Petra Diamonds’ stock price fell to its lowest in a year after the diamond miner said full-year production would fall 8-9% short of its earlier guidance of 4.4 million carats due to a slower-than-anticipated build-up of its expansion programme.

Shares in the company fell as much as 10.1% on Wednesday.

The company, which owns the Cullinan mine that has produced the two largest diamonds in the British Crown Jewels, said revenue would also be between 8 and 9% lower than market consensus.

Investors look to diamonds, which tend to hold value when basic commodities fall in price, to shield their portfolio from volatile markets.

Petra said it had initial discussions with its lender group and was “confident” that the likely shortfall in the upcoming ratio measurement, arising from the lower production levels, would not be an issue.