FTSE 250-listed miner Petra Diamonds has recovered an "exceptional" 122.52-carat blue diamond at its Cullinan mine in South Africa."The rarity of a blue diamond of this magnitude sets it apart as a truly significant find," the company said in a statement.Petra said that the stone will require further analysis in order to work out its potential value. Therefore, it will not be sold before the end of the current financial year (June 30th).Cullinan, located in the foothills of the Magaliesberg mountain range near Pretoria, is said to be one of the world's most important sources of blue diamonds.The mine, bought by Petra in 2008, is well known for the discovery of the Cullinan diamond in 1905, the largest rough gem diamond ever found at 3,106 carats, which forms part of the Crown Jewels in the Tower of London.BC