(IDEX Online) - Gem Diamonds reports a six per cent dip in its full-year revenue for 2022, down to $188.6m.The UK-based miner, which owns 70 per cent of the Letseng diamond mine, in Lesotho, says average per carat prices also dropped slightly, down four per cent to $1,755.Letseng, renowned for its large, top color, exceptional white diamonds, is the highest average dollar per carat kimberlite diamond mine in the world.But recoveries of large diamonds were below average in 2022 - four +100-cts compared with an average since 2008 of eight per year, and lower figures for smaller stone categories.The highest price achieved during the year was $34,170 per carat for a 42.98 carat white diamond that sold for $1,469,000, one of four diamonds that sold for $1m.Gem Diamonds noted in its Q4 2022 Trading Update that was 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 PLC.Gem's CEO Clifford Elphick said power outages had disrupted production and increased costs. Output for 2022 was 106,704 carats, down seven per cent on the previous year.An exceptional Type II 245-carat diamond recovered from Letseng mine, in Lesotho, last June