SYNTHETIC diamonds have been around for 40 years, but methods of creating jewels in the laboratory have become so sophisticated that it is now harder and harder to distinguish between the fake and the real thing. The Hong Kong Jewellery and Watch Fair is to hold a seminar on the subject of ''Gems - Synthetic and Treated''. An industry spokesman said the naked eye was unable to detect a false emerald, ruby or sapphire, although certain tests could prove whether a stone was natural or not. Tests include dipping the stone into a chemical solution or viewing it under a microscope. The fake diamond industry took a new turn recently when American Thomas Chatham and his company Chatham Created Gems launched a lab stone specifically for the jewellery industry which, Mr Chatham claimed, was exactly the same as the real thing. The diamonds are being created in Russia, where they are cheaper to manufacture. According to Asia Precious, a trade magazine for the jewellery industry, 100 carats of rough diamond are produced every month at a Moscow factory. The rough takes 40 days to grow. The process takes longer for emeralds, sapphires and rubies. When developed, they are ready for cutting, either in Russia or Thailand. Mr Chatham said diamonds produced this way sold at one tenth the cost of natural ones. ''People who can't afford a diamond of the quality they want, can now get something else,'' he said. The cultivated ruby is another favourite. J. O. Crystal, a Californian-based company, has tripled its production in 10 years. The Ramaura Cultured Ruby grows by ''spontaneous nucleation'', a process similar to the way a natural ruby grows, according to its creator, Judith Osmer, a trained chemist with 25 years' experience as a crystal grower. Anne Paul, president of the Hong Kong Gemological Association, said there was a huge market for synthetic jewels. ''They offer a uniformity of colour that you don't get with the natural gems. ''Also, not everyone can afford an emerald but they love the look of the stone. As long as there is demand for synthetics, there's no problem.'' The price varies according to the method used. Mrs Paul said: ''I was in Burma recently and was offered synthetic rubies for US seven cents a carat for cut stones. ''I thought that was incredibly cheap. But they were made from the flame fusion process that takes a matter of hours. ''Different processes take longer and, therefore, cost more - anywhere between US$100 and $300 a carat. ''The real thing can cost anywhere between $1,000 and $15,000 a carat.'' At Friday's seminar, Jeanette Fiedler-Schwab, manager of the German Institute of Diamonds, will discuss synthetic diamonds.