Shares in Petra Diamonds sparkled after the diamond producer said received strong prices from the sale of two diamonds.'Two Cullinan white diamonds of 58 and 53 carats were sold in early November for US$2.8m,' it said.'The strong prices achieved bode well for the values that could be realised from the exceptional 507 and 168 carat white diamonds, which were recovered in the same production series at the Cullinan mine in September this year.'Petra also announced that it has increased its ownership in Cullinan, which is in South Africa, to 74% and confirmed that it had doubled its stake.