Irradiation:
Irradiation of diamond by high-energy particles (electrons, ions, neutrons or gamma rays) knocks off carbon atoms and produces vacancies in the diamond lattice. Those vacancies produce green color centers in pure transparent diamond and yellow-green color in yellow diamonds. Yellow diamonds have it color mostly due to the nitrogen impurity and they constitute the majority of all natural diamonds. Heating those irradiated diamonds to temperatures above 600 °C results in brown color associated with aggregation of the vacancies, with or without nitrogen involved.
Such irradiation and annealing treatment can occur in nature because diamonds are often accompanied by uranium-containing ores which emit alpha particles. However, the thus produced color is restricted to a thin surface layer of few micrometers.Homogeneous color can be produced if the treatment was performed artificially, using electrons, neutrons or gamma-rays. Radiation treatment induces characteristic sharp optical absorption lines which can be easily detected by spectroscopic techniques
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