Lab-grown diamonds are generally produced by chemical vapor deposition (CVD) or high pressure high temperature (HPHT) methods.
They are the purest, highest-quality diamonds.
HPHT, used since the 1950s, puts a pure, solid carbon source under high pressure and high temperature. In the presence of a catalyst, the pressurized and heated carbon crystalizes into diamond. HPHT is sometimes used to improve the color of mined or lab-grown diamonds.