What is Gemstone Luminescence?

gemstone luminescence
Some minerals glow or fluoresce under ultraviolet light, such as some shown here. “Apatite, Quartz, Orthoclase Feldspar, and Muscovite under UV light” by Géry Parent. Licensed under CC By-SA 3.0.

If you’ve ever seen a gem appear to glow, 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.

Gemstone luminescence can help gemologists differentiate between certain natural and synthetic gems. However, for gem identification purposes, gemologists usually analyze luminescence in conjunction with other tests.

What’s the Difference Between Fluorescence and Phosphorescence?

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.

What Kind of Energy Causes Luminescence?

X-rays, visible light, and even heat can provide the energy to excite the electrons in minerals. However, gemologists most commonly use ultraviolet light (UV) to trigger gemstone luminescence.

tugtupite - gemstone luminescence
Collectors prize tugtupite, found in Greenland, for its intense displays of gemstone luminescence under UV light. “Tugtupite” by Géry Parent. Licensed under CC By-SA 3.0.

What is Longwave and Shortwave Ultraviolet Light?

There are two kinds of UV light, longwave (LW) and shortwave (SW). LW UV light has a wavelength of 3660 Å (or 366 nm). Fluorescent lamps can generate LW UV light. SW UV light has a wavelength of 2587 Å (or 258.7 nm). Special quartz tubes can generate SW UV light.

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.

andalusite - gemstone luminescence
Andalusite from Brazil will fluoresce dark green or yellow-green under shortwave ultraviolet light. However, andalusite from Lancaster, MA, like the variety sometimes called chiastolite shown here under normal white light, will fluoresce brown under shortwave ultraviolet light. “Andalusite.” © Rob Lavinsky, www.iRocks.com. Used with permission.