What makes them glow? To understand the mechanics of fluorescence, let's revisit our high school science class. Now, don't panic, we'll keep this simple. Do you remember the atom? Remember that it's made up of three main constituents -- protons, neutrons, and electrons? And, do you remember that neutrons and protons form the center of the atom, called the nucleus, while the electrons orbit around the nucleus somewhat akin to the planets orbiting around the sun? Let's focus on those orbiting electrons.
The orbital paths occupied by electrons are called electron shells. Electrons will occupy specific shells, or orbital paths, based on the amount of energy they have. Electrons in shells near the nucleus have lower levels of energy than those in shells farther away. Sometimes, when energy is applied from an outside source (like a black light), it will add energy to electrons and they will fly away from their electron shells, similar to a hyperactive kid on too much sugar! Anyway, when this happens, an electron from a higher shell jumps down to the lower shell to fill the void left by the hyperactive electron. Since this replacement electron has more energy than it needs, it releases some of it's energy as visible light during the transition, producing the pretty colors you see emitted from the mineral. Ta-da -- fluorescence!
Rock hounders use ultraviolet lamps to provide the outside energy source that will excite the hyperactive electrons. Ultraviolet light is divided into long-wave, medium-wave, and short-wave. Each type of ultraviolet light may produce different colors in the same mineral! Sometimes only on type of ultraviolet light will work on a mineral, and, in general, short-wave light will work on more minerals than long-wave. The long-wave source lamps are readily available, and are akin to the black lights you might have hanging next to your favorite velvet poster of Elvis. Short-wave lamps are generally more expensive. They are also more dangerous and can damage your skin and eyes if you are not careful.
Minerals that fluoresce may have certain impurities in them, called "activators," that assist in boosting fluorescence. Activators must be present in the proper concentration ( or quantity) in order for the mineral to fluoresce. If there is too little or too much of the impurity, the mineral simply won't fluoresce. Conversely, some impurities, such as iron, tend to inhibit fluorescence. These impurities are called "quenchers".
One clarification, regarding the terms fluorescence and phosphorescence. If a mineral stops "glowing" immediately upon removal of the ultraviolet lamp, the mineral is fluorescent. If, however, it continues to glow for a period of time, then it is phosphorescent.
Minerals and the discovery of fluorescence are historically linked. The property of fluorescence was discovered by Sir George Stokes in the mid-1800's when he noticed that the mineral fluorite changed colors from green in the shade to blue in the sunlight. He must have been pretty impressed by that piece of fluorite, because he named this particular property (fluorescence) after it. Later, when mines started using electric motors and lights, the miners noticed that the initial spark from electric switches caused some of the surrounding minerals to temporarily glow.
In the early 1900's, the New Jersey Zinc mines used ultraviolet lights to help determine ore grades. Rocks containing high zinc ore glowed a very bright green and rocks with calcite and no zinc ore tended to glow a bright red-orange. By the way, Franklin, N.J. is one of the most famous fluorescent mineral locations in the world. (I wonder if the producers of "The Sopranos" knew that?) During World War II, when tungsten from China was inaccessible, many deposits of scheelite (a tungsten mineral) were found in the west using short-wave ultraviolet light.
So, why the renewed interest in fluorescent minerals? There are a number of factors. Ultraviolet lamps are readily available, costing anywhere in the range of $10 to $1,000 or more. LED ultraviolet lamps are now easily found as well. Most of the ones used by hobbyists are between $150 and $600. Fluorescent minerals may cost less than other types of mineral specimens. Be forewarned, however, that they are getting more expensive, and there are the rather pricey upper-end fluorescents out there.
Many of the traditional mineral collecting locations that have "played out" of the classic mineral specimens are being rediscovered for the variety or type of fluorescent minerals present. In fact, don't be surprised if you approach an old mine dump, only to find a rather peculiar person lying under a tarp prospecting with an ultraviolet lamp!
OK, so your appetite is whetted and you're ready to go hunting for "them pretty-colored glow in the' dark rocks". Where can you find them? First, let me say that there hasn't been a lot of prospecting for fluorescent minerals. So, if you are aware of some old mine dumps that are safely and legally accessible, they might be worth checking out. There are some fairly common minerals, like calcite, that tend to fluoresce and phosphoresce. Calcite is a common gangue, or non-ore, mineral in many of the local deposits. An area noted for unique fluorescent calcite and another mineral, spurrite, is the Tres Hermanas mountains south of Deming. The fluorite from Cooke's Peak also tends to fluoresce. The fluorite from the Gila River area and Burro Mountains doesn't work so well, possibly because coatings of druzy quartz tend to inhibit the penetration of ultraviolet light. The local chalcedony, a form of quartz, can also fluoresce. Areas with lots of chalcedony include Round Mountain near Lordsburg and the Mule Creek, NM area. As always, have fun and be safe out there.