Who first made synthetic diamond in a reproducible commercial experiment?

The first reproducible, commercially viable synthesis of diamond was achieved in December 1954 by Tracy Hall, a scientist working for the General Electric Company.1

While earlier researchers, such as James Ballantyne Hannay and Henri Moissan, claimed to have synthesized diamonds in the late 19th century, their experiments could not be successfully repeated. The breakthrough by Hall utilized a belt-type apparatus to subject a mixture of carbon and a ferrous metal catalyst to pressures of approximately 6 gigapascals (GPa) and temperatures of 1,500 °C. This success built upon the foundational high-pressure research of Percy Williams Bridgman, whose work demonstrated that the pressures necessary for the graphite-to-diamond transition could be sustained. Following this development, General Electric standardized the process, and industrial-scale production of synthetic diamond began in 1960.21