Most synthetic diamonds are produced using high temperature/high pressure techniques developed by the General Electric Company and based on work by the American physicist Percy Williams Bridgman.
This technique was first used to produce diamonds in 1955. Due to their cost and number of flaws, synthetic diamonds are rarely, if ever, used as gems. However their hardness means they find many uses in items such as drill bits and record player cartridges.
The pressures used are generally of the scale of 1 million psi with temperatures running to 1,900°K. A metal catalyst is normally used. Other methods of creating diamond include decomposition of methane at high temperature and explosive shock techniques. More information on synthetic diamond can be found at De Beers Industrial Diamonds FAQ page