Heat Treatment
The color of some gemstones changes when they are subjected to heat. Gems thus treated are sometimes called "fired" or "burnt" stones. Such changes may be temporary, resulting in immediate reversion to the original color as the temperature returns to normal. Thus ruby, which changes to green at high temperatures, reverts to its original color upon cooling. In other cases, reversion to the original color, or to one somewhat similar, may be gradual or may not be detectable even after many years. For instance, some zircons that have been heat treated to produce a blue color revert partially over a period of time; others seem to be permanently colored. In some heat induced color changes, the result may be permanent every time; the brownish topaz-quartz that is produced by heating some amethyst colored quartz is an example.
Dyed Stones
A few gemstones are permanently changed in color by chemical treatment; heat is also usually applied during the treatment. This change of color is sometimes permanent, as in the case of chalcedony stained black. More often the color changes gradually as it does in blue-dyed quartz. On the other hand, many stones are stained with dyes that change color quickly.
Loss of Color
Exposure to heat or light may result in gradual and complete loss of color. Examples are the production by heat treatment of colorless zircons from colored specimens of that gem mineral, and the gradual loss of color in rose quartz when exposed to daylight over a long period of time. The majority of dyed stones lose color under either heat or light.
COLOR IN METALS
Under ordinary conditions of illumination, the precious metals have a distinctive color. This color is often greatly modified by light reflections from the metal's surfaces. Pure gold under bright lights appears a different color than under very diffused light. Usually, the distinctive color of a metal is apparent in the color of any alloy of which it is an important constituent, such as the color of copper in 10 karat gold. However, mixtures of metals may result in an alloy that retains little of the distinctive color of the metal that is its principal constituent. This is true of 18 karat white gold used in jewelry.
