There two main ways to make a lab grown diamond: High Pressure High Temperature (HPHT) and Chemical Vapor Deposition (CVD).
During the HPHT process, pure carbon and a tiny piece of diamond (referred to as a starter seed) are placed in a high pressure, high temperature chamber. As the carbon is heated, it melts around the starter seed and a diamond is created.
A diamond made using CVD begins with placing a narrow slice of diamond seed in a sealed chamber. By filling the chamber with hydrocarbon gas mixtures and ionizing them with lasers, microwaves and other tools, the molecular bonds in the gases are broken and the pure carbon sticks to the diamond seed.
Both techniques successfully replicate the same diamond-making process that happens inside the earth with natural diamonds. The rough diamond is then cut and polished to meet our exacting standards.