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 commonly 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 fabrics glowing as they react to UVA. Fortunately, most glass and certain materials can block UVB and UVC, which are more harmful to humans.

In this context, the words light, photons, and electromagnetic radiation all describe the same physical phenomenon. “Light” is the everyday term, “electromagnetic radiation” is the scientific umbrella term, and “photons” are the individual packets of that energy. Whether we say ultraviolet light or ultraviolet radiation, we are talking about the same thing—energy traveling through space at specific wavelengths.

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—much like how a hot object releases heat to cool back down to equilibrium. In a dark room, the only light source you’ll see is coming from the rocks themselves, glowing with fluorescence.

Pure ultraviolet light is not visible to humans, but UV bulbs are not perfect. While emitting UV radiation, 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 use special filters to refine the light output to a desired spectrum, such as 254 nm (nanometers) for UVC.

UVA

UVA causes relatively low direct DNA damage compared with UVB/UVC, but it still contributes to skin aging, pigment changes, and can cause sunburn at shorter UVA wavelengths (closer to 315 nm). About 95% of the UV from sunlight that reaches Earth is UVA, so strong UVA lamps can deliver an exposure profile that feels “like being outside,” especially at close range or over long sessions.

UVB

Only a small fraction of UVB makes it through the ozone layer, yet UVB is the main driver of sunburn and a major contributor to skin cancer risk over time. UVB also causes eye injury with direct exposure (think painful irritation and inflammation rather than just “bright light discomfort”).

UVC

UVC is strongly absorbed by outer tissues, making it effective for sterilization but causing rapid skin burns and eye injury (like "welder's flash"). Symptoms—including intense eye pain, tearing, light sensitivity, and a gritty feeling—start hours after exposure and are typically temporary, resolving within 1–2 days for eyes and about a week for skin/eye surface irritation. While UVC's shallow penetration makes long-term issues like skin cancer or cataracts less likely than with UVB, the short-term injury can be severe.

The Visible Light Spectrum

The visible light spectrum is the section of the electromagnetic radiation spectrum that is visible to the human eye.

The different wavelengths within the visible spectrum are responsible for the different colours we see. An imaginary helper named ROY G. BIV is often used to remember the order of visible colours: red, orange, yellow, green, blue, indigo, and violet.

According to the spectrum, if light with a wavelength of 550 nm shines toward our eyes, it appears green. Coloured objects look the way they do because of reflected light. When sunlight shines on a green leaf, wavelengths such as violet, red, and orange are absorbed, while the remaining wavelengths are reflected. Those reflected wavelengths are what our eyes perceive as green.

Black and white objects represent the extremes of colour behaviour. Black objects absorb nearly all incoming light, reflecting none back to our eyes, so we perceive black—the absence of visible colour. White objects reflect nearly all wavelengths of visible light, which is why we perceive them as white.

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 within nanoseconds to microseconds once the light source is removed.

Phosphorescence, in contrast, extends the drama. When certain minerals absorb light, they retain 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 can persist for seconds, minutes, or even hours after the initial excitation.

When you hold a phosphorescent stone such as the Red River selenites from Manitoba, charge it, and then remove the light, it can feel like you are holding a ball of energy. The delayed emission appears 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. These materials absorb light energy and then release it slowly over time, making them visible in the dark.

The colour we perceive as black is fundamentally the absence of visible light. Although black lights do emit light, ultraviolet radiation lies outside the visible spectrum and cannot be seen by human eyes, so the light is effectively “black” to us. A lamp that emits only ultraviolet light would leave a room in apparent darkness, even though radiation is present.

There are just over 5,000 known mineral varieties. Fluorescence or phosphorescence can be detected with scientific equipment in roughly 700 of them. The number of minerals that produce a visually striking or commercially interesting reaction, however, is far smaller.

The beauty of a fluorescent mineral display comes from its range and intensity of colour. Many minerals glow vividly under ultraviolet light, while many others do not. Some reactions are dull, either due to low photon emission or muted colour saturation.

The most common fluorescent minerals found on the global market include aragonites, calcites, chalcedonies, fluorites, hackmanites, hyalite opals, rubies, and sodalites. Beyond these are region-specific minerals, such as those from Franklin, New Jersey (USA) and Greenland, which are renowned for unique and intense fluorescent assemblages. There are also numerous obscure or rarely encountered mineral varieties that are seldom seen and not readily available at gem shows or rock shops.

Certain trace elements can interact with a mineral’s crystal lattice, creating energy transitions that result in fluorescence or phosphorescence. The specific response can vary widely depending on chemical composition, crystal structure, and the type and concentration of impurities present.

A wide range of elements from the periodic table can act as these activators. When present as trace impurities, they can alter how a material absorbs and releases energy, producing fluorescent or phosphorescent behavior that would not occur in a chemically pure crystal.

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.