Some minerals shine under ultraviolet light due to the presence of certain elements in their crystalline structure, such as activating impurities or defects in the crystal lattice, which react to UV light by emitting visible light, a phenomenon called fluorescence.
When certain minerals are illuminated with ultraviolet (UV) light, some electrons inside receive a significant boost of energy. As a result, they get temporarily projected onto higher energy levels, somewhat like jumping a floor. But since they don't stay there comfortably for long, they eventually fall back down, and as they return, they release the excess energy in the form of visible light. This phenomenon is called fluorescence. What’s nice is that the emitted color directly depends on the difference between the energy levels crossed. Therefore, depending on the atomic structure of the mineral, you can observe a wide variety of shades, such as fluorescent green, red, purple, or blue. Most of the time, it is small impurities or trace elements trapped in the mineral's structure that make this light emission possible. It's a bit like hidden mini-switches that activate this beautiful display of colors under ultraviolet light.
What makes minerals shine under ultraviolet light are generally certain elements they contain, such as manganese, uranium, or impurities called activators. Just a tiny amount is enough, and the mineral illuminates. On the other hand, other elements like iron or copper act as party poopers by absorbing ultraviolet light without re-emitting visible light, thus preventing the phenomenon of fluorescence. Essentially, it all depends on the chemical cocktail: some ingredients make things shine, while others completely dim the show.
Some minerals are particularly popular among collectors due to their ability to glow under ultraviolet light. Fluorite, for example, clear and discreet to the naked eye, shines in blue or purple under UV. Autunite, a yellow-green uranium mineral, displays an impressive flashy green fluorescence. Another interesting example is calcite, which is sometimes beige or white, but suddenly reveals orange or pink hues under UV. The same goes for scheelite, which turns bright blue or bluish white, making it truly attractive. Willemite is also easily found, usually dull and brown, but bursts into a vivid bright green once under UV. Even more surprisingly, the famous diamond can emit a delicate blue or bluish light when it reacts under ultraviolet, hence its interest in identifying gemstones.
The crystalline structure of the mineral plays a significant role: anything that disrupts this structure can directly alter the fluorescence. Imperfections such as cracks, impurities, or internal defects change the way UV light is absorbed and then emitted, sometimes making the effect more intense or, conversely, weaker. The quantity and type of chemical impurities present, such as manganese or uranium, also strongly influence the intensity of the observed fluorescence. You have probably noticed that the intensity also varies depending on the specific wavelength of the ultraviolet light used: some rocks shine very brightly under short UV light and are almost dull under longer UV light. Finally, the surface condition matters: a well-polished mineral will often be brighter under UV than a rough or unrefined stone.
The fluorescence of minerals isn't just for impressing friends with a UV light. In mining exploration, it helps easily locate deposits of interesting minerals like scheelite (tungsten ore), which glows under UV light. In criminology, certain fluorescent minerals found in soils or dust can help identify the geographical origins of evidence at crime scenes. Not to mention jewelry: UV light is used to check the quality of gemstones like diamonds to detect any impurities or treatments. It's also very useful in art restoration to determine authenticity, origin, or previous restorations thanks to the fluorescent mineral particles present in pigments or ancient materials. Finally, geologists use fluorescence to better map the types of rocks in a region, quickly and without hassle.
Some scorpions also glow under ultraviolet light due to fluorescent compounds in their exoskeleton, making them easier for scientists to observe at night.
Ultraviolet fluorescence is used by geologists to quickly identify certain minerals in the field, thus simplifying mining research and environmental assessments.
Unlike phosphorescence, which continues to emit light after the light source has been removed, the fluorescence of minerals stops immediately when the ultraviolet light is turned off.
The colors that minerals display under ultraviolet light are closely related to the chemical impurities or structural defects present in their crystal lattice, rather than necessarily their natural visible color.
Generally, the intensity of fluorescence remains constant, but some minerals may gradually lose their fluorescence when exposed for extended periods or under certain environmental conditions (humidity, excessive temperature, etc.).
Although fluorescence can be a useful criterion for identification, it is not sufficient alone to reliably recognize a mineral. Use it in combination with other methods such as hardness tests, natural color, or chemical tests.
Observing minerals under UV light is generally safe by taking a few simple precautions: avoid direct prolonged exposure of your eyes or skin to the UV lamp, wear UV safety glasses, and limit the exposure time.
No, only certain fluorescent minerals react to ultraviolet light. This property depends on the mineral's chemical composition and the impurities present, known as fluorescent activators.
Fluorescence refers to an immediate light emission as long as the mineral is exposed to the UV source. Phosphorescence describes light that the mineral continues to emit for a short time after the UV source has been turned off, due to a delayed emission of the absorbed energy.
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