When exposed to ultraviolet (UV) light, some diamonds exhibit a phenomenon known as fluorescence, where they emit a visible glow. Not all diamonds display this characteristic; some remain unaffected by UV light, while others illuminate with varying colors and intensities.
The Phenomenon of Diamond Fluorescence
Diamond fluorescence occurs when specific trace elements within the diamond’s crystal structure absorb invisible UV radiation and then re-emit it as visible light. The most common cause is the presence of nitrogen atoms, which typically lead to a blue glow. Boron is another trace element that can cause fluorescence, often resulting in yellow or green hues. These elements were incorporated into the diamond during its formation deep within the Earth.
This optical effect is not universally present. Approximately 25% to 35% of natural diamonds exhibit some degree of fluorescence when exposed to long-wave UV light, such as that found in sunlight or special lamps. The intensity of this glow can vary significantly from one diamond to another.
Colors and Intensity of Fluorescence
While diamonds can fluoresce in various colors, blue is by far the most prevalent, accounting for over 95% of all fluorescent diamonds. Other less common colors include yellow, green, white, orange, and red. The specific color depends on the trace elements present and their configuration within the atomic lattice.
Gemological laboratories grade the intensity of diamond fluorescence on a standardized scale. This scale typically includes classifications such as None, Faint, Medium, Strong, and Very Strong. A “None” rating means no observable fluorescence, while “Faint” means a very subtle glow that is usually not perceptible without a UV light source. Diamonds graded as “Medium,” “Strong,” or “Very Strong” show a progressively more noticeable glow under UV light.
Impact on Diamond Appearance and Value
For most diamonds, especially those with faint to medium fluorescence, this characteristic has no discernible impact on their visual appearance under normal lighting conditions. The glow is typically only visible when the diamond is exposed to a strong UV source, such as a blacklight or direct sunlight. Therefore, most consumers would not notice the fluorescence in everyday wear.
In specific instances, strong or very strong blue fluorescence can influence a diamond’s appearance. In a small percentage of cases, particularly in diamonds with higher color grades (D-F), strong blue fluorescence might cause the diamond to appear “milky,” “oily,” or “hazy.” Such occurrences are rare, affecting fewer than 0.2% of fluorescent diamonds submitted to the Gemological Institute of America (GIA). Conversely, blue fluorescence can sometimes be beneficial for diamonds with a slight yellowish tint, typically in the G-J color grades. The blue light emitted by the fluorescence can counteract the yellow, making the diamond appear whiter or more colorless.
The presence and intensity of fluorescence can influence a diamond’s market value. Diamonds with strong to very strong fluorescence, especially in higher color grades, may be discounted compared to non-fluorescent diamonds of similar quality. This is often due to historical perceptions or the rare possibility of a hazy appearance. However, for diamonds with faint or medium fluorescence, there is generally little to no impact on value. For diamonds with lower color grades, medium to strong blue fluorescence can sometimes command a slight premium due to its ability to visually enhance the stone’s color.
Fluorescence and Diamond Identification
Beyond aesthetic implications, diamond fluorescence serves as a practical tool in gemology. Jewelers and gemologists routinely use UV light to observe fluorescence as an identifying characteristic. This can help distinguish natural diamonds from certain diamond simulants, such as cubic zirconia, which may exhibit different fluorescent reactions or none.
Fluorescence can also provide clues in differentiating between natural and some types of synthetic or treated diamonds. While fluorescence alone is not a definitive test for authenticity, it is one of many properties examined by gemological laboratories. Diamond grading reports, such as those from the GIA, include information on a diamond’s fluorescence intensity.