Diamond formation requires high temperature and high pressure conditions. It takes around 2,500 degrees Fahrenheit and 825,000 pounds per square inch in pressure to form a raw diamond. These extreme conditions are naturally found about 100 miles below the Earth's surface. Diamond formation occurs at depths greater than about 150 km, with the possibility of formation still existing down to about 1500 km.
Diamonds are usually associated with the rock Kimberlite, where they crystallize when the Kimberlite is in magma form. The diamonds are then transported when the Kimberlite is driven upwards by excess pressure exerted by CO2. After being ejected near the Earth's surface, diamonds can remain in the Kimberlite eruption or be distributed into alluvial deposits near rivers and the sea through natural erosion processes.
Synthetic diamonds can also be created using high-pressure, high-temperature (HPHT) manufacturing techniques. There are three basic processes used: the belt press, the cubic press, and the split-sphere (BARS) press. Each process aims to create an environment of extremely high pressure and temperature where diamond growth can occur. A small diamond seed is placed in carbon and subjected to these conditions to facilitate diamond growth.
The belt press, for example, uses upper and lower anvils to generate pressure over 1.5 million pounds per square inch and temperature above 2,000 degrees Celsius. In this environment, pure carbon melts and begins to form into a diamond around the starter seed. However, it should be noted that most man-made diamonds produced today are not of gem quality and are primarily used for industrial applications.
Efforts have also been made to synthesize diamonds at lower temperatures and pressures. In 1953, diamond growth was achieved on the surface of a natural diamond seed crystal at a temperature of about 900 degrees Celsius using thermal decomposition of carbon-containing gases. This technique preceded the high-pressure, high-temperature (HPHT) method developed by GE in 1955.
In laboratory conditions, diamonds can be synthesized using various carbon sources, ranging from zero-dimensional (0-D) to three-dimensional (3-D) structures. The choice of carbon source determines the synthesis method used, which can range from high-pressure synthesis to chemical vapor deposition (CVD). It is important to note that the properties of laboratory-synthesized diamonds are almost identical to those of naturally occurring single crystal diamonds.
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