Many diamonds exhibit a temporary glow when exposed to ultraviolet (UV) light, a phenomenon known as fluorescence. This occurs when the diamond absorbs energy from the UV radiation and immediately emits it as visible light. The visible light produced is typically a soft blue color, which is the most widely recognized form of diamond fluorescence. The glow stops almost immediately once the source of the UV light is removed.
The Atomic Cause of Fluorescence
The ability of a diamond to fluoresce is rooted in its atomic structure and the presence of trace impurities trapped within the carbon lattice. A diamond is composed almost entirely of carbon atoms, but during its formation, small amounts of other elements were sometimes incorporated. The most common impurity responsible for fluorescence is nitrogen, although boron can also play a role. These trace elements create “defects” or energy centers within the diamond’s structure.
Ultraviolet light consists of photons that are absorbed by the electrons in these defect centers, exciting them to a higher, unstable energy level. The electrons quickly drop back down to their stable ground state. In the process of returning to the lower energy level, they release the absorbed energy as a photon of visible light, which is the glow we perceive as fluorescence.
The most common blue fluorescence is caused by a cluster of three nitrogen atoms known as an N3 center, a common defect in Type Ia diamonds. The energy released from these centers falls within the blue spectrum. This instantaneous emission is distinct from phosphorescence, a much rarer phenomenon where the diamond continues to glow briefly after the UV source is removed.
Measuring Intensity and Color Variation
While blue is the predominant color, the specific trace elements and their configurations determine the exact color of the emitted light. Approximately 95% of fluorescent diamonds glow blue, but a rare few may exhibit colors such as yellow, green, or white. Yellow fluorescence, for instance, can be caused by aggregates of nitrogen atoms called platelets. The color variation is a direct result of the specific energy level changes dictated by the different types of atomic defects.
The intensity of this phenomenon is standardized by gemological laboratories using a five-level scale based on the strength of the glow observed under long-wave UV light:
- None
- Faint
- Medium
- Strong
- Very Strong
About 25% to 35% of all natural diamonds exhibit some degree of fluorescence. However, only about 10% of those that fluoresce display a Medium or higher intensity level. The color of the fluorescence is noted on grading reports if the intensity is Medium, Strong, or Very Strong.
How Fluorescence Impacts Diamond Value and Appearance
For the majority of diamonds, fluorescence at a Faint or Medium intensity level has no noticeable effect on the stone’s appearance in typical lighting conditions. The UV component in most lighting, including standard indoor and outdoor daylight, is usually too low to trigger a visible reaction. Fluorescence is not considered a grading factor like the 4Cs, but it is an identifying characteristic noted on professional grading reports.
The primary discussion regarding appearance centers on diamonds with strong blue fluorescence. For diamonds that naturally have a faint yellowish tint, such as those in the I, J, or K color grades, the blue glow can optically counteract the yellow, making the stone appear whiter or more colorless. This effect occurs because blue is the complementary color to yellow, subtly enhancing the perceived color of the diamond.
In rare instances, particularly with Strong or Very Strong fluorescence, the glow can lead to an undesirable hazy, milky, or oily appearance. Gemological studies suggest this haziness is not directly caused by the fluorescence itself, but rather that the strong fluorescence can intensify the effect of pre-existing light-scattering structural defects. This rare hazy effect, combined with a historical trade bias, typically causes high color-grade diamonds (D to H) with Strong or Very Strong fluorescence to be discounted in the market. Conversely, the perception of improved color in lower-grade diamonds means those with a Medium or Strong blue fluorescence may sometimes command a slight premium.