(IDEX Online) - Petra Diamonds has recovered a 342.92-carat Type IIa white diamond from its Cullian mine, in South Africa.The UK-listed miner said it was of exceptional color and clarity.In March, it sold a smaller rough stone from the same mine - a 299.3-carat white gem, also Type IIa - for $12.18m.It was bought by Stargems DMCC, Dubai-based arm of the Antwerp diamond supplier.Earlier this month Petra sold a 39.34-carat blue rough diamond, also from Cullinan, to a De Beers/Diacore partnership for $40m.The company says it expects to sell the newly-recovered 342.92-carat stone (pictured) at its tender in September.