Many natural diamonds exhibit a temporary glow under ultraviolet (UV) light, a phenomenon known as fluorescence. This light emission usually appears as a soft blue hue, though other colors can occur. UV light is a form of electromagnetic radiation whose high-energy photons interact with a diamond’s atomic structure. This reaction causes the diamond to temporarily emit visible light, which is the “color change” people observe. Not all diamonds fluoresce, but a significant portion of the world’s diamonds possess this characteristic.
The Scientific Basis of Diamond Glow
The light emission observed in diamonds results from trace elements incorporated into the crystal lattice during the stone’s formation. While diamonds are composed of pure carbon, minute impurities create structural defects that act as activator centers. Nitrogen is the most common trace element responsible for fluorescence, often replacing carbon atoms within the diamond’s structure.
When exposed to high-energy UV photons, electrons associated with these impurity centers absorb the energy and jump to a higher energy state. To return to their stable, lower energy state, the electrons must release the absorbed energy. This energy is released as a lower-energy photon, which falls within the visible light spectrum, creating the diamond’s glow. Blue is the most frequently observed color because specific nitrogen-related defects typically release blue light.
Understanding the Types of Light Emission
The light emission from diamonds under UV exposure is categorized into two forms: fluorescence and phosphorescence. Fluorescence is the immediate glow that appears only while the diamond is actively exposed to the UV light source. The glow ceases the instant the ultraviolet radiation is removed.
Approximately 25% to 35% of all natural diamonds exhibit some degree of fluorescence. Blue is the dominant color, accounting for over 95% of all fluorescent diamonds, but other hues like yellow, green, orange, or red can occur.
Phosphorescence is a much rarer phenomenon, defined by a lingering glow that persists after the UV light source has been removed. This afterglow can last from a few seconds to several minutes. Phosphorescence is often caused by the presence of boron atoms and is more commonly observed in certain types of diamonds, including some blue diamonds or lab-grown diamonds.
How Fluorescence Affects Diamond Grading and Value
Fluorescence is not considered one of the traditional “4Cs” (Cut, Color, Clarity, Carat Weight), but major laboratories like the Gemological Institute of America (GIA) measure and report its intensity. The GIA uses a five-step scale to grade the intensity of this reaction under long-wave UV light: None, Faint, Medium, Strong, and Very Strong. The color of the fluorescence is also noted if the intensity is Medium or higher.
The impact of fluorescence on a diamond’s market value is complex and highly dependent on its intensity and color grade. In the highest color grades (D through H), diamonds with Strong or Very Strong fluorescence may trade at a discount. This is due to a rare visual effect where intense fluorescence causes the diamond to appear hazy or oily in normal daylight.
Conversely, fluorescence can sometimes enhance the appearance of diamonds in the lower color grades (I through M). Since the glow is most often blue, this color acts to visually counteract the faint yellow tint naturally present in these stones. This effect can make the diamond appear whiter or more colorless to the naked eye, sometimes resulting in a slightly higher valuation.