Diamond fluorescence describes the visible light some diamonds emit when exposed to ultraviolet (UV) rays. The diamond typically glows blue, though other colors can occur. This glow lasts only as long as the diamond is under the UV light source.
What Causes Diamonds to Glow Under UV Light
The glow in diamonds under UV light originates from the presence of specific trace elements and structural defects within their crystal lattice. Diamonds are primarily composed of carbon, but during their formation, other elements can be incorporated. The most common element responsible for blue fluorescence, observed in over 95% of fluorescing diamonds, is nitrogen. Nitrogen atoms absorb UV light energy and then re-emit it as visible blue light.
Boron is another trace element that can cause fluorescence, sometimes resulting in a blue or even red glow, although its presence is much less common than nitrogen. Other rare colors like yellow, green, or orange can also occur due to different impurities or structural arrangements within the diamond. Not all diamonds exhibit fluorescence; only about 25% to 35% of natural diamonds show some degree of this effect. This is because the necessary trace elements might not be present in sufficient quantities, or their atomic configurations may not be conducive to light emission.
How Fluorescence Affects Diamond Appearance
Diamond fluorescence is assessed by its intensity under long-wave UV light. Grading laboratories use a scale that typically includes categories such as None, Faint, Medium, Strong, and Very Strong. While fluorescence is a distinct characteristic, its impact on a diamond’s visual appearance under normal lighting conditions is often subtle.
For diamonds in the higher color grades, specifically those considered colorless (D-H), strong or very strong blue fluorescence can sometimes cause a hazy, milky, or oily appearance. This effect, though rare, can diminish the transparency and brilliance of the diamond. This haziness is not caused by fluorescence alone, but can be intensified in diamonds with existing light-scattering defects.
Conversely, blue fluorescence can have a positive effect on diamonds with a slight yellowish tint, typically those in lower color grades like I-M. The blue light emitted by the fluorescence can counteract or mask the yellow tones, making the diamond appear whiter or more colorless. This can visually improve the diamond’s perceived color by up to a full grade, especially under lighting conditions containing UV, like natural daylight. The overall effect on appearance largely depends on the intensity of the fluorescence and the diamond’s original color.
Assessing Diamond Fluorescence
Professional diamond grading laboratories, such as the Gemological Institute of America (GIA) and the American Gem Society (AGS), routinely assess and document fluorescence as an identifying characteristic on their grading reports. If the fluorescence is Medium, Strong, or Very Strong, the color of the glow, usually blue, will also be specified.
The assessment process typically involves observing the diamond under controlled long-wave UV light sources in a laboratory setting. Under these specific conditions, the glow becomes apparent, allowing gemologists to determine its intensity. For most diamonds, fluorescence is not noticeable under typical viewing conditions, such as indoor lighting, unless the intensity is very strong. Consumers considering a diamond should review its grading report for fluorescence information and, if possible, observe the diamond in various lighting environments to understand its appearance firsthand.