corundum, treatments.6, irradiation, gems

Recently in New York, we were shown a ring containing a natural colorless sapphire that the client said had been a pleasant “Ceylon” yellow before a recent repair job. Dr. Kurt Nassau had suggested that the client bring the ring to us for the purpose of exposing it to X-rays. On two occasions in the past, Dr. Nassau had restored the color to red tourmaline—mistaken for ruby—that had lost its color due to overheating. In those cases, the color, apparently permanent, was restored using gamma rays. To control our experiment with the colorless sapphire, we exposed the ring and another colorless sapphire from our own collection. When the two stones reached a medium dark brownish yellow, we placed them in a sunny window for about four hours. Upon removal, our loose stone had become colorless again, but the ring stone had turned a pleasant yellow, which the owner stated resembled the original hue. Evidently, the natural color center differs from an induced center, although heat can render both colorless. We anticipate that the restored color will be permanent under normal conditions.