Creating Man-Made Diamonds
Currently, there are two primary technologies that can create diamonds: HPHT and CVD.
HPHT - High Pressure High Temperature
The high pressure, high temperature process recreates conditions found deep inside the earth, in which diamonds naturally grow under. There are multiple designs for HPHT growth units.
A belt press was the first successful design, and some belt presses can be many stories tall and weigh thousands of pounds. This design has two anvils that press together and can grow many diamonds in one cycle, though conditions cannot be controlled as easily, creating varying quality within a single cycle. These presses are most commonly used for industrial diamonds and powder, though are capable of producing gem-quality stones.
A cubic press uses six different anvils pressing onto a cube. These presses can vary greatly in size and are most commonly used to produce industrial diamond powder.
At the core of the HPHT machines is a growth cell. This cell contains all the elements and materials necessary to grow the diamond. This includes a tiny diamond seed, highly refined and purified graphite (carbon) and a catalyst of mixed metals and powders that facilitates the diamond growth. The growth cell is placed at the center of the machine and is heated to over 1,300 degrees Celsius and over 50,000 atmospheres of pressure is applied. As the temperature and pressure increase, the catalyst turns into a molten metal solution.
Once the ideal conditions are reached, the graphite dissolves into this solution. Through a controlled cooling process over the course of many days, the carbon atoms slowly build upon the crystal structure of the diamond seed. The diamond normally grows in a truncated octahedron or a hexa-cubic shape, depending on parameters of the growth process. Once the machine runs its full cycle, the growth cell is removed. The new diamond is cleaned and ready to be cut and polished, just like any other rough diamond.
During the growth, the temperature and pressure need to be maintained within a very strict set of parameters. If there is any fluctuation, the diamond can either stop growing, or become so heavily included, the usable gem-quality size is quite limited. It is not possible to see the diamond during growth, so in most cases the machine runs a complete planned cycle, even though the diamond may have become heavily included or stopped growing part way through the cycle.
The different colors grow at different rates, which is why the size availability varies based on the color. It is exponentially more difficult to grow larger sizes, as the longer the cycle runs, the more prone it is to failure.
CVD - Chemical Vapor Deposition
Chemical vapor deposition is a very different process from HPHT, though they both ultimately grow diamond. The environment inside a CVD machine is a small fraction of one atmosphere of pressure. Carbon-based gasses, commonly methane, are added to the environment and are heated to many hundreds of degrees Celsius which causes the gasses to break apart and release their carbon atoms. The carbon then "rains" down onto a diamond substrate, in which it slowly grows in layers (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.
Most CVD diamonds grow with brown or black hues and must be put through an additional color treatment process to turn them near-colorless.