by Mark Isaacs
Fluorescent minerals are natural; they are not painted or artifically treated in any way. Of over 3600 known mineral species only about 500 are know to fluoresce. There is no way to know by looking at a rock under regular, visible light whether it will fluoresce under ultraviolet.
Invisible ultraviolet light coming from lamps in the display cases causes electrons within the mineral molecule to jump to a higher energy level. As the electrons fall back to their normal energy level they give off the extra energy in the form of visible light which appears to our eyes as many different colors. Some minerals will fluoresce only one color while others may fluoresce different color (or not fluoresce at all) depending on the presence of minute amounts of different "activators".
Most minerals do not fluoresce, and there is no way to know by looking at a rock under regular light whether it will fluoresce. Fluorescence is not the same thing as radioactiviity. Some radioactive minerals fluoresce, and some do not.
Ultraviolet light has a range of wavelengths, and some minerals will fluoresce under one wavelength but not another. Interestingly, some minerals will change color when viewed under different wavelengths of ultraviolet. "Long wave" (LW) ultraviolet with a wavelength of 366nm is the same as the UV coming from blacklights like those in hardware stores and dance clubs. It is relatively harmless. "Short wave" (SW) UV at 254nm, however, can cause burns on exposed skin and eyes. Most fluorescent minerals are sensitive to short wave (SW) UV while only about 15% are sensitive to LW, and for that reason most of the lights in the display cases are SW. Special glass and plastic are used on the front of the cases to block SW UV from being transmitted, making displays safe to view.
Mark Isaacs is Northern California's Regional Vice-President of the Fluorescent Mineral Society and past guest speaker at our general education meeting.