De Beers, the world's largest miner of rough diamonds, warned that the US economic slowdown was affecting demand for gems but said there was still a shortage of good quality stones.

The private company, which is 45 per cent-owned by UK-listed mining group Anglo American, saw a slight drop in diamond production in the six months ending June 30 after the sale of two South African mines. But a 10 per cent rise in diamond prices boosted revenues to $3.74bn (£1.86bn), compared with $3.4bn in the same period of 2007.