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Firestone Diamonds (LON:FDI) described first quarter production from its Liqhobong Mine in Lesotho as “good” as it revealed it uncovered an unbroken 74 carat yellow gem.
It also unearthed a 39 carat light yellow gem and a 26 carat white stone. However, both were fragments of much larger stones.
In all, 67,148 carats were sold for US$71 a carat in the period.
Separately, Firestone said modifications made to its pilot plant mean it is now consistently operating at 100 tons an hour.
A total of 45,491 carats were recovered in the first three months at 35.4 carats per hundred, which is slightly below forecasts.
Further plant modifications are planned for early June aimed at reducing diamond breakage of larger stones.
Elsewhere, a definitive feasibility study for a main treatment plant a Liqhobong is on track for completion in June.
Chief executive Tim Wilkes said: "We have had a good first quarter in terms of production at Liqhobong. The first maintenance shut-down of the pilot plant was successful and the benefits of the increased throughput are already visible.
“The second maintenance shut-down planned to be completed in June will address the remaining challenges we have in reducing the breakage of the larger diamonds.
“We are delighted to have recovered the largest unbroken 74 carat light yellow gem from Liqhobong to date which further supports the high quality of our ore body.
“The results of the recent diamond tender in Botswana showed strong pricing for diamonds in the better quality categories, however the smalls, browns and near gems remain under pressure.
“With the restructuring that was undertaken in Q1, we are continually focusing on increasing Liqhobong's revenue per carat in 2012, by reducing breakage, and in developing the main treatment plant in order to reach our target of producing over 1 million carats per annum by 2015.
“The feasibility study has gained significant momentum and we are still on track for completion by the end of June 2012.”
Dr Ryan Long, at broker Northland, highlighted that it was "positive" the firm was continuing to find large diamonds.
"The presence of the large stones within the deposit is expected to increase the average price per carat significantly," he said.
"Firestone’s main problem at Liqhobong continues to be diamond breakage. However, the plant is being improved to reduce the breakage with modifications expected to be complete in June 2012.
"The recovery of a 74ct unbroken stone is seen as a step in the right direction. The average price per carat $71/ct was lower than last June 2011’s tender ($123/ct) due to a larger proportion of small stones and near gems that were not previously sold.
"With a strategic decision on exploration expected shortly and further improvements at the plant the next few months will be interesting for Firestone."
As at 12.35 pm, Firestone shares stood at 6.38 pence each.