London--Rio Tinto reported Thursday that revenues from its diamond mining and sales unit increased 15 percent year-over-year, from $741 million in 2012 to $852 million in 2013.

The diamond unit recorded net earnings of $53 million in 2013, compared with a net loss of $25 million last year.

Rio Tinto said that polished diamond prices were “relatively stable” throughout 2013 while the prices for rough diamonds saw slightly greater volatility.

Rio Tinto currently owns 100 percent of Australia’s Argyle mine and holds a 60 percent interest in the Diavik mine in Canada and a 77.8 percent stake in the Murowa mine in Zimbabwe.

Total diamond production for the company last year rose by 22 percent, up from 13.1 million carats in 2012 to 16.0 million carats in 2013. 

Rio Tinto attributed the increase mostly to an increase in the tons processed and higher grades at Argyle following the opening of the underground portion of that mine, which was once an open-pit operation only.

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The underground mine is currently ramping up to full capacity, Rio Tinto said, and the project has extended the life of the mine through at least 2020.

In addition, the mining company has transitioned Diavik to a fully underground mine with all three of its pipes at full production.

For 2014, Rio Tinto predicts that its diamonds production will be flat at 16 million carats. 

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