JOHANNESBURG (Commodity Online): Petra Diamonds will be putting up 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. 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.

The stone is one of the 20 largest high-quality rough diamonds ever to be found. Its sale will enable the company's annual financial results to be spurred. The stone is named as the Cullinan Heritage.

The level of interest in the Cullinan Heritage stone shows a recovery in the diamond market, which began in early October as jewellers started restocking.

It is rare for a diamond of such size and quality to be sold by competitive tender.  There has been very strong interest in viewing the stone and in being involved in bidding, said Petra.

The company, which is listed on the London Alternative Investment Market, said it had swung to a first-half profit on higher production and sales, and an adjustment related to its bigger stake in the Cullinan mine.