gems, topaz, causes of color, spectroscopy

Bank discusses the causes of color in topaz and the changes that may be produced by natural or artificial influences. He examines the behavior of differently colored topaz and considers the effects of irradiation and subsequent heating on color, including the production, alteration, and loss of color centers within the crystal. The paper relates these changes to the structural and chemical characteristics of topaz and discusses their significance for understanding naturally occurring colors as well as artificially induced coloration. The study is of gemological importance in demonstrating that topaz color may be unstable or modified by treatment and that observed color does not necessarily represent the original state of the material.