When a gemstone appears to glow with an inner light, it is exhibiting a phenomenon known as fluorescence. This captivating effect occurs when a gem absorbs light energy from a specific, non-visible source, such as ultraviolet (UV) radiation, and immediately re-emits that energy as visible light. The resulting illumination, often a vivid blue, red, or green, reveals a hidden property of the mineral that is not apparent under normal lighting conditions. The ability to fluoresce is a unique characteristic of only a small percentage of all natural gemstones.
The Science of Gem Fluorescence
Fluorescence is initiated at the atomic level when a gem’s structure interacts with high-energy UV photons. These photons are absorbed by electrons within the atoms of the mineral, causing the electrons to jump momentarily to a higher, more energetic orbit or “excited state.” Because this excited state is unstable, the electrons quickly fall back down to their original, stable energy level, known as the ground state. The energy released during this drop is lower than the energy originally absorbed, and this lower energy corresponds to a longer wavelength of light, which falls within the visible spectrum.
The specific color of the visible glow depends on which trace elements are present within the gem’s crystal lattice. These impurities include elements such as manganese, chromium, and various rare earth elements. For instance, the presence of chromium is known to cause a strong red fluorescence in rubies. Conversely, the presence of iron, even in small amounts, can suppress or “quench” the fluorescence entirely, preventing the visible light emission.
Practical Application: Types of UV Light
Gemologists rely on specific UV light sources to trigger and observe fluorescence, as a stone’s reaction is entirely dependent on the wavelength used. The two most commonly employed sources are Longwave (LW) UV and Shortwave (SW) UV. LW UV light, often referred to as “black light,” operates around 365 nanometers (nm) and is generally considered safer for prolonged use. SW UV light, with a wavelength near 254 nm, is much higher in energy and requires special quartz tubes to be generated, as regular glass blocks it.
A third, less common light source, Midwave (MW) UV, has a wavelength around 312 nm. Because SW UV radiation is energetic enough to cause damage to skin and eyes, proper protective measures, such as wearing UV-blocking eyewear, are necessary when conducting fluorescence testing.
Key Fluorescent Gemstones
Many gemstones exhibit a distinctive fluorescent signature that aids in their identification.
- Fluorite: The mineral from which the phenomenon takes its name, Fluorite often displays a blue-violet glow under LW UV light, with the color varying widely based on the specific rare earth elements present as activators.
- Ruby: Ruby frequently shows an intense red fluorescence under LW UV, due to its chromium content, a reaction so strong it can sometimes make the stone appear to glow even in daylight.
- Diamond: Approximately one-third of all natural Diamonds exhibit fluorescence, most commonly a blue glow, which is generally stronger under LW UV than under SW UV. This blue color is often attributed to defects involving nitrogen atoms within the crystal structure.
- Calcite: This common mineral is well-known for its vibrant red or orange fluorescence under SW UV, a reaction caused by the inclusion of manganese.
- Zincite: Certain specimens, especially those from the Franklin, New Jersey deposits, exhibit a moderate yellow glow under LW UV.
- Scapolite: This gem often glows a bright yellow or orange under either SW or LW UV, helping to distinguish it from other similar-looking yellow stones.
Beyond Fluorescence: Phosphorescence and Identification
While fluorescence is the immediate emission of light that ceases the moment the UV source is removed, some gemstones also exhibit the related phenomenon of phosphorescence. Phosphorescence is a delayed glow that persists for a period of time—ranging from a few seconds to several minutes—after the stimulating UV light has been turned off. This lingering afterglow occurs because the excited electrons become temporarily trapped in an intermediate energy state before slowly releasing their remaining energy as visible light.
Both fluorescence and phosphorescence are invaluable tools in the field of gemology. The specific color and intensity of the glow, as well as the presence of phosphorescence, can help gemologists distinguish between natural and synthetic stones. For example, some synthetic rubies and lab-grown diamonds often exhibit a stronger or different color fluorescence compared to their natural counterparts. Analyzing these luminescent properties can also help detect treatments like heating or fracture-filling.