Today, lab-grown diamonds are produced by two methods: chemical vapor deposition (CVD) or high pressure, high temperature (HPHT).
While both offer a controlled environment, the HTHP is more costly and less energy efficient. The thermodynamic process involves placing a diamond seed into pure carbon, then exposing it to intense pressure and heat. The carbon attaches to the diamond seed in 14 different directions and forms a rough diamond with a complex cuboctohedron shape.
CVD is a more economical and sustainable process, using vacuum chambers to convert carbon-heavy gasses into plasma with less energy consumption. Kinetics allow the carbon atoms inside the chamber to build on top of the diamond seed in a single vertical direction, so the rough diamond grows into a cleaner square shape.