Demand for the world's biggest, most perfect diamonds may fade as the credit crunch and the worsening economic outlook force buyers worldwide to tighten their pursestrings.

Flawless large Class D diamonds can command a price per carat of more than 20 times the average for other stones. The price nearly doubled last year - rising between 50 per cent and 100 per cent, depending on size - as strong sales in Asia and the Middle East offset falling American sales. As the credit crunch and economic slowdown has caused a drop in diamond sales in the United States, there are fears that buyers elsewhere could also be put off.

Last week, Sotheby's failed to sell a $12 million (£6.06 million) diamond in Hong Kong, the first sign of weakness in the rare gem market.

Large stones will retain their value as they are increasingly rare, Charles Wyndham, director of WWW International Diamond Consultants, said. “The very bottom end of the rough and polished diamonds market struggled through 2007 and continues to struggle in 2008. However, fancy-coloured diamonds and top-quality large and small D-colour stones still attract top prices,” he said. Gem Diamonds, the FTSE 250 miner, said that the market for large stones would not be affected significantly by the credit crunch.

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