If you’ve ever seen a gem that appeared to be glowing, you’ve witnessed gemstone luminescence. This phenomenon occurs when electrons in certain atoms of a crystallized mineral absorb energy and then release it in small amounts over time. If the absorbed energy is released almost immediately, the effect is called fluorescence. If there is a delay in the release of the energy (ranging from seconds to hours), the effect is called phosphorescence.
While luminescence is useful for gem identification, especially for differentiating between certain natural and synthetic gems, gemstone luminescence testing is best used in conjunction with other tests.
X-rays, visible light, and even heat can provide the energy to excite the electrons in minerals. However, ultraviolet (UV) light is most commonly used to trigger gemstone luminescence. There are two kinds of UV light, longwave (LW) and shortwave (SW). LW UV light has a wavelength of 3660 Å (or 366 nm) and is generated by fluorescent-type lamps. SW UV light has a wavelength of 2587 Å (or 258.7 nm) and is generated by special quartz tubes.
Some gemstone minerals display luminescence in LW, some in SW, some in both, and some in neither. In many cases, UV light won’t excite a mineral unless it contains an impurity element that functions as an activator. The element manganese, for example, plays such a role in many minerals. Conversely, the element iron quenches fluorescence in most minerals.