Two processes have established themselves on the market for the production of laboratory-grown diamonds: The high-pressure, high-temperature process (HPHT) and chemical vapour deposition (CVD).
The high-pressure, high-temperature process (HPHT)
HPHT stands for "High-Pressure-High-Temperature" and is one of the most important methods for producing diamonds in the laboratory. In this traditional diamond growing method, laboratory diamonds are made from carbon material in apparatus that mimics the extreme heat and pressure conditions in the earth under which natural diamonds grow. The HPHT process takes place at pressures of 5-6 GPa and temperatures of 1300-1600 °C.
The HPHT process in detail:
- The formation of the lab-grown diamond takes place in a small capsule inside a device that can generate very high pressures.
- Inside the capsule, a carbon feedstock (e.g. graphite) dissolves in a molten flux consisting of metals such as iron, nickel or cobalt, lowering the temperature and pressure required for diamond growth.
- The carbon material then migrates through the flux towards the cooler diamond nucleus and crystallises on it to form a diamond crystal.
- Crystallisation takes place over a period of several days to weeks to grow one or more crystals.
Laboratory diamonds show cubic growth patterns with the HPHT process.
While natural diamond crystals are usually octahedral in shape, lab-grown HPHT diamond crystals usually have not only octahedral but also cubic faces. As a result, natural and lab-grown HPHT diamond crystals have different growth patterns. These growth patterns are among the most reliable methods to distinguish mine diamonds from lab grown diamonds.
Why the HPHT process has a bad reputation
Lower quality diamonds, whether natural or lab grown, can also be subjected to the HPHT process to improve colour and clarity. With this process, diamonds can be coloured not only more colourless, but also pink, blue or yellow, for example. In this case, the diamond must then be marked as a "treated" diamond. Especially in the past, this additional "treatment" of diamonds may have been insufficiently identified by some lab grown manufacturers. For this reason, many industry participants and customers view this process critically due to the lack of transparency.
Chemical Vapour Deposition (CVD)
CVD stands for Chemical Vapour Deposition and is another method of producing lab grown diamonds. With this new technique, diamonds can be produced in the lab at moderate temperatures (700°C to 1300°C) and lower pressure. Carbon-containing gas is pumped into a vacuum chamber where it is deposited on a diamond seed and crystallises as a lab-grown diamond. The size of the diamond then depends only on the growth time.
The CVD process in detail:
- The diamond seeds, also known as seed crystal, are placed in a chamber that is filled with a gas containing carbon (for example methane).
- At the same time, the chamber is heated to around 900-1200 °C.
- By means of a microwave beam, the carbon dissolves from a plasma cloud and settles on a seed crystal.
- The diamonds are removed every few days to polish their surface and remove "non-diamond" carbon.
- Each batch of diamonds may require several stop/start cycles
- The entire growth process can take three to four weeks.
- After the raw lab diamonds are removed, they can be cut and polished into the final product.
More specifically, CVD diamond growth takes place in a vacuum chamber filled with a gas containing hydrogen and carbon, such as methane. An energy source, such as a microwave beam, splits the gas molecules and the carbon atoms diffuse towards the colder, flat diamond seed plates. Crystallisation takes place over a period of weeks, and several crystals grow simultaneously. The exact number depends on the size of the chamber and the number of nucleation plates.