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Minerals and rocks, which glow under ultraviolet light, are usually perceived as a miracle. Minerals and rocks can completely change their colors by being subjected to UV light or black light. This phenomenon is reinforced by the colors observed. New colors are so bright and vivid that it is hard to say they are natural. But it is.
The most common minerals and rocks that glow under UV light are fluorite, calcite, aragonite, opal, apatite, chalcedony, corundum (ruby and sapphire), scheelite, selenite, smithsonite, sphalerite, sodalite. Some of them can glow a particular color, but others can be in a rainbow of possible hues.
You can observe this phenomenon even at home! Unlike some other physical and optical phenomena, which can hardly be observed without any specific microscopes or chemical solutions, fluorescence can be easily seen even at home with the help of a UV flashlight. Let us guide you through this miracle of nature!
Reading this article, you will find how to observe mineral glow or fluorescence at home. What minerals are needed? Are they rare? What kind of light source should you use? No doubt, you will be surprised how easily this phenomenon can be achieved.
If you are looking for the best UV light for rockhounding only, I and other members of the reputable Facebook group about rockhounding recommend buying Convoy 8+ UV light (Amazon link).
What Types of Rocks Glow?
Mineral and rock glow is scientifically explained by the fluorescence phenomenon. The answer to why some minerals and rocks glow is hidden in their chemical composition.
A list of specific elements (activators) can be an essential part of a crystal structure or just an impurity. Fluorescent minerals combined with other rock-forming minerals produce rocks that glow.
Rocks that contain fluorescence minerals can glow under UV light. They are rocks with a high concentration of calcite, which are limestone, marble, and travertine. Granites, syenites, and granitic pegmatites with a high concentration of fluorescent feldspar and low iron content can also glow.
Many of the light-colored silicate minerals in low-iron igneous rocks (particularly granites, syenites, and granite pegmatites) can fluoresce.
Albite and the potassium feldspars typically fluoresce in shades of red, which may range from barely perceptible to moderately bright.
Red fluorescence in feldspar has been ascribed predominantly to Fe3+, although REE, Pb2+, and many other metals may also play a role.
It’s noteworthy that fluorescence can be present or not even in one mineral species. For instance, it’s easy to encounter fluorite and calcite, which will be inert to UV light.
It means that they will not glow. The explanation is that the samples you are testing have no necessary impurities to activate fluorescence. That is why scientists do not strongly rely on fluorescence as a diagnostic feature.
Are Glow Stones Natural?
The fluorescence color is so dramatic, so naturally evokes thoughts that the material is not natural. For example, very common yellow calcite can turn bright red, or common grayish-white chalcedony will show up in acidic green.
Stones that glow under UV light are natural. Their unique ability to glow even helped geologists open new deposits of such critical elements as tungsten and uranium in the previous century. Fluorescent or glowing minerals contain specific elements (activators) that help them to glow.
Fluorescence has practical uses in mining, gemology, petrology, and mineralogy. The mineral scheelite, an ore of tungsten, typically has a bright blue fluorescence.
Geologists prospecting for scheelite and other fluorescent minerals sometimes search for them at night with ultraviolet lamps.
Before World War II, the United States had no deposits of tungsten. Tungsten has been imported from China. As soon as the War began, the country began to experience an acute shortage of this critical metal used for armor production.
The government made a massive advertising company and asked people to “Look for tungsten.”
Thousands of people rushed to search for scheelite in mined tunnels and quarries of non-ferrous metals. Very soon, a large scheelite deposit of Yellow Pine was discovered.
Gemologists commonly use another application of the fluorescence effect. The presence or absence of fluorescence or its different colors helps gem-experts to differentiate synthetic gemstones from natural ones.
BTW: Do you want to know more about rock and mineral identification? The books listed below are the best ones you can find on the internet (Amazon links):
- Smithsonian Handbooks: Rocks & Minerals
- Gemstone & Crystal Properties (Quick Study Home)
- Ultimate Explorer Field Guide: Rocks and Minerals (National Geographic Kids)
Can Crystals Glow?
Crystals are perfectly crystallized minerals with even facets and edges. They have the same chemical composition, so fluorescence will be observed in crystals also. It’s even more charming to have mathematically perfect crystals, which additionally have fluorescence phenomenon. Bingo!
Crystals, which are perfectly even crystallized minerals, can also produce different color glows under UV light. The most common glowing crystals are well-formed cubic crystals of fluorite, scalenohedral calcite, prismatic apatite, corundum (ruby and sapphire), aragonite, scheelite, and selenite gypsum.
- Fluorescence of mostly blue, blue-violet, greenish-white, and rarely red color can be observed in perfectly formed cubic or octahedron crystals of fluorite.
- Well-formed prismatic, rhombohedral, and scalenohedral calcite crystals can show a rainbow of color possibilities. In most cases, calcite crystals will glow orange, yellow, white, and green. Occasionally calcite crystals glow blue, red, and pink.
- Nicely shaped hexagonal crystals of apatite, which may be prismatic, dipyramidal, and stubby, produce yellow, violet, blue, white, or pink colors under UV light.
- Barrel-shaped crystals of corundum and its two varieties, ruby, and sapphire, can glow bright red.
- Acicular and prismatic crystals of aragonite glow yellow, blue, green, or white.
- Pseudo-octahedral crystals of (calcium tungstate) scheelite captivate with bright blue, bluish-white, and sometimes yellow colors under UV light.
- Selenite, satin spar, desert rose, and gypsum flower are crystal habit varieties of the mineral gypsum. Tabular crystals with rose-like shapes can produce surprising lime-green and blue colors under UV light.
TIP: Rubies and sapphires are considered precious gemstones. Do you know what the difference is between precious and semi-precious gemstones? Check out the main differences (and explanation) in the article below:
Precious & Semi-Precious Gemstones: Explanation & Difference
What Causes Fluorescence in Minerals?
Fluorescence is an exciting physical phenomenon that can be observed in minerals and rocks. A mineral can significantly change its color while exposed to UV radiation.
What makes minerals glow? Do they glow the same color? What causes different colors of glow? We are glad to reveal it to you!
The fluorescence phenomenon is caused by specific elements (usually tungsten, lead, boron, titanium, uranium, chromium, and rare earth elements, which are present within the mineral. UV light provokes energy fluctuations within atoms. Differences in energy result in fluorescence we can observe in minerals.
It’s necessary to get the principles of the fluorescence phenomenon.
The fluorescence occurs when a chemical (atoms in mineral) absorbs light at one wavelength (UV light wavelength ranges from 10 nm to 400 nm) and re-emits light at a different longer wavelength (visible light), having wavelengths in the range of 400–700 nanometres (nm).
As soon as an electron of the activator element receives energy from UV light, its orbit changes. Simply put, it jumps to a higher energy level. But it’s not its typical position, so it falls back.
The difference in energy between these two energy levels should be somehow realized. In the case of fluorescence, this excessive energy is reemitted in the form of visible light we can observe.
Fluorescent materials almost immediately cease to glow when the source of radiation (UV lamp) ceases, unlike phosphorescent materials that tend to emit light for some time.
Minerals that are sometimes phosphorescent (glow even after the UV lamp is turned off) include calcite, celestite, colemanite, fluorite, sphalerite, and willemite.
Over 500 minerals have been discovered that exhibit some sort of fluorescence when exposed to ultraviolet light.
Fluorescence usually occurs when specific impurities, known as “activators,” are present within the mineral. These activators are typically cations of metals such as tungsten, molybdenum, lead, boron, titanium, manganese, uranium, and chromium.
Rare earth elements such as europium, terbium, dysprosium, and yttrium are also known to contribute to the fluorescence phenomenon. Fluorescence can also be caused by crystal structural defects or organic impurities.
Different chemical elements (activators) produce different glow colors:
- Trivalent, low-concentration chromium (Cr3+) is the source of red fluorescence in ruby and spinel.
- Divalent europium (Eu2+), when seen in the mineral fluorite, is the source of blue fluorescence.
- Trivalent lanthanides (terbium (Tb3+) and dysprosium (Dy3+)) activate the creamy yellow fluorescence shown by the mineral fluorite and contribute to the zircon’s orange fluorescence.
- The tungstate (WO42-) molecular ion is the source of white and yellowish-white to yellow fluorescence for scheelite (calcium tungstate).
- Uranyl ion (UO22+) produces an acidic green fluorescence color for chalcedony, calcite, and sometimes gypsum.
- This red fluorescence is emitted by divalent manganese (Mn2+), the activator for which (Pb2+) commonly acts as a coactivator in calcite.
- Apatite fluorescence can be due either to divalent manganese (Mn2+) (yellow fluorescence) or to various combinations of REE, including europium (Eu2+), cerium (Ce3+), samarium (Sm3+), and dysprosium (Dy3+), producing blue, white, or pink fluorescence.
- Yellow to orange to red fluorescence of sodalite and related feldspathoids occurs due to the sulfide (S2–) molecular ion.
TIP: Did you know that glowing minerals can be radioactive, too? It is always good to know what minerals are radioactive to protect you and your relatives when rockhounding:
What are Radioactive Minerals? 6 Common Radioactive Rocks
What Rocks and Minerals Glow in UV Light?
The minerals, which glow under UV light, have a very specific element (activator) within their atomic structure. This activator produces the glow we are admiring. The list of activators and, as a result, the list of minerals that glow is quite limited.
The most common minerals that glow under UV light are calcite, fluorite, selenite, scheelite, chalcedony, and corundum. Rocks, which contain these minerals, will also glow. Limestone, marble, and travertine can glow because of calcite presence. Granite, syenite, and granitic pegmatite rocks can also glow.
Minerals are combined into rocks. The rock, which has fluorescent minerals in its composition, will glow, too.
For example, such sedimentary rocks as limestone, marble, and travertine are mostly composed of calcite, so sometimes, the whole rock can glow under UV light.
Potassium feldspar can also glow sometimes. If this mineral becomes a part of rock, we can include granite, syenite, or granitic pegmatite in a list of occasionally fluorescent rocks, too.
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Why Do Some Diamonds Glow in UV Light?
Even such a popular gemstone as a diamond can sometimes display fluorescence. Water-clean, transparent, absolutely colorless, and perfectly cut diamonds can emit various strength blue glow under UV light.
Diamonds can also glow under UV light. Usually, diamonds emit a bluish light or, more rarely, a yellow or orange light. This phenomenon is usually caused by nitrogen, boron, or aluminum impurities present in the diamond’s automatic structure. The intensity of the glow is taken into account while grading a diamond.
Fluorescence is a very important feature of diamonds. It can significantly decrease the price of the stone. Even if two similar stones (color, clarity, cut, and carat weight are equal) are in comparison, the one that has fluorescence will lose in price (Source).
Some gemologists believe that fluorescence in diamonds deteriorates their clarity and negatively influences color (Source).
What Rocks and Minerals Glow Under Black Light?
First of all, black light is just synonymous with long-wave (low energy) UV light. There is no “black” light in nature.
Only 10-15% of all known mineral species can glow under UV light. It’s a small percentage, but fortunately, some of those minerals are widely distributed and can already be in your collection. Hurry up and check your fluorites and calcites under black light (long-wave UV light).
Here is the list of the most common minerals and colors that can be observed.
| Mineral | Color under the UV light |
|---|---|
| Fluorite | Mostly blue, blue-violet, greenish-white, rarely red. |
| Calcite | Any color. Mostly orange, yellow, white, and green. Occasionally blue, red, and pink. |
| Selenite | Blue, occasionally lime-green. |
| Apatite | Yellow, violet, blue, white, or pink. |
| Aragonite | Yellow, blue, greenish, or white. |
| Chalcedony | Green, yellow, white. |
| Corundum (ruby and sapphire) | Red. |
| Opal | Green. |
| Scheelite | Blue, bluish-white, white, and yellowish-white to yellow. |
| Smithsonite | Red. |
| Sodalite | Orange. |
| Willemite | Green. |
| Spodumene (kunzite) | Blue, white. |
| Potassium feldspar | Pink. |
| Zircon | Yellow. |
| Spinel | Red. |
TIP: Fluorite is one of the most common rocks that glow under UV light. You can also buy it online quickly and easily. That’s why you should be able to identify real and fake fluorites. Find out more in the article below:
Real vs. Fake Fluorite: Focus on These 5 Differences
Why Do Rocks Glow Under Black Lights?
Blacklight is synonymous with long-wave (low energy) UV light. Sometimes, the black light term can cause colossal misunderstanding, and such legends as mineral glow in darkness can arise.
Rocks glow under black light (long-wave UV light) because of the minerals they are composed of. Rocks with high content of calcite (limestone, marble, and travertine) will glow under UV light. Some granites contain potassium feldspar, which sometimes can also glow under UV light.
Black light is the same light source as long-wave UV light, so the same minerals and rocks will glow under it.
The small difference is that UV light is usually divided into two groups, such as long-wave UV light, which is synonymous with Blacklight, and short-wave UV light with higher energies.
FAQ About Rocks Glowing Under UV Light
Still, have you not found the answer to your questions about glowing rocks? Find frequently asked questions in the section below:
Does Fluorite Glow Under UV Light?
Fluorite glows under UV light. The fluorescence phenomenon is named after fluorite, which was first observed by George Gabriel Stokes in 1852. Fluorite can glow in a variety of colors, but most often, it occurs in vibrant blue or green colors under UV light.
Does Selenite Glow?
Selenite, or gypsum, glows under UV light, displaying bright blue or sometimes green colors. Uranyl ion (UO2)2+ is believed to be a fluorescence activator in the selenite structure. Sometimes, selenite can also be phosphorescent and glow for a few seconds or minutes, even after UV light is turned off.
Does Ruby Glow in the Dark?
No, ruby doesn’t glow in the dark, but it can glow vibrant red under UV light, which is sometimes called black light. But it’s a huge misconception to say that rubies glow in the dark. Because of chromium impurities, corundum (ruby and sapphire) can glow bright red under UV light.
Do Sapphires Glow Under UV Light?
A very small amount of natural sapphires (sapphires can be any color except red) can glow under UV light. Natural colorless, pink, or occasionally blue sapphires with low iron content (iron is a fluorescent quencher) can glow red, pink, or orange in long-wavelength UV light.
Do Agates Glow Under UV Light
Some agates, together with closely related chalcedony, can glow in extremely bright and lime-green to yellow color. The fluorescence activator for agate is believed to be uranyl ion (UO2)2+ as an impurity. Appealing examples of fluorescence can also be observed in agate and chalcedony geodes.
TIP: Do you know how to cut and polish agates you have found? It is not so tricky; you only need to follow a few simple steps. Find out more in the article below:
How to Cut and Polish Agates? Follow These Simple Steps
Conclusion
The fluorescence phenomenon displays how perfectly balanced chemistry and physics are in minerals. Only about 15% of minerals glow under UV light.
However, fluorescent mineral species are quite common and can be easily obtained. The most common minerals which display fluorescence effect are:
- fluorite,
- calcite,
- aragonite,
- opal,
- apatite,
- chalcedony,
- corundum (ruby and sapphire),
- scheelite,
- selenite,
- smithsonite,
- sphalerite,
- sodalite.
Gathered together with other rock-forming minerals, the minerals above form different types of rocks. Rocks that can glow belong to all three groups of rocks: sedimentary, igneous and metamorphic.
The rocks that can glow are:
- limestone,
- marble,
- travertine,
- granite,
- syenite,
- granitic pegmatite.
With the help of a UV lamp or black light, you can observe fluorescence phenomena even at home. The lamps can be easily found on the Amazon or purchased during mineral shows. It’s even easier to find fluorescent minerals than UV lamps.
Double-check your collection with the list of minerals and rocks above. Maybe you already have some nice samples. Just take calcite or fluorite crystal to reveal a true miracle of nature.
TIP: So you already know the most common rocks and minerals that glow under UV light. Finding glowing rocks is great fun. Check out the ultimate guide about rockhounding with UV light in the article below:
GUIDE: Rockhounding with UV Light & 3 Best UV Lights (2021)