G.I.A.: 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.
Richard Hart G.G.