When rays from an ultraviolet lamp called a black light strike certain minerals, their atoms become excited (rapidly vibrate) and reradiate energy at a lower wavelength, often providing strikingly brilliant colours. This effect is termed fluorescence, named after the mineral fluorite, which often glows blue-violet when a black light is shone on it. Fluorescence is the ability of an object to absorb energy such as ultraviolet light and reradiate that energy at a different wavelength, usually in the visible range.
Ultraviolet light has a very short wavelength that cannot be seen by the naked eye. However, some minerals when exposed to a black light emit light at a longer wavelength that is visible only as long as the light source remains. A good source of ultraviolet light of short wavelengths is a quartz lamp, whereas argon light produces longer ultraviolet rays. Other sources of fluorescent energy are X-rays and cathode rays, which are high-energy electrons. The overwhelming majority of fluorescent minerals are not very attractive until a black light is turned on them, when the ultraviolet rays produce vivid and intense colours. The distinctive colours are produced by a variety of minerals.
Some minerals might fluoresce in one locality but not in another. The property of fluorescence aids the geologist in prospecting for minerals and petroleum at the surface and the hobbyist in discovering this exciting field of mineralogy.
Fluorescent minerals usually contain a small amount of impurities such as manganese, which are called activators. These enhance a mineral’s ability to glow in the presence of ultraviolet rays. If a mineral continues to glow for a short time (up to several minutes) after the ultraviolet light source has been removed, it is phosphorescent. These minerals are much less common than those that fluoresce. In addition, some minerals only fluoresce in shortwave ultraviolet, whereas others only fluoresce in long-wave ultraviolet. However, many fluorescent minerals glow in both ultraviolet wavelengths. Therefore, when exploring for fluorescent minerals, one is advised to select a black light that uses both ultraviolet wavelengths.
Willemite, which is a zinc ore, was first discovered to have fluorescent properties at the Franklin mine in New Jersey, where spectacular fluorescent specimens are perhaps the finest in the world. Miners observed a brilliant green glow from willemite when they exposed it to the ultraviolet rays of an arc lamp in a dark mine shaft. Franklin calcite also fluoresces, providing a brilliant red colour.
Sphalerite, another ore of zinc, is also fluorescent and offers another quality by emitting flashes of light when scratched in the dark.
Scheelite, the most important tungsten ore in North America, is one of the few minerals that always fluoresce. Geologists explore for scheelite by shining a black light on suspected outcrops at night and look for a characteristic blue glow. If the mineral glows yellow, molybdenum, an important mineral for hardening steel, is present. Scapolite, a complex silicate metamorphic mineral, glows an appealing yellow under a black light. Opal from the western states glows green as a result of the presence of a small amount of uranium impurities. Uranium ores, besides being detected by their radioactivity, produce strikingly beautiful hues of yellow and green under ultraviolet light. Even diamonds will fluoresce under a black light with various hues of blue.