When we use UV lights, we are bombarding minerals with electromagnetic radiation (photons) within the Ultraviolet range (see scale below). There are three different types of UV light used in the mineral world: UVA, UVB, and UVC. UVA is generally considered less harmful to humans and can penetrate some materials like glass. You may notice your teeth, fingernails, eyes, and sometimes socks glowing as they react to UVA. Fortunately, most glass and certain materials can block UVB and UVC, which are more harmful to humans.

As frequencies increase beyond UVC, the wavelengths of electromagnetic radiation continue to shorten. This is when you’d hear Star Trek’s Scotty saying “We’rrre apprrroaching crrritical levels of rrradiation, sirrr!”

When electromagnetic radiation in the UV range is absorbed by minerals, the minerals become excited due to the excess energy and emit light in longer wavelengths. This phenomenon is known as "fluorescence." The minerals' molecules balance the extra energy input from the UV lamps (following the law of conservation of energy) by emitting their own photons within the visible spectrum. In a dark room, the only light source you'll see is coming from the rocks themselves, glowing with fluorescence.

Pure UV light is not visible to humans, but UV bulbs are not perfect. While emitting UV light, they often emit some visible light as well. Low-quality UV lamp fixtures can "pollute" a display with visible violet light. Higher-quality lamp fixtures have special filters that refine the light output to a desired spectrum, such as 254 nm (nanometers) for UVC.

UVA

While causing minimal genetic damage to tissues, UVA radiation can also lead to sunburn through its shorter wavelengths. Roughly 95% of the sun’s UVA rays reach Earth, making exposure to a UVA lamp almost equivalent to being outdoors.

UVB

While only 5% of the sun’s UVB rays breach the ozone layer and reach the Earth, UVB is the primary culprit for sunburn and skin cancers. Sunburn is a sign of short-term overexposure, while premature aging and skin cancer are side effects of prolonged UV exposure. Avoid looking directly at a UVB light source to avoid sunburning your eyes.

UVC

UVC radiation can cause severe burns of the skin and eye injuries (photokeratitis). Avoid direct skin exposure to UVC radiation and never look directly into a UVC light source, even briefly. Skin burns and eye injuries from UVC exposure usually resolve within a week with no known long-term damage. Due to UVC’s shallow penetration, the chances of skin cancer, cataracts, or permanent vision loss are considered low. UVC-related eye injuries (welder’s flash) result in intense pain and a gritty sensation, potentially affecting vision for 1-2 days, even after brief exposure.

Fluorescence and phosphorescence are captivating behaviors exhibited by certain minerals when exposed to light. In fluorescence, these minerals swiftly absorb ultraviolet or visible light and promptly emit it, often in a different colour. It’s a vivid response to a temporary burst of energy. This effect, seen in minerals like fluorite and calcite, creates a striking glow under specific lighting conditions. Fluorescence typically ceases nanoseconds to microseconds once the light source is removed.

Phosphorescence, in contrast, extends the drama. When certain minerals absorb light, they hold onto that energy, releasing it more gradually over time. This lingering luminosity, observed in minerals such as willemite and sphalerite, is akin to a gentle afterglow. Phosphorescent materials store energy and release it at a leisurely pace. Phosphorescence persists seconds, minutes or even hours after initial excitation.

When you hold a phosphorescent stone such as the Red River selenites from Manitoba, charge them and then remove the light, it will feel like you are holding a ball of energy. The delayed emissions appear paradoxical and the stuff of science fiction—much like holding an illuminated lightbulb that is not plugged in.

Glow-in-the-dark materials are often phosphorescent. The materials absorb light and then release it slowly over time, making them visible in the dark.

The color we perceive as black is fundamentally the absence of any light frequencies of the visible spectrum. Although black lights emit light, ultraviolet light is not visible to human eyes, so the light is "black" as far as your eyes are concerned. A light that only gives off ultraviolet light would leave a room in apparent total darkness.

There are just over 5000 known mineral varieties. Fluorescence or phosphorescence can be detected with scientific equipment in about 700 of those. The number of “interesting" reactive minerals is far fewer. A fluorescent mineral display’s beauty comes from the magnificent show of colours, and there are many that can glow vividly. There are also many that do not. Their glow reaction to the UV light will be dull in either emission of photons or colour saturation.

The most common minerals on the global market that fluoresce are aragonites, calcites, chalcedonies, fluorites, hackmanites, hyalite opals, rubies, and sodalites. Next are the region- specific minerals such as ones from the township of Franklin, New Jersey (USA) and Greenland. There are still other obscure mineral varieties that aren’t commonly seen nor readily available at gem shows and rock shops.

These elements can interact with the crystal lattice of minerals or compounds, leading to energy transitions that result in fluorescence. Keep in mind that the specific fluorescent properties can vary widely depending on chemical composition, crystal structure, and impurities present in the material.

Various trace elements from the periodic table can cause phosphorescence or fluorescence. The presence of certain elements as impurities in materials can lead to phosphorescent behavior.

The ability of materials to fluoresce has a broad range of applications across various fields. Here’s a list of some scientific, industrial, and artistic purposes for harnessing fluorescence.