Diamond company De Beers has missed analysts' earnings forecasts as higher global interest rates dented demand.
De Beers plans to spend $100m searching for uncut diamonds
Analysts also said that spending had been affected by record oil prices that had pushed up petrol and energy costs.
De Beers, the world's biggest producer of diamonds, said sales rose to $3.25bn (£6bn) during the first six months of 2006, up from $3.2bn a year earlier.
The company, 45% owned by mining firm Anglo American, said it would add $238m to its owner's first-half profit.
Despite the slower demand, analysts said they were optimistic about the long-term outlook for diamond sales, because many companies said they were finding it difficult to replace mined deposits.
Earlier this year, De Beers announced plans to spend $100m searching for new diamond seams in countries including Botswana, Angola and the Democratic Republic of Congo.
At the same time, De Beers has been increasing its marketing in an effort to lure customers.
The company said it still expected demand for diamond jewellery to rise from last year's levels.