(ShareCast News) - Diamond miner Gem Diamonds reported a decline in production and sales in the first half of the year.The London-listed company said it recovered 50,019 carats of diamonds in the first six months of the year, a 7% decline year-on-year, as grades in the first half declined to 1.61 carats per tonne from 1.69.Sales declined 15% from the corresponding period in 2014 to 46,981 carats to generate $106.3m in sales, which was made worse by a 3.1% drop in average prices to $2,264 per carats.The Africa-focused group said it recovered 13 rough diamonds worth approximately $1m each at its Lesteng mine in South Africa, although it warned the development at the Ghaghoo mine in Botswana was progressing slower than planned."The development of the Ghaghoo mine is progressing slower than planned due to difficult ground conditions which have hampered slot development in the first five production tunnels and constrained production ramp-up," said group chief executive Clifford Elphick.Gem Diamonds shares were down 6.18% to 8.25p at 1559 BST on Thursday.