Petra Diamonds putting a 507-carat egg-sized diamond on sale this week, which it discovered at its Cullian mine in South Africa. The company is expecting the 100-gram diamond to sell for around $25 million, reports say. Originally, analysts had placed the diamond price at around $20 million, but the rising rough diamond prices and higher buyer interest cited for the stone, could raise the diamond’s sale price, reports add. %% The stone is one of the 20 largest high-quality rough diamonds ever to be found, reports say. Its sale will enable the company’s annual financial results to be spurred. The stone is named as the Cullinan Heritage.