Petra Diamonds shares jump on 425-carat discovery at Cullinan

However, they acknowledge that there are no signposts yet in the market that suggest mineral prices will rebound soon.

The company is said to be wanting to offload its Barro Alto nickel mine and the vast Minas-Rio iron ore complex.

The miners concerted effort to limit global diamond supply is forecast to be offset by stable Russian production, new mines, and production increases by Dominion Diamond and Petra Diamonds.

The besieged miner plans to lower costs at its iron ore mines after boosting total annual output for the steelmaking ingredient.

Rough diamonds sales rose to 7.1 million carats in October through December. This compares with 4.9 million carats of sales in the third quarter.

The ratings agency expects physical supply/demand imbalance to widen further, leaving miners in extremely weak conditions.

The discovery of the 55-carat diamond confirms the presence of large stones in the area, said the company.

Top miners BHP Billiton, Rio Tinto, Vale and Anglo American are among the firms at risk of rating downgrade.

In a fresh attempt to counter weakening demand, the diamond giant is said to have cut prices by as much as 7% at the Jan. 18 sale.