A black light emits ultraviolet (UV) radiation, which is invisible to the human eye. When UV light strikes certain gemstones, it can cause them to temporarily emit visible light, a phenomenon known as fluorescence. This “glow” helps gemologists and collectors identify and distinguish a stone’s characteristics. A related effect, called phosphorescence, occurs when the gem continues to glow for a short time after the UV source is removed.
The Mechanism of Gem Fluorescence
The ability of a gemstone to fluoresce depends on its atomic structure and the presence of specific trace impurities. When a high-energy UV photon enters the gem, its energy is absorbed by an electron within the atom. This absorbed energy temporarily boosts the electron to a higher, unstable energy level.
The electron immediately returns to its original, more stable energy state, releasing the excess energy. Because some energy is lost as heat, the emitted light has a longer wavelength than the absorbed UV light, placing it within the visible light spectrum. This lower-energy, visible light is the characteristic glow observed under the black light.
The specific color and intensity of the glow are determined by trace elements known as “activators.” Common activators include ions of manganese, chromium, and uranium, which integrate into the gem’s crystal lattice during its formation. Conversely, other elements, such as iron, can act as “quenchers” that suppress or eliminate any potential fluorescence.
Gemstones That Exhibit Fluorescence
The diamond is the most famous example of a fluorescent gem; approximately one-third of natural stones display blue fluorescence. This blue emission is typically caused by trace amounts of nitrogen atoms clustered within the carbon lattice. Less commonly, diamonds may fluoresce yellow, green, or red, depending on specific structural defects or other impurities.
Rubies, a red variety of the mineral corundum, exhibit strong red fluorescence due to chromium. The chromium ions responsible for the stone’s red body color also activate the fluorescence, enhancing the gem’s apparent redness. This intense glow can sometimes be used to distinguish natural rubies, which often have some iron to dampen the effect, from synthetic rubies that lack iron.
The mineral fluorite is so well known for this property that it gave fluorescence its name. Fluorite specimens display an astonishing range of colors, including blue, green, purple, yellow, and white, depending on the specific rare-earth elements or yttrium present as activators. A related fluorescent mineral is scheelite, a tungsten ore that consistently glows a bright blue or yellowish-blue color under UV light due to the tungsten content.
Opal, a hydrated form of silica, can show a distinct glow, particularly the variety known as hyalite opal. This type of opal often contains trace amounts of uranium and will fluoresce a vivid, striking green under a black light. Another highly fluorescent mineral is calcite, which can glow in a wide spectrum of colors, including red, pink, blue, or orange, with the specific hue depending on its impurities, such as manganese.
Sodalite, a deep blue mineral, frequently displays a vivid orange or red fluorescence, an effect attributed to sulfur present within its chemical structure. The rare gemstone benitoite often shows a brilliant blue-white fluorescence, a property that aids in its identification.
Using UV Light for Gem Identification
Observing fluorescence is a practical, non-destructive tool used by gemologists to gain insight into a stone’s origin and potential treatments. The process requires testing with two main types of UV light: longwave UV (LWUV), typically around 365 nanometers, and shortwave UV (SWUV), typically around 254 nanometers. Many gems respond differently to these two wavelengths, providing diagnostic clues.
For example, observing the difference between LWUV and SWUV response is useful in separating natural diamonds from certain laboratory-grown types. Natural diamonds that fluoresce typically glow brighter under LWUV, whereas some synthetic diamonds often exhibit a stronger glow under SWUV. A lingering glow after the UV light is turned off, known as phosphorescence, is also a marker, as it is rare in natural diamonds but common in HPHT-grown synthetics.
Fluorescence also helps detect various treatments applied to a gemstone. Fracture-filled emeralds may show a different color glow from the filler material, such as blue or yellow, contrasting with the inert stone. Similarly, the resins and waxes used to treat jadeite can fluoresce under UV light, revealing the presence of artificial enhancement. When using shortwave UV light for testing, it is important to take precautions, as this high-energy radiation can cause temporary eye damage without proper protective eyewear.