Waco Gem and Mineral Club, Waco, Tx

18.1 What is Fluorescence and Why Do Some Minerals Fluoresce


18.1: What is 'Fluorescence' and Why Do Some Minerals Fluoresce?

Light moves in waves and comes in different forms depending on the wavelength. Some of these forms are infrared, visible, and ultraviolet (UV). We‘re most familiar with visible light. UV light moves in waves too short for human eyes to detect, but we can see its effects with certain minerals. What appears to be a gray rock in visible light may glow orange or green under UV light. Or a mineral of one bright color under visible light may appear a different color under UV; for instance, purple fluorite may turn green or blue. Still other minerals may stay the same color, but appear more vivid, as with red ruby. In all these cases, under UV light the minerals seem to glow from within.

The first person to describe this phenomenon was English scientist Sir George Stokes in 1852. He worked with fluorite, so he called the effect fluorescence. Some minerals containing impurities called activators will absorb UV light, then emit longer, visible light waves which we see as colors. At the atomic level, UV light causes electrons in some molecules from the 'activators' to jump to a higher energy level. In falling back to their normal level, they give off the extra energy in the form of visible light. UV light is usually divided into short wave (SW) and long wave (LW). Most fluorescent minerals are sensitive to SW. Some will change color as you switch from SW to LW. Fluorescent lamps, especially SW, are very expensive, but you can use a less expensive alternative for LW with a black-light tube readily available at hardware stores, the kind of light tubes people use to make posters glow and as grow-lamps for plants.

What I‘ve provided above is an extremely brief and simplistic explanation of fluorescence. Some great books for teaching kids more about fluorescence and fluorescent minerals are:

Stuart Schneider, Collecting Fluorescent Minerals, 2004. Not only does this book have great opening chapters about fluorescent minerals in general and collecting them, it also serves up a feast for the eyes with a colorful photographic atlas of fluorescent minerals from around the world filling most of the pages.

Stuart Schneider, The World of Fluorescent Minerals, 2006. Schneider picked up from his earlier book with even more colorful photos in this follow-up volume.

Manuel Robbins, Fluorescence: Gems & Minerals under Ultraviolet Light, 1994. Robbins describes fluorescence, overviews significant localities, and provides full chapters devoted to individual minerals, as well as a chapter on the activators that cause fluorescence of different colors. While not as colorful as the Schneider books (it contains a small color insert), it‘s still chock full of good information.

Harry Wain, The Story of Fluorescence, 1965. This little paperback has been around for decades. The Raytech company manufactures fluorescent lamps and packages a copy with each lamp they sell.

Finally, you can also get great information from the web site of the Fluorescent Mineral Society.