Diamond mine operator Petra Diamonds saw sales jump by 16% in the first six months of the year, to reach $214.8m, but warned that it may not meet analysts' expectations for full-year profits.The miner's increased sales in the period was in part thanks to the sale of two exceptional diamonds for a combined revenue of $38.7m.However, first half production decreased 2% to 1,601,069 carats.In its trading update the mining outfit explained that in the reporting period the market for rough diamonds had been softer than usual.Nonetheless, the company increased its full-year production guidance from approximately 3.2 Mcts to about 3.3 Mcts.Despite that, Petra Diamonds warned that adjustments to its estimates for diamond grades and pricing may lead to full-year results coming in below the current market consensus.To take note of, the weaker Rand - South Africa's currency - is also currently having a favourable effect on Petra's operating costs in US dollar terms.Costs remain well controlled and in-line with guidance. At $125.2m the firm's capital expenditures were also in-line.Net debt had decreased sharply by 31 December 2014, to stand at $45.8m versus the $108.8 at the end of the prior year.The company's cash position came in at $129.6m, while its $66.9m in debt facilities were undrawn and available.