Two Cullinan white diamonds of 58 and 53 carats were sold this month for $2.8-million (R20.8 million), Petra Diamonds said on Wednesday.

"The company is very encouraged by the excellent prices achieved for the 58.49 and 53.31 carat white diamonds, which sold for $1.82-million and $960 000 respectively," it said in a statement.

"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 announced an update with regards to the group's tender prices for the last four months, a period which saw a consistent strengthening in rough diamond prices, especially at Cullinan, Koffiefontein and Williamson.

Petra said it continued to assess the route to market for the 507 carat diamond in order to maximise revenues.

"Expert analysis so far indicates that this stone is of exceptional colour and clarity."

The 168 carat diamond would be sold on tender "soon" and the company would announce the value realised in due course. Petra said diamond prices realised at its tenders had continued to strengthen.

"The most recent tenders in October (Williamson mine in Tanzania) and early November (SA production) saw particularly strong prices."

It had sold a number of "special" diamonds from the Cullinan and Koffiefontein mines in the last four months.

According to Petra CEO Johan Dippenaar, the rough diamond market had improved significantly in recent months.

"Our recent tenders have been particularly successful and this positive price environment bodes well for the sale of the exceptional 168 carat white Cullinan diamond, as well as for the overall earnings of our operations as we continue to increase group production."

Locally, Petra had interests in five producing mines ? Cullinan, Koffiefontein, Helam, Sedibeng and Star ? and agreed to acquire from De Beers the Kimberley underground mines.

In Tanzania, Petra has an interest in the Williamson mine.

Petra is listed on the alternative investment market of the London Stock Exchange.