cullinan Diamond has been cut into two stones - the First Star of Africa and the Second Star of Africa - that form part of Britain's Crown Jewels held in the Tower of London.

Other notable diamonds discovered at the mine are the 25.5 carat Cullinan blue diamond found in 2013 and sold for $16.9 million, and the Star of Josephine diamond found in 2008 and sold for $9.49 million.

Shares in Petra Diamonds were little changed at 148.1 pence at 14:48 SA time on Friday on the London Stock Exchange. - Reuters