Lab grown diamonds are grown using two methods, High Pressure High Temperature (HPHT) and Chemical Vapour Disposition (CVD).
The HPHT method involves placing a diamond seed crystal in a purpose built press or growth chamber where extreme heat and pressure is applied. This then allows carbon atoms to precipitate on the diamond seed crystal and enables the lab grown diamond to grow. Once grown to a required size it will then be cut and polished. As part of the growth process HPHT diamonds use a metal catalyst and as such can have metal inclusions.
The CVD process involves a diamond seed crystal being placed in a diamond growth chamber where it is filled with carbon containing gas. The chamber is then placed under extreme heat causing carbon to precipitate and deposit on the diamond seed crystal slowly forming the diamond. Once grown to a required size it is then cut and polished. CVD diamonds do not contain metal inclusions but as part of the growth process can come with graphite or other mineral inclusions. Some CVD diamonds may also have post pressure and temperature treatment applied to them after they are grown and will have this listed on the lab diamond certificate.