The device used by GE to synthesize diamond was termed a belt device because tungsten carbide rams were driven into a cavity contained by a doubly-tapered carbide cylinder, contained in turn by a steel jacket - termed a belt. Between the rams is a cylinder of graphite - a furnace - containing the material to be raised to high temperature and pressure. Around the furnace assembly and between the anvils and belt is a compressible material to contain the pressure and accept the deformation; it has traditionally been a natural clay called "pipestone clay" for its alternative use in tobacco pipes. A hydraulic press, capable of perhaps 50 tons, drives the rams into the belt cavity, amplifying the force at the interior to high pressure. An electrical current is passed between the rams and through the conductive graphite, which heats in response; the clay acts as a thermal insulator as well as a container for pressure.

Photograph courtesy IBM Research Laboratories and Lamont-Doherty Earth Observatory; taken by George E. Harlow