The diamond industry's worst nightmare, genuine synthetics which are impossible to pick from the real thing, has raised its ugly head again with a report in a New York daily newspaper that two manufacturers of the imitation are stepping up their production and marketing.

The New York Post told its readers yesterday that Gemesis was planning to sell yellow diamonds, made on an old Russian heat-and-pressure synthetic manufacturing machine, at just 10% of the cost of the natural diamonds.

Apollo Diamonds, using a new chemical vapour process to grow its diamonds from a single crystal, could be marketing "stones" for US$5 a carat.

According to the Post, which took a synthetic for valuing at a New York jewellery store, the Gemesis diamonds are hard to pick from naturals but can be identified using a special De Beers machine.

Apollo diamonds, using the crystal-growing technique, are virtually impossible to detect.

In its street-test, a 1.7 carat yellow synthetic received a jewellery store valuation of US$7000.