An important problem is a distinction between yellow sapphires that are color stable and those that fade under normal conditions. Yellow sapphires have surged in popularity in recent years, which has prompted further study into their properties and, especially, the effects of treatment by heat or irradiation. One unexpected finding was that type 1 material can fade on heating even below 200☌, but thin the color is restored by exposure to light. The authors examined more than 150 samples of yellow sapphire to confirm the seven types and document their different reactions to light, heat, and/or irradiation. The authors determined that there are actually seven types of "yellow" sapphire on the market today, which differ in the cause of color and the stability of the color to light: type I -natural, light-stable color center type 2-natural or irradiation- produced, fading color center type 3-iron containing, not heated, light stable type 4-iron containing, heated, light stable type 5-surface-diffused additive, light stable type 6 -synthetic, light stable and type 7-synthetic irradiated, fading.
Vinyard, of Lake Oswego, OR and from the Gemological Institute of America, Santa Monica, CA. Acknowledgments: The authors gratefully acknowledge the loan of study material from R. Valente is vice-president of American Gem Market System, Moraga, California. Nassau is a research scientist residing in Bernardsville, New Jersey Ms. THE SEVEN TYPES OF YELLOW SAPPHIRE AND THEIR STABILITY TO LIGHTĭr.
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