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.
By Joel E. Arem, Ph.D., FGA 2 minute read

Some minerals glow or fluoresce under ultraviolet (UV) light, such as some shown here. Apatite, quartz, orthoclase feldspar, and muscovite under normal white light and UV light. Photos by Géry Parent. Licensed under CC By-SA 3.0.

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. Collectors also prize crystal specimens with spectacular luminescent displays.

tugtupite - gemstone luminescence

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

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 then called phosphorescence.

phosphorescing diamonds - gemstone luminescence

These yellow diamonds fluoresce magenta or green under UV light. When the UV light is turned off, they phosphoresce a soft green color afterwards. Diamonds phosphorescing (crystals approximately 1 to 1.5 mm in size), Democratic Republic of the Congo. Photo by James St. John. Licensed under CC By 2.0.

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.

Some gemstones fluoresce even in normal visible light or daylight. When backlit with daylight or halogen light, the transparent areas of this lavender fluorite crystal fluoresce an intense “grape juice” purple. 7.7 x 5.7 x 5.3 cm, Greenlaws Mine, Daddry Shield, Weardale, North Pennines, Co. Durham, England. © Rob Lavinsky, www.iRocks.com. Used with permission.

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. Gems from different sources may also have different luminescent properties.

andalusite - gemstone luminescence

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

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.

This pink manganoan (manganese-rich) calcite fluoresces a vibrant reddish orange. 13.5 x 10.5 x 10.1 cm, Huanggang Mines, Keshikteng, Ulahad Bnner, Inner Mongolia A.R., China. © Rob Lavinsky, www.iRocks.com. Used with permission.