A diamond is a form of carbon in which the carbon atoms are arranged in a crystal structure such that each carbon atom is surrounded by four other carbon atoms to form a perfect tetrahedron. This structure is extremely stable, and diamonds are the hardest naturally occurring substance. Coal is an amorphous form of carbon that can be transformed into diamond with a high pressure, high temperature (HPHT) process. This occurs in nature over millions of years and there are several different processes that can accomplish this artificially.
Use HPHT processes to produce very small diamond chips within a few hours. These diamonds are not suitable as gemstones and are primarily used to coat the edges of cutting tools.
Make diamonds with a belt press. Upper and lower anvils press on an inner cell containing carbon. The pressure is usually confined internally with steel bands, although this may also be accomplished with hydraulic pressure. Belt presses were the first technique for creating diamonds, and they are still in use today, although modern belt presses are now much larger than the original design.
Use the cubic press to synthesize diamonds. A cubic press has six anvils to create pressure on all six faces of a cube. This type of press is usually smaller than a belt press and can achieve the pressure and temperature required to create a diamond more quickly. The primary disadvantage of the cubic press is that it cannot scale up as easily as the belt press. This is due to the fact that the amount of force needed on the anvils must increase in order to achieve the required pressure.
Employ the split-sphere press. This method is commonly known as the BARS method, which is a transliteration of a Russian acronym for this technique. A split-sphere press uses a ceramic cylindrical capsule of about 2 cubic centimeters in volume. This capsule is placed into a cube of some material that transmits pressure, such as pyrophyllite ceramics. Each side of this cube has an inner anvil which is enclosed by two hemispheres, each of which contains 4 outer anvils. This entire assembly is locked in a barrel which is filled with oil and heated. The oil expands in response to the heat, which provides the required pressure.