This is a large, transparent, purple fluorite cleavage octahedron from the Hill-Ledford mine, in Cave-In-Rock, Hardin County, Illinois. Under UV, this specimen shows a deep, saturated blue fluorescent response. The octahedron measures 54 mm on an edge.
The Hill-Leadford Mine was one of the many notable fluorspar mines in the Illinois/Kentucky fluorspar district, and was operated by the Ozark-Mahoning Company from the late 1950’s into the 1970’s.
As shown in the photograph above, this cleavage octahedron from the Hill-Leadford mine in Southern Illinois, fluoresces blue under short wave UV (254 nm).
Fluorescence under long wave UV (365 nm) produces an intense, saturated, deep blue color. Although not evident in the photograph above, the brightest response is seen under long wave UV, but is difficult to accurately depict due to limitations of the camera's image sensor. This blue fluorescence is activated by the presence of trace amounts of rare earth elements in the crystal structure.
Same specimen showing its natural transparent purple color under visible light.