(IDEX Online) - Gem Diamonds saw its revenue from rough sales at Letseng, in Lesotho, down 19 per cent to $71.6m in the first half of this year.The UK-based miner blamed the ongoing downturn, fewer recoveries of high-value diamonds, and power outages interrupting ore processing.Letseng, known as the highest dollar per carat kimberlite diamond mine in the world, saw average prices fall 22 per cent in the six months to 30 June, from $1,765 to $1,373. Gem said it sold a dozen $1m-plus diamonds during H1 2023, for a total of $21m.The highest price achieved for any diamond was $282,889 per carat for a 6.63 carat pink. The highest price for a white diamond was $34,441 per carat for a 58.07 carat Type IIa.Carats recovered and sold remain within original guidance due to improved grade performance, but operating and treatment costs increased."During the period increasing grid electricity interruptions caused a reduction in volumes of ore processed," the company said in a trading update."In addition, higher than expected internal basalt dilution in certain domains of ore that was treated impacted throughput."Gem Diamonds, which owns 70 per cent of the Letseng mine, in the process of selling its 100 per cent share in its other mine, Ghaghoo, in Botswana, which has been mothballed since 2019, to Botswana-based Okwa Diamonds, owned by Vast Resources.Pic courtesy Gem Diamonds shows a 163.91-carat yellow diamond recovered last month.