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Glowing Rocks - Minerals That Fluoresce

Humans have been taking advantage of minerals' marvelous ability to fluoresce for a long time.

Glowing Rocks - Minerals That Fluoresce
Fluorescent minerals Hgrobe/Wikimedia Commons

All light is created by some type of energy. Two forms of light are incandescence and luminescence. Incandescence is visible light produced from heat energy, and examples include our sun, incandescent light bulbs, and fire.

Luminescence is the emission of light without heat, and examples include the light created by a chemical reaction within a lighting bug's abdomen, LEDs, glow sticks, and the phosphorescent paint on luminous watch dials.

Luminescence created by the absorption of electromagnetic radiation is called photoluminescence, and it is divided into two categories: fluorescence and phosphorescence. In both cases, atoms in the materials become excited by the energy, which is then emitted in the form of light. The difference between the two is that phosphorescence continues for a period of time even after the source of electromagnetic radiation has been withdrawn.

Over 500 minerals fluoresce when viewed under various wavelengths of ultraviolet, or UV, light. These include aragonite, calcite, fluorite, powellite, scheelite, sodalite, willemite, and zircon. Minerals that are phosphorescent include calcite, celestite, colemanite, fluorite, sphalerite, and willemite.

How fluorescence works

Photons in UV light excite electrons within the atoms of a mineral, and the electrons temporarily jump to a higher orbital, or shell. When they return to their original orbital, the electrons give off excess energy by emitting photons of visible light. Visible light has a longer wavelength than that of UV light.

How fluorescence works
How fluorescence works Source: Marcia Wendorf

This phenomenon is called Stoke's shift, after the Irish physicist George Gabriel Stokes who published a paper in 1852 describing the process. The mineral that Stokes viewed fluorescing was fluorite, which glowed a deep blue when exposed to either shortwave or longwave ultraviolet light, and Stokes gave the name of the mineral to the phenomenon.

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Fluorite fluorescing
Fluorescing fluorite from Boltsburn Mine, County Durham, England Source: Didier Descouens/Wikimedia Commons

Ultraviolet light, which is invisible to the human eye, appears just above violet visible light on the electromagnetic spectrum. Shortwave ultraviolet light is within the range of 100 to 280 nanometers, midwave UV light is between 280 and 315 nm, and longwave ultraviolet is between 315 and 400nm.

Minerals that fluoresce

Some rubies and emeralds fluoresce red when viewed under longwave UV light, while diamonds usually fluoresce blue under UV light, and also fluoresce when bombarded with X-rays.

Most pure minerals do not fluoresce, with scheelite being an exception, because its impurities — called "activators" — within the mineral cause it to fluoresce. Activators include tungsten, molybdenum, lead, boron, titanium, manganese, uranium, and chromium.

Even rare earth elements, such as europium, terbium, dysprosium, and yttrium, can be activators. However, the presence of iron or copper impurities, called "quenchers", has a dampening effect on fluorescence.

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Many minerals fluoresce only a single color, while others fluoresce in multiple colors. The mineral calcite can fluoresce red, blue, white, pink, green, or orange. Minerals that fluoresce in multiple colors have what is known as "banding", which reflects multiple stages of the mineral's growth.

Fluorescence has many practical uses. Miners prospecting for tungsten often do so at night because they are looking for the fluorescence produced by the mineral scheelite. Scheelite is an ore of tungsten, and it fluoresces a bright blue color.

Geologists in the oil and gas industry examine drill cuttings and drill cores with UV lamps because small amounts of oil within rocks will fluoresce under UV illumination. The color of the fluorescence can even tell the geologists the grade of the oil within the rock, with darker colors of fluorescence indicating heavier oils and lighter colors indicating lighter oils.

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Miners in underground mines often use UV lights to look for ore-bearing rocks. One particular example is the gemstone opal, which fluoresces blue under UV light. 95 percent of the world's opal is mined in Australia, and places such as Coober Pedy in South Australia and Lightning Ridge in New South Wales are riddled with human-sized "gopher" holes where miners have dug down searching for the gem.

Fluorescence can even be used to identify where a gemstone has been mined. Light yellow diamonds that fluoresce a strong blue come from South Africa's Premier Mine, while colorless stones that also fluoresce a strong blue, are produced in South Africa's Jagersfontein Mine. The fluorescence of diamonds has resulted in the use of controlled lighting conditions to aid in color grading the stones.

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Where to find fluorescent minerals

If you're going to go out hunting fluorescent minerals, the black lights sold at novelty stores won't cut it, because they only emit longwave UV light. You'll need to use a more specialized UL lamp, but you should keep some safety points in mind. Never look into the UV lamp or shine it directly onto your skin. Never shine the light into the face of another person, and it's best to wear UV blocking glasses or safety glasses.

Calcite perfume jars from Tutankhamun's tomb, 1323 BC
Calcite perfume jars from Tutankhamun's tomb, 1323 BC Source: Frank Rytell/Wikimedia Commons

Below is a list of some of the more well-known minerals that fluoresce:

  • Fluorite - originally known as fluorspar, fluorite fluoresces blue-violet, sometimes white or cream, and rarely red
  • Calcite - the most common fluorescent mineral, calcite is found throughout the world; it fluoresces in an array of colors
  • Aragonite - along with calcite, it is another common carbonate mineral formed in both salt and freshwater environments, it fluoresces yellow, white or blue, green or white
  • Chalcedony - one of several varieties of silicon dioxides consisting of crystals of quartz and moganite, it is common in geodes and hydrothermal mineral deposits and fluoresces green, yellow, or white
  • Corundum - an aluminum oxide, it includes rubies and sapphires, rubies fluoresce red under longwave UV
  • Selenite - a crystallized form of gypsum, it is referred to as satin spar, desert rose, and gypsum flower, its crystals are also phosphorescent, it fluoresces bluish-white
  • Sphalerite - a zinc sulfide, sphalerite is the most common and most important zinc ore in the world, it can fluoresce in orange or in a rainbow of colors

Rockhounding is becoming an increasingly popular hobby, and a quick search will most likely lead you to a group located in your area. If you know which minerals fluoresce, not only can you find a specimen that looks good during the day, but one that will light up the night.

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