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Fluorescence in minerals is a selective transformation of incoming ultraviolet energy, which is invisible to the human eye, into emitted visible light. Ultraviolet means "beyond violet," and is the name given to that region of the electromagnetic spectrum just shorter in wavelength (and higher in energy) than violet light. Mineral fluorescence takes place when the ultraviolet energy is absorbed by a mineral and is immediately given off as visible light. Fluorescent colors are exceptionally vivid and present the viewer an eerie glow because they are emitted, not reflected, light. The emission is often confined to a narrow band of light, thus yielding a pure, intense color. This phenomenon has nothing to do with radioactivity, although some radioactive materials fluoresce. The willemite, calcite and other local minerals that fluoresce are not radioactive.

Mineral fluorescence was discovered near the end of the 19th Century, when mines were electrified. Sparks from knife switches and the mine "trollies" emitted ultraviolet light, revealing the fluorescent quality of willemite and other minerals. Before long, crude iron-arc spark devices were being used to sort willemite in the mills, and to further analyze fluorescence and phosphorescence. By the 1930's, portable filtered mercury-arc lamps, direct ancestors of those used today by collectors, were being taken underground. There, geologists and surveyors could trace the ore at Sterling Hill and Franklin by fluorescence of willemite.

A substantial number of local minerals fluoresce; they emit visible light when subjected to ultraviolet radiation. The fact that there are more spectacularly colored fluorescent minerals found at Franklin-Sterling Hill than anywhere else in the world prompted the New Jersey State Legislature on September 12, 1968 to declare the Borough of Franklin the "Fluorescent mineral Capital of the World."

The introduction of low-cost, low-pressure, mercury-vapor, portable ultraviolet lamps stimulated a whole new emphasis in mineral collecting at Franklin. This branch of local mineral collecting emerged energetically and fostered intense collecting and preservation of fluorescent mineral specimens on a grand scale. The bountiful supply of material fed an increasing demand and ensured the permanent role of fluorescence in the public aspects of Franklin's mining history and mineralogy.

To most collectors of fluorescent minerals, "Franklin" is typified by the red-and-green-fluorescing combination of calcite and willemite. While these two are found together at a dozen or more other localities, at none of them is this combination abundant or typical. At Franklin and Sterling Hill it was a substantial proportion of the millions of tons of ore mined there.

At present, 84 fluorescent mineral species are known from the Franklin and Sterling area. Of these, about 10 are of sufficient brilliance that specimens of them are widely regarded as "classics." Some of the factors which make Franklin specimens the best know in nature are the intensity, depth and brilliance of their colors of fluorescence; the sharp contrasts where several fluorescent species are present on one specimen (commonly two to four, but up to seven); the textural variations for a variety of species; and the large size of some specimens.