A synthetic gemstone that resembles the natural diamond, produced at very high temperature and under great pressure. The earliest experiments were made before1880 by a Glasgow chemist, J.B. Hannay, who produced some small stones of doubtful authenticity. Experiments by F.F.H. Moissan in the 1890s and by others later proved inconclusive. In 1955 a successfulprocess was developed using techniques devised by Dr Percy W. Bridgman (1882-1961) for the General Electric Co. in the United States, producing by high temperature and enormous pressure some very smallstones (1,2mm in length). As a result, this firm and others in Sweden and elsewhere now produce quantities of small synthetic diamonds for industrial use, but they are not large enough for gemstonesand are more costly than natural stones.By 1970 General Electric Co. had produced gem-quality synthetic diamonds that were colourless or that showed yellow or blue, but they show technical deviations from the natural stones and under amicroscope have a 'dusty' internal look. Some man-made stones are sold as simulants of diamonds but are not synthetic diamonds, e.g. cubic zirconia;yttrium-aluminium-garnet, and strontium titanate.