Fluorescence is the luminescence produced by determined minerals when they are exposed to the action of certain rays (X rays, ultraviolet and visible rays among others). This radiation is transformed by the mineral into light waves of a longer wavelength than the rays which fall on it. In contrast to cases of phosphorescent luminosity, in fluorescence the luminous emission ceases the instant the exciting radiation is eliminated.
Opal, fluorite and some calcites are minerals which emit fluorescence.

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Screens made from fluorescent materials produce scintillations, twinkles or sparkles, when X rays fall on them.

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