Fluorescence is a startling property of certain minerals: when you shine an ultraviolet "black light" on them in a darkened place, they respond by glowing in strangely bright colors. For collectors of minerals, fluorescence adds a whole new dimension to their enjoyment. How does it work?
Ordinary illumination happens when photons (light "particles") meet a substance and bounce off in every direction including our eyes. Exactly how that happens is explained by quantum physics, but for our purposes the result is that the photon comes out with the same energy it went in with. Because light behaves as both particles and waves at the same time, we can also say that the photon's wavelengthits coloris unchanged.
Fluorescence happens when photons are briefly absorbed by electrons in a substance, then spat out again. An electron that absorbs a photon enters an excited state and takes up a larger orbit. Within a few nanoseconds it emits another photon that takes away the energy of excitation, and the electron slips back into its low-energy state.
That fluorescent photon is not the same as the initial one; it has a longer wavelength (and lower energy). The incoming and outgoing energy don't match because excited electrons also lose energy in other ways. The geometry of the electron's orbit precisely controls the wavelength (and energy) of the photon it emits, therefore the color of fluorescent light is very pure. It's as if your bank could accept checks larger than 100 dollars, but you could only cash that money in hundred-dollar bills. The extra money gets lost in bank chargesor in the case of excited electrons, they lose small amounts of energy as heat (vibrational energy) and other forms that can't turn back into photons.
Light of shorter wavelength, then, excites fluorescence of longer wavelength. Ultraviolet (UV) light excites fluorescence at visible wavelengths. So do X-rays. Visible light excites fluorescence at infrared wavelengths, which can be seen using night-vision scopes. But fluorescent minerals are almost always found and enjoyed using UV. X-rays are used by professional chemists, for whom fluorescence is a serious research tool.
Long-Wave versus Short-Wave UV
Mineral collectors use two types of UV light to test for fluorescence. The common "black light" is long-wave UV, with wavelengths around 365 nanometers. (Visible light ranges from violet at about 400 nm to red at about 700 nm.) Long-wave UV is the default among collectors: the equipment is inexpensive and there's no particular hazard in using it (although you should keep it from your eyes like any bright light). You can get a pocket flashlight using ultraviolet LEDs for a few dollars. It leaks a lot of violet light as well, but it's a good screening tool if you're walking through a cave or are out at night exploring a tailings pile. More serious black lights filter out the visible part and can produce spectacular light shows when your eyes are dark-adapted.
Short-wave UV, with wavelengths around 250 nm, excites fluorescence in a whole different set of minerals. The lamps are much more expensive, though. Moreover, the light itself is hazardous. Short-wave UV causes skin burns and eye damage. Although all light is technically radiation, this stuff is really radiation. Short-wave UV is what's used in sterilizing lamps and UV water purifiers. (The sun's short-wave UV radiation is filtered out by the ozone layer, high in the stratosphere.) These factors make short-wave UV fluorescence a minority hobby.
The best place to get acquainted with fluorescent minerals is at a large rock and mineral show. That's where the fluorescence freaks get a darkened room to show off their wares.
Common Fluorescent Minerals
Some fluorescence colors are predictable, but many are not. Most fluorescence arises from impurities in a mineral, so a mineral from one locality may show it while specimens from elsewhere do not. Other impurities, notably iron atoms, can quench fluorescence.
Here's a brief list of fluorescent minerals you may find in the field. None of these is always fluorescent, so fluorescence is not usually considered a diagnostic property of minerals. Naturally, collectible minerals are exceptional; a good list is at Amethyst Galleries.
- Albite, a feldspar mineral, may sometimes fluoresce in blue.
- Amber often fluoresces in a dull greenish color that makes it look opaque.
- Apatite may fluoresce in yellow-orange, especially after heating it.
- Calcite may display a wide variety of colors in UV, including white, but is most commonly red. So is its cousin aragonite.
- Corundum may fluoresce in yellow, orange or red. Some rubies have a very strong red fluorescence that reinforces their color.
- Diamond is sometimes fluorescent in short-wave UV, which can help in distinguishing a stone's source.
- Fluorite gave fluorescence its name: it most commonly displays purple colors.
- Gypsum is notable for yellow fluorescence colors that highlight the hourglass-shaped interior structure of its crystals. Several other sulfate minerals fluoresce.
- Halite (rock salt) may fluoresce in red.
- Quartz may occasionally fluoresce in green.
- Sphalerite fluoresces in various colors if it has a low iron content.
- Spodumene may fluoresce in pink or violet.
- Zircon very commonly fluoresces in orange or yellow.
Fluorescence in Industrial Geology
Oil drillers use long-wave UV to detect traces of petroleum in cores and cuttings and gauge its quality: light oil fluoresces blue-white, and the fluorescence color changes through yellow to brown with increasing density (that is, reduced API gravity).
Prospectors may use UV as a quick survey tool, depending on the mineral they seek. The tungsten ore scheelite, for instance, fluoresces brightly in short-wave UV, blue when it's pure and yellow if significant amounts of molybdenum are present. Uranium fluoresces with a bright yellow-green color in the secondary mineral autunite, as well as antique glass.