FTSE 250 miner Petra Diamonds reported a doubling of production in its third quarter, driven by its purchase of the Finsch mine in South Africa.The firm said production rocketed 126% to 622,509 carats during the period, with Finsch exceeding management expectations and contributing 343,051 carats.Revenue for the quarter was $98m, almost equal to the whole of its first half ($101.4m), which Petra said boded well for full year revenues."These results demonstrate Petra's credentials as a high growth company, as we continue on our path to significantly ramp up annual production, with the goal of reaching over 5m carats per annum by [the end of full year] 2019," said Chief Executive Johan Dippenaar."We are on track to enjoy a strong second half to the financial year, due to the seasonality of our tender dates, the inclusion of the strongly performing Finsch mine for a full six month period and an overall improvement in the rough diamond market," he added.The figures were helped by Petra selling four 'special' stones exceeding $1m each during the period.Two of the giant diamonds came from Finsch, one from Cullinan, and one from Koffiefontein, totalling $5.2m.The firm said the rough diamond market had stabilised in December 2011, and pricing showed a slow but steady upward trend during the third quarter, though tempered by some volatility.It put the stronger market down to a recovery in confidence after the initial impact of the Eurozone debt crisis abated. Steady demand was being seen from both the US and Asian markets, it added.