Colorless to yellow to intense orange sapphire will react quite differently to light or heat depending on the nature of the material and the origin of the color. 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 1—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. 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. One unexpected finding was that type 1 material can fade on heating even below 200°C, but that the color is restored by exposure to light.
