A Brief History of Diamond Synthesis
The synthetic diamond industry was in its infancy around World War II. At that time, the machines were very large "belt" presses, in which two anvils apply heat and pressure to a central growth cell. These early machines could only maintain the necessary temperature and pressure for minutes or hours at a time, which limited the size of the diamonds. For many years, the output remained small diamond grit in yellow and brown colors, which were commonly used for industrial cutting applications.
By the 1970's and 1980's, the processes were further refined and controlled, and a "cubic" press was developed, which is much smaller than a belt press and has six anvils applying pressure to each side of a growth cube. By this time, single stones could be grown to gem-quality sizes, though many were still heavily included or had undesirable colors. The technology to create blue and colorless diamonds were developed but not refined, so most lab-created diamonds were still brown and yellow for economic reasons, and exhibited strong fluorescence.
During this time a very different approach was also developed. Chemical vapor deposition uses heated gasses, commonly including methane, in a low-pressure atmosphere. Plasma breaks apart the gasses into their individual elements, one of which is carbon. The carbon then "rains" down onto a diamond substrate, in which it slowly grows (a few micrometers per hour) as a crystal. This technology is best suited for applications utilizing thin layers of diamonds, such as optics and semiconductors. CVD is capable of producing gem-quality diamonds, though the size of the polished diamond is limited by the thickness of the diamond wafer.
In the early 1990's, Russian scientists developed a high-pressure, high-temperature "BARS" press which uses eight spherical outer anvils and six inner anvils to apply hydraulic pressure to the growth cell. One cycle of a BARS machine produces one diamond crystal. These BARS presses are to-date, the most effective process for gem-quality diamonds, while belt and cubic presses are better suited for high-volume industrial-quality diamonds.
By the late 2000's, the BARS processes have been refined where fancy yellow and blue diamonds can easily be grown to produce one carat and larger gem-quality polished diamonds. Growing gem-quality colorless diamonds is still experimental, though some are available.