Annealing type Ib or mixed type Ib-Ia natural diamond crystal

An annealing process for converting type Ib nitrogen to type Ia nitrogen in natural diamond type Ib crystal or natural diamond mixed type Ib-Ia crystal, each of said crystals having a minimum size of one micron as measured along the longest edge dimension of the crystal, which consists essentially of subjecting a specimen consisting of said crystal to an annealing temperature ranging from about 1500.degree. C. to about 2200.degree. C. under at least a pressure which prevents significant graphitization of said crystal at said annealing temperature for a period of time ranging from about one minute to about 50 hours and sufficient to convert at least about 20% of the total amount of type Ib nitrogen present in said crystal to type Ia nitrogen, the minimum pressure ranging from 48 kilobars at said annealing temperature of 1500.degree. C. to a minimum pressure of 70 kilobars at said annealing temperature of 2200.degree. C., and recovering the resulting annealed free diamond crystal wherein the dissolved nitrogen detectable by ultraviolet, visible and infrared absorption spectra and electron paramagnetic resonance consists of types Ib and Ia nitrogen, said type Ib nitrogen always being present in said annealed crystal in at least a detectable amount, said annealing process making no significant change in the physical dimensions of said diamond crystal.