The various techniques in use today to alter colorless topaz to produce brown, pink, and most commonly, blue stones are discussed. Special attention is given to the three irradiation sources -gamma rays, high-energy electrons, and neutrons - used to convert the colorless stones to hues in the yellow to brown range and the subsequent heat treatment required in most cases to turn the irradiated stones blue. The blue color produced by irradiation and subsequent heat treatment is stable to light. Currently, no routine gemological tests are available that can definitively determine whether the color of the topaz is natural or the result of treatment.
