The "exceptional" 29.6 carat blue diamond is the latest rare stone to be found in the Cullinan mine
A rare blue diamond has been discovered in a mine in South Africa.
The 29.6-carat stone was recovered by Petra Diamonds at its Cullinan mine, about 40km (25 miles) north-east of Pretoria.
"This stone is one of the most exceptional stones recovered at Cullinan during Petra's operation of the mine," the company said.
Petra unearthed a 25.5 carat blue diamond which sold for $16.9m (£10.3m) in 2013.
'Class of its own'The latest discovery is also expected to sell for a high price.
The Cullinan mine is famed for the production of blue diamonds
"The stone is an outstanding vivid blue with extraordinary saturation, tone and clarity, and has the potential to yield a polished stone of great value and importance," Petra said in a statement on Tuesday.
Cullinan mine has produced hundreds of large stones and is famed for its production of blue diamonds - among the rarest and most highly coveted of all diamonds.
The mine was acquired in 2008 by Petra Diamonds, which also operates in Botswana and Tanzania.
A similar 26.6-carat blue rough diamond discovered by the company in May 2009 was cut into a near perfect stone and fetched just under $10m at a Sotheby's auction.
Another deep-blue diamond from Cullinan was auctioned for $10.8m in 2012 and set a world record for the value per carat.
In 1905, the renowned Star of Africa blue diamond - the world's second largest cut diamond - was discovered at the Cullinan mine.
The pear-shaped 530-carat stone was presented to King Edward VII and became part of the British crown jewels.
Ann Marie