Luminescence in Gems


Certain electrons in atoms within the crystal structure of a mineral may be able to absorb energy and release the energy at a later time. This creates the phenomenon known as luminescence. If the absorbed energy is released almost immediately, the effect is called fluorescence; if there is a delay (ranging from seconds to hours) in the release of the energy the effect is called phosphorescence. The energy applied to the mineral (or gemstone) that causes the excitation in the atoms of the mineral may be X-rays, visible light, or even heat, but the most widely used energy source is ultraviolet light. Ultraviolet (UV) light is generated by several different kinds of lamps. The two most common types are longwave (LW) UV at 3660 Å, which is generated by fluorescent-type lamps, and shortwave (SW) UV, at 2587 Å, which is generated by by special quartz tubes.

Some minerals react in LW UV, some in SW UV, some in both, and some in neither. In many cases a mineral is not excited by UV light unless it contains an impurity element that acts as an activator. The element manganese (Mn) plays such a role in many minerals. Conversely, the element iron (Fe) quenches fluorescence in most minerals.

Luminescence effects are very useful in gemstone identification, especially in certain cases in distinguishing synthetics. However, luminescence is best used in conjunction with other gemological tests.

Another type of luminescence is Triboluminescence. This is an optical phenomenon in which light is generated when material is pulled apart, broken, ripped, scratched, crushed, or rubbed through the breaking of chemical bonds in the material. The phenomenon is not fully understood, but appears to be caused by the separation and reunification of electrical charges. The term comes from the Greek word τριβείν meaning to rub and the Latin word lumen meaning light. Triboluminescence can be observed when breaking sugar crystals (especially Wint-O-Green Life Savers) in the dark.

Reference: The Color Encyclopedia of Gemstones; Second Edition by Joel E. Arem



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