The colour changes of corundum crystals annealed in hydrogen, oxygen and argon, and exposed to an UV irradiation is described. The comparison of samples doped with Be, Mg, or Fe is given. The change from pink colour to brown of rubies as well as the yellow colour of sapphires is attributed to the O− centre formation which causes an increase of the absorption between 400 to 480 nm.
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Kvapil et al. (1973) studied this question.
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