Adjacent amethyst and citrine colors in the same crystal of quartz have been reported from South America, although there appears to be uncertainty as to the exact locality of origin. Such material has also been produced by synthesis. Several previous studies on the impurities contained in natural and synthetic quartz and the effects of irradiation and heating, as well as new heating and irradiation experiments with the current natural amethyst-citrine quartz and with amethyst, are described. It is concluded that there are presently no known tests to distinguish natural bicolor material that was heated or irradiated (or both) in nature from natural amethyst heated or irradiated (or both) by man or from the equivalent synthetic material.
