When a diamond glows under a black light, it is exhibiting fluorescence. A black light emits long-wave ultraviolet (UV) radiation, which is invisible to the human eye. This UV light causes many natural diamonds to temporarily emit visible light, perceived as a “glow.” This reaction is caused by trace elements trapped within the diamond’s crystal structure during its formation.
The General Physics of Fluorescence
Fluorescence is a type of luminescence that occurs when a material absorbs light energy and immediately releases it as a different color of light. This process begins with the absorption of high-energy UV photons from the black light. A photon is absorbed by an electron, which temporarily boosts the electron to a higher, unstable energy level.
This excited state is short-lived, typically lasting only a few nanoseconds. The electron immediately drops back down to its lower energy level, releasing the excess energy it absorbed. Because some energy is lost as heat, the re-emitted photon has less energy and a longer wavelength than the original UV photon. Since the re-emitted light is in the visible spectrum, the diamond appears to glow. The glow stops almost instantly once the UV light source is removed, which distinguishes fluorescence from phosphorescence, where the glow lingers.
Trace Elements and Defects Causing Diamond Glow
The reason a diamond fluoresces is due to imperfections in its otherwise pure carbon crystal lattice, primarily the presence of nitrogen atoms. Diamonds are categorized into types based on the presence and arrangement of these impurities, with Type Ia diamonds—which contain nitrogen clusters—making up about 98% of all natural diamonds. When the nitrogen atoms are arranged in a specific cluster of three, known as the N3 center, they create a structural defect that acts as the primary activator for fluorescence. This N3 center defect absorbs the UV radiation and re-emits it as a visible blue light, which is the most common color of fluorescence seen in diamonds.
Other trace elements and defect configurations can cause different colors of light, though they are much rarer. For instance, single isolated nitrogen atoms replacing carbon can sometimes cause an orangey-yellow glow. The presence of boron atoms in a different type of diamond, Type IIb, is known to cause a reddish fluorescence, such as in the famous Hope Diamond.
Fluorescence and Diamond Grading
Gemological laboratories measure and grade a diamond’s fluorescence based on its intensity when exposed to long-wave UV light. The standard grading scale used by organizations like the Gemological Institute of America (GIA) includes categories such as None, Faint, Medium, Strong, and Very Strong. Less than 35% of natural diamonds exhibit any measurable fluorescence, and only about 10% of those have a strong enough reaction to potentially affect their appearance.
For diamonds with lower color grades (those with a subtle yellowish tint), a Medium or Strong blue fluorescence can sometimes be beneficial. The blue light counteracts the yellow, making the diamond appear whiter in certain lighting. Conversely, Strong or Very Strong fluorescence can rarely cause the stone to exhibit a hazy or milky appearance in daylight. This possibility leads to a general market discount for diamonds with stronger fluorescence grades, even though the effect occurs in less than 0.2% of fluorescent diamonds.