There are two ways to make lab grown diamonds - HPHT and CVD. Read about each technological process below:

HPHT (High Temperature High Pressure) mimics the environment that a natural diamond undergoes beneath the Earth’s surface. Temperatures exceeding 1300 degrees Celsius and 50,000 atmospheres of pressures are required to turn a carbon source into carbon atoms that start to form into a rough diamond crystal.

CVD (Chemical Vapour Deposition) is a process whereby a hydrocarbon gas is placed into a vacuum chamber over a tiny diamond seed. Carbon gasses are heated to 800 degrees Celcius, causing them to break apart from their carbon atoms which then form on top of the diamond seed, slowly growing into a rough diamond crystal.