Johannesburg - A 232 carat white diamond has been found at Petra Diamonds' Cullinan mine, east of Pretoria, the company said on Tuesday.
Analysts predicted that it could fetch between $10m and $16m.
Shares in the diamond miner climbed as much as 6.5% in early Tuesday trading, the biggest rise on Britain's FTSE-250 Midcap Index.
The find is Petra's largest of a white diamond since it unearthed the 507-carat Cullinan Heritage diamond in 2009 from the same mine, the source of many large diamonds.
"The stone is a D colour Type II diamond of exceptional size and clarity, and is a magnificent example of the large, high quality diamonds for which the mine is known," it said in a statement.
The mine is renowned for the discovery of the Cullinan diamond in 1905, which was the largest rough gem diamond ever found.
Type II diamonds have no measurable nitrogen impurities, meaning they are often of top quality in terms of colour and clarity.
"We estimate a sales price in the order of $10m to $15m, given Gem Diamonds' recent sale of Type II white at about $70 000 per carat and assuming some losses for cutting/polishing," Numis Securities analysts said in a note.
The diamond miner, with five producing mines in South Africa and one in Tanzania, said it expected the diamond to be sold in October to December, the second quarter of its financial year.
View images of the diamond supplied to Fin24 by Petra Diamond Limited.
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"The company will update the market when finalised on the expected timing for the sale of this stone, which is expected to be in the second quarter of this financial year."
Petra bought the mine from Anglo American Plc's [JSE:AGL] De Beers unit, the world's biggest diamond producer by value.
The miner's stock was up 6.1% to 187.6 pence at 09:30 on the London Stock Exchange.
The two stones cut from it, the Great Star of Africa and the Second Star of Africa, are part of the Crown Jewels of the UK.
In June, the company found a rare, 122.52 carat blue diamond at the mine.
Last year, Petra sold a 25.5 carat blue diamond for US16.9m.
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