Petra Diamond’s (LON:PDL) shares sank this morning after they said they expected revenue to be below consensus expectations. 

The South African diamond miner said that the revenue miss would be caused by the lower grade of diamond their mines were now producing. They were confident that they would meet their 3.2 million carats production target but were disappointed that the grade of these carats would lead to lower revenue. 

Their mines are old and the remaining stones are small. Petra is in desperate need of expansion and their plans to increase output to 5 million carats by 2019 cannot come soon enough for shareholders.

The falling price of diamonds will be of little comfort either.

Shares trade down 7.5% at 157.6p at 14:47 London time.