(IDEX Online) - Gem Diamonds saw revenue and average per carat prices down by around a quarter during 2023 amid ongoing "downward pressure" in the rough market.The UK-based miner operates Letseng, in Lesotho, the highest dollar per carat kimberlite diamond mine in the world.It says total sales for the year were $139.4m, down 26 per cent, and the average per carat price was down 24 per cent to $1,334.The highest price it achieved during the year was $33,745 per carat for a 117.09 carat white diamond which sold for $4m. It also sold six $1m-plus diamonds for a total of $13.8m.Revenue for Q4 was $36.4m, up 14 per cent on Q3, according to its Q4 2023 Trading Update published on 1 February). Gem says it recovered 32,142 carats in Q4, up 16 per cent on Q3.Recoveries of large diamonds in 2023 were similar to 2022, but below the mine's average since 2008. The decrease in prices achieved in 2023 negatively impacted overall revenue achieved during the year.Letseng is 70 per cent owned by Gem and 30 per cent by the Lesotho government.Pic shows a 117.47-ct stone recovered last October, courtesy Gem Diamonds.