For a low-key business, the colored diamond market has seen a lot of action lately?though not necessarily the kind of action it would wish. In the last year, two companies?NovaDiamond in Provo, Utah, and Bellataire Diamonds Inc. of New York (formerly Pegasus Overseas Ltd.)?have unveiled plans to sell treated green diamonds. And other companies hope to sell fancy-colored synthetics?mostly yellows, though other colors are said to be on the way. (Synthetics typically are treated to deepen their colors.) All the innovators use high-pressure/high-temperature technology and have a connection to either Russian diamond technologists or synthetic diamond pioneer General Electric. There is also talk of synthetics coming from Sweden and elsewhere. ?This technology has been around for a while, and people have finally realized it can be commercialized,? says Carter Clarke, president of Gemesis in S