The glow of diamonds under a blacklight, or ultraviolet (UV) light, is a natural characteristic rooted in their atomic structure. Understanding this glow involves delving into the science of light and the unique composition of these precious stones. This article explains the scientific reasons behind this luminescence and clarifies what it signifies for a diamond.
Understanding Fluorescence
Fluorescence describes a process where a substance absorbs invisible ultraviolet (UV) light and instantly re-emits it as visible light. When UV light strikes certain materials, it excites electrons within their atoms. These excited electrons temporarily jump to a higher energy level.
As they quickly return to their original, lower energy state, they release the absorbed energy as visible light. This emission occurs almost immediately after UV light absorption, distinguishing fluorescence from phosphorescence, which continues to emit light after the UV source is removed. The color of the emitted light depends on the substance’s chemical composition.
The Diamond’s Unique Glow
Diamonds exhibit fluorescence due to specific trace elements, or impurities, within their crystal lattice. As diamonds form deep within the Earth, under immense heat and pressure, microscopic amounts of other elements can become trapped within their carbon structure.
Nitrogen atoms are the most common impurities responsible for blue fluorescence, found in approximately 75% of all diamonds. Boron is another trace element that can cause a diamond to fluoresce, typically resulting in yellow or green light. These impurities absorb UV light, exciting their electrons to a higher energy state. When these electrons return to their stable state, they release this excess energy as visible light. Only diamonds containing these trace elements in sufficient quantities will display this characteristic.
Colors and Intensities of the Glow
The glow emitted by a fluorescent diamond can appear in various colors, with blue being the most prevalent due to nitrogen impurities. Less frequently, diamonds may glow yellow, green, or even white, depending on the specific trace elements present, such as boron.
The intensity of a diamond’s fluorescence also varies. Gemological laboratories, like the Gemological Institute of America (GIA), classify it using specific grades: None, Faint, Medium, Strong, and Very Strong. “None” indicates no observable fluorescence, while “Faint” means a very slight glow, usually unnoticeable to the unaided eye. Medium, Strong, and Very Strong describe progressively brighter glows under UV light.
What the Glow Means for Your Diamond
Fluorescence in a diamond is a natural characteristic, not an indicator of quality like the 4Cs (color, clarity, cut, and carat weight). For most diamonds, even at medium or strong levels, fluorescence has no noticeable impact on their appearance in normal lighting conditions.
A common misconception suggests fluorescence always causes a diamond to appear hazy or milky. This “overblue” hazy effect is rare, occurring in fewer than 0.2% of fluorescent diamonds submitted to GIA. In some instances, particularly for diamonds with a faint yellowish tint (lower color grades), blue fluorescence can make the stone appear whiter or more colorless. This happens because the blue light emitted can counteract the yellow tones. Conversely, in diamonds of very high color grades (D-F), strong blue fluorescence is sometimes perceived as less desirable in the trade, though studies indicate average observers often do not detect a difference.
Regarding value, diamonds with higher levels of fluorescence might be priced slightly lower than non-fluorescent diamonds, especially in the top color grades. This is largely due to market perceptions rather than a widespread negative impact on appearance. For many diamonds, particularly those with faint or medium fluorescence, this characteristic can offer good value without compromising visual beauty.