"Heating
Heat altered gem material is changes or improves the color. Some heat treatment is permanent and can lighten, darken, or completely change the color of the gem. Some heat treatment is unstable and can revert to the original pretreated color with time. Zircon can be unstable and after heat treatment the stones can be exposed to sunlight for several days and then stored in the dark up to a year to remove the unstable stones (Hurlbut and Kammerling, 1991, p. 169).
Heat treatment may change crystal inclusions within the gem, causing them to melt or explode. This may be detected with magnification by a skilled person, although it may be difficult to definitively state any color is natural when the gem material is flawless.
Temperatures used for heat treatments vary, depending on the material and desired color. Sometimes low temperature, such as that from an alcohol lamp, will change brown topaz to pink; very high temperatures, as high as 2050 degrees C, are needed for other alterations, such as titanium-rich milky sapphires to blue."
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