Diamond fluorescence is a natural characteristic that causes some stones to emit a visible glow when exposed to ultraviolet (UV) radiation. This phenomenon is most easily observed under a blacklight, which produces long-wave UV light. The reaction is not universal, as only about 25% to 35% of all diamonds exhibit fluorescence. When a diamond does fluoresce, the color most commonly observed is blue.
The Science Behind Diamond Fluorescence
The glow observed when a diamond is exposed to a blacklight occurs when energy is absorbed and then re-emitted as visible light. Trace elements were incorporated deep within the diamond’s crystal structure during its formation. These elements are not part of the pure carbon lattice and act as “activators” for the fluorescence.
Nitrogen atoms are the most common of these trace impurities, and their presence is directly linked to the blue light emission. The blue glow is often attributed to the N3 center, a defect consisting of three nitrogen atoms clustered around a vacant lattice site. When UV light strikes these nitrogen centers, the electrons within the centers absorb the energy, moving to a higher energy state.
This excited state is unstable, and the electrons immediately return to their normal, lower energy state. The excess energy is released in the form of a photon, a particle of visible light. Because the re-emitted light has less energy than the absorbed UV light, it falls into the visible blue spectrum, creating the characteristic glow. This reaction stops immediately once the UV source is removed.
Common Colors and Intensity Levels
While a blue glow is by far the most frequent reaction, occurring in over 95% of fluorescent diamonds, other colors can appear due to different trace elements and structural defects. Less common hues include yellow, green, white, and in very rare cases, orange or red. Yellow fluorescence, for instance, can be caused by specific nitrogen aggregates known as platelets.
The intensity of the glow is important for characterizing the phenomenon. Gemological laboratories use a standardized five-point scale to describe the strength of a diamond’s fluorescence under long-wave UV light. This scale includes the designations of:
- None
- Faint
- Medium
- Strong
- Very Strong
Only about 10% of fluorescent diamonds exhibit a strength of Medium, Strong, or Very Strong. A “Faint” rating means the glow is barely perceptible. Conversely, a “Very Strong” rating indicates a highly saturated and prominent glow, which may be visible even in indirect sunlight. The specific color of the reaction is only noted on a grading report if the intensity is Medium, Strong, or Very Strong.
The Role of Fluorescence in Diamond Evaluation and Pricing
Fluorescence is noted on a diamond grading report as an identifying characteristic, not a quality grade like cut or clarity. Laboratories like the Gemological Institute of America (GIA) include the intensity level to fully describe the stone’s properties. The presence or absence of fluorescence can influence a diamond’s perceived quality and market value.
In diamonds with a high color grade, such as D, E, or F, a strong or very strong blue fluorescence can sometimes be viewed negatively. In a small fraction of cases (fewer than 0.2%), the intense fluorescence may cause the diamond to appear slightly hazy, milky, or oily in daylight. This adverse visual effect often leads to a price discount for otherwise colorless stones.
The opposite effect is observed in diamonds with a slight yellowish tint, typically those graded I, J, or K. Since blue is the complementary color to yellow, the blue fluorescence can subtly counteract the faint yellow body color in natural light. This effect can make the diamond appear whiter or more colorless to the naked eye. Consequently, diamonds in these lower color grades with medium to strong blue fluorescence may trade without a discount or command a slightly higher price.