Fluorescence and, a related phenomena called phosphorescence, are properties of minerals that emit visible light when exposed to ultraviolet (UV) light and/or continue to emit such light after exposure to UV light. Some minerals may fluoresce under both wavelengths with the same or a similar color, while some may show different colors under each. Most respond best to only one of the two. Over 4000 mineral species have been identified at this time. Over 15% of these species are known to fluoresce visibly in some specimens.

The phenomenon known as fluorescence occurs at the subatomic level by a process called electron excitation. Electrons are subatomic particles that orbit the nucleus of an atom at specific distances known as electron shells. These shells are arranged in layers around the nucleus, the exact number of electrons and their shells depending on the type of atom (element). The electrons contained in the shells nearest the nucleus carry less energy than the electrons in the outer shells.

When certain atoms are exposed to ultraviolet (UV) light, a photon (particle of light energy) of UV will cause an electron residing in a lower-energy inner electron shell to be temporarily boosted to a higher-energy outer shell. In this condition, the electron is said to be excited. It will then drop back to its original inner electron shell, releasing its extra energy in the form of a photon of visible light. This visible light is the fluorescent color that our eyes perceive. The exact color depends on the wavelength of the visible light emitted, with the wavelength itself being dependent on the type of atom undergoing the electron excitation. The specific atoms which undergo the fluorescence are known as activators. They are usually present as impurities in the normal molecular structure of the mineral, but sometimes are an intrinsic part of the mineral's composition. In fluorescent minerals, very often the activators are cations, which are atoms or molecules which carry a net positive charge (due to the loss of one or more electrons, each of which display a negative charge). Because the activators are usually impurities, the same mineral species may fluoresce in some locations and not others, depending on whether the activator was present when the mineral was formed. It also may contain different activators depending on location, and therefore fluoresce in various colors. The intensity of the fluorescence depends on the concentration of the activator in the mineral, but too much activator may actually block fluorescence.

There are a few minerals that will fluoresce when pure. These are called "self-activated" minerals, and include scheelite, powellite, and several uranium minerals. Others suspected of being self-activated include benitoite, cerussite, anglesite and perhaps many other lead minerals.

Photo Courtesy of: The Warren Museum of Fluorescence