Why minerals glow
Fluorescence happens when a material absorbs ultraviolet light, which is invisible to us, and re-emits part of that energy as visible light of a longer wavelength. The colour you see depends on the mineral and, very often, on traces of other elements, so two specimens of the same species can glow differently or not at all.
The word itself comes from a mineral. In his 1852 paper, the physicist George Gabriel Stokes described how fluorspar, the old name for fluorite, turned invisible ultraviolet into visible light, and coined 'fluorescence' after it. Phosphorescence is the related effect where the glow continues after the lamp is switched off.
Long-wave and short-wave UV
Mineral lamps come in long-wave (around 365 nm), mid-wave and short-wave (around 254 nm) versions, and many minerals respond to only one of them. Scheelite, for example, glows bright blue to white under short-wave UV. Cheap 'black light' torches are long-wave only, so a mineral that seems dead under one may glow strongly under a short-wave lamp.
Minerals known for fluorescence
The species most often seen glowing under a UV lamp:
| Mineral | What to expect |
|---|---|
| Fluorite | Many specimens fluoresce, often blue to violet; the mineral that named the effect |
| Calcite | May fluoresce red, blue, yellow and other colours under short- and long-wave UV; some phosphoresce |
| Willemite | Strongly fluorescent; may phosphoresce |
| Scheelite | Bright blue to bluish white under short-wave UV |
| Cerussite | Some specimens fluoresce; Wikipedia shows a Mibladen cerussite under UV |
| Aragonite | Some Moroccan stalactitic aragonite is sold as phosphorescent |
| Autunite | Strong yellow-green fluorescence under UV; radioactive, so collect with care |
Fluorescent minerals from Morocco
Several Moroccan species can fluoresce: fluorite from El Hammam, calcite, cerussite from Mibladen and some aragonite. Because the response depends on trace elements, it has to be checked piece by piece. When we test a specimen under UV, we say so in its listing; if a listing does not mention it, assume it has not been tested.
Displaying and photographing them
A few practical tips:
- Use a dark room or a closed case; even dim daylight washes out the glow.
- Short-wave UV is harmful to eyes and skin: wear UV-blocking glasses and do not shine it at people.
- Photograph on a tripod with a long exposure, and take a daylight shot of the same view to show both faces of the specimen.
- The Franklin Mineral Museum in New Jersey has a 32-foot display of fluorescent minerals, a good reference for what a collection can look like.
Questions collectors ask
Which minerals glow in the dark?
Under UV light, many fluorite and calcite specimens, willemite and scheelite glow strongly. A few, such as some calcite, keep glowing briefly after the lamp is off, which is phosphorescence.
Does barite fluoresce?
Some barite does, but not reliably. Fluorescence depends on trace elements, so each piece has to be tested.
What is the difference between long-wave and short-wave UV?
Long-wave lamps (about 365 nm) are common and cheap; short-wave lamps (about 254 nm) make different minerals glow and are more hazardous to eyes and skin.
Why is it called fluorescence?
George Gabriel Stokes named it in 1852 after fluorspar, the old name for fluorite.
Sources
- Wikipedia, Fluorescence (Stokes 1852, the word from fluorspar)
- Wikipedia, Calcite (fluorescence and phosphorescence)
- Wikipedia, Scheelite (short-wave fluorescence)
- Wikipedia, Willemite
- Wikipedia, Franklin Mineral Museum (fluorescent room)
- Wikipédia, Cérusite (Mibladen specimen under UV)
- Wikipedia, Autunite (fluorescence, radioactivity)