Rocks that glow under black light exhibit fluorescence, a property where certain minerals temporarily emit visible light after absorbing invisible ultraviolet (UV) radiation. This phenomenon occurs in only a small fraction, estimated to be about 15%, of all known mineral species. The color and intensity of the glow depend on the mineral’s internal structure and the specific wavelength of UV light used to excite it.
The Science Behind Mineral Fluorescence
Fluorescence begins when a mineral absorbs high-energy photons from ultraviolet light. This absorbed energy transfers to electrons within the mineral’s crystal lattice, causing them to jump to a higher, unstable energy level. This excited state is momentary, and the electrons immediately fall back down to their stable positions. The excess energy released during this drop is emitted as a new photon of light, which is at a longer wavelength and falls within the visible spectrum, allowing the human eye to perceive it as a glow.
The specific color of the emitted visible light is determined by trace impurity elements, known as “activators,” incorporated into the mineral structure. Elements like manganese, uranium, and certain rare earth elements act as these activators, absorbing the UV energy and dictating the glow color. For a mineral to be considered fluorescent, the visible glow must stop almost instantaneously—within nanoseconds—the moment the UV light source is removed. If the glow persists for a measurable period after the UV source is turned off, the mineral is exhibiting phosphorescence, a related process where the energy is temporarily trapped.
Essential Equipment and Observation Safety
To observe this phenomenon, a source of ultraviolet light, commonly called a black light, is necessary. UV light is categorized primarily by wavelength; the two most common types used for mineral observation are Longwave UV (LWUV) and Shortwave UV (SWUV). Longwave UV, typically around 365 nanometers (nm), is found in most household black lights and is relatively safe for casual use. Many minerals will only fluoresce under one specific wavelength, so having access to both types is important for collectors.
Shortwave UV, around 254 nm, is much higher in energy and is often required to produce the most brilliant colors. This higher energy poses a greater risk to human tissue. It is imperative to use UV-blocking safety glasses to protect the eyes from potential damage when operating a shortwave lamp. Prolonged exposure to Shortwave UV on exposed skin should also be avoided, as this wavelength is capable of causing a sunburn-like reaction.
Notable Examples of Glowing Minerals
Many famous glowing rocks contain the mineral calcite, which typically fluoresces in shades of red, orange-red, or pink due to the presence of manganese as an activator. Calcite often requires Shortwave UV to show its brightest colors; specimens from Franklin, New Jersey, are renowned for this vivid glow. Red calcite is frequently found alongside willemite, a zinc silicate mineral that fluoresces brilliant green under the same Shortwave UV light. This combination creates a striking contrast under the black light.
Willemite’s bright green emission is also caused by a manganese activator within its structure, and it frequently displays phosphorescence, causing the glow to linger after the lamp is switched off. Another well-known fluorescent mineral is fluorite, which gave the phenomenon its name. Fluorite most commonly glows blue or violet under Longwave UV, with the activation often traced to trace amounts of the rare earth element Europium.
Sodalite has recently gained widespread attention for its fluorescence, particularly the variety found in “Yooperlite” rocks. This mineral typically fluoresces bright orange or yellow under Longwave UV. The orange glow is generally attributed to the presence of a sulfur compound within the mineral’s structure, rather than a metallic element.