Rocks can fluoresce, or glow, under ultraviolet light.
Rock hounds can identify certain minerals by the glow they give off under ultraviolet (UV) light. We can't see UV light directly, though the minerals absorb its energy and glow with vivid colors. The rocks are producing their own light.
Ultraviolet Light
The light we normally see lies in a narrow band of the electromagnetic spectrum. Visible light waves have wavelengths that range from 400 to 750 nanometers (nm), or billionths of a meter. Ultraviolet light waves are shorter than visible light, ranging from 3 to 300 nm.
Energy
The energy of light is inversely related to its wavelength. Shorter waves carry more energy, so UV light has more energy than visible light. This energy causes certain minerals to glow.
Fluorescence
When UV light hits a special mineral, its atoms absorb the UV and give off a longer wavelength of visible light. Scientists call this action fluorescence. Only certain kinds of atoms are sensitive to UV light, so not every substance glows.
Short and Long UV
For looking at minerals, you can use two wavelengths of UV light. Long-wave UV, also called UVA, has wavelengths from 315 to 400 nm. Short-wave UV, called UVC, has waves of 100 to 280 nm.
Minerals and Colors
Of the hundreds of minerals in the Earth, several dozen have a noticeable UV glow. Fluorite, gypsum, and colemanite glow bluish-white under UVA light. Calcite and halite have a red glow. Agate glows green. Under UVC, fluorite glows yellowish-white, ruby and halite glow red, and diamond glows green.
Warning
Ultraviolet light, having more energy than visible light, is harmful to your eyes and skin. You can tolerate exposure to UVA for several minutes, since its energy is somewhat lower. Never look directly into a UVC light source, as its energy is much stronger.
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