Petra Diamonds forecast full-year revenue below market expectations as the smaller diamonds mined from late stage ores at its Finsch and Cullinan mines in South Africa have been fetching lower prices.

The miner’s third warning this year sent its shares down as much as 12.4 per cent, making it the second largest percentage loser on the FTSE-250 Midcap Index on Friday.

Petra said it expected revenue of about $430 million (€381 million) for the year ended June 30, almost 9 per cent lower than a year earlier and below an average market estimate of $442 million (€392 million), according to three analysts.

The company had warned twice earlier this year that its results would be below consensus due to variability in grade and production mix and softness in the diamond market earlier in the year.

Petra has been mining at mature caves at Finsch and Cullinan and has therefore seen a lower incidence of the high quality stones that have traditionally boosted the company’s results. – (Reuters)