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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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