PETRA Diamonds continues to grow output, saying in a detailed trading update on Tuesday that it had lifted its full-year production by 21%, and forecasting a further rise in the coming year as it rolls out its expansion plans.

Petra, which is listed in London but has all but one of its mines in South Africa, reported production of 2.67-million carats for the full year to end-June, a 21% increase.

It predicted it would generate between 2.9-million and 3-million carats in its 2014 financial year and said it would give the market a more detailed view on August 12. Petra posted a 27% increase in revenue, to $402.7m, and said costs were "well controlled".

It had cash of $26m at the end of June and had no reason to raise any more, CEO Johan Dippenaar said. Petra’s debts amount to $75m after it held its largest tender of diamonds shortly before its year-end and it holds diamond inventories worth about $35m.

It would curtail sales until the end of August, given that July and August were traditionally quiet months for rough diamond sales, Mr Dippenaar said.

Petra reported an 87% increase in the number of diamonds sourced from tailings and alluvial deposits. It produced 630,190 carats from these sources in financial 2013 and was on track to produce about 5-million carats by financial 2019, he said.

The increase in 2013 production was largely as a result of Petra reporting the Finsch mine’s output for a full year after buying it from De Beers. Williamson mine, which was also bought from De Beers, has hit its planned production levels after Petra rebuilt its processing plant.

The Finsch mine is the largest within Petra and contributed 1.4-million carats, an improvement over the previous year’s 1.1-million carats.

Cullinan, the second-biggest mine in the company, notched up a marginal increase in production. It generated 868,975 carats, up from 2012’s 867,780 carats. This was because of a delay in the tailings expansion project and a slower grade from the mine. The tailings project was supposed to treat 2.7-million metric tonnes of material but managed only 1.5-million tonnes because of labour problems in the first half of the financial year.

The Kimberley underground mine reported a 69% increase in production to 115,400 carats and Williamson more than trebled production to 150,342 carats.

"Overall price performance for 2014 is expected to remain steady due to constrained supply and a firmer US market, the world’s major market for polished diamonds, as well as continued growth in China, albeit at a lower rate than over recent years," Mr Dippenaar said.