When a diamond appears to glow under a black light, you are witnessing a natural phenomenon known as fluorescence. This glow is an emission of visible light by the diamond when it is exposed to invisible ultraviolet (UV) light. It is a characteristic found in some, but not all, diamonds and is not considered a grading factor like color or clarity.
Understanding Fluorescence
Fluorescence is a general scientific principle where a substance absorbs energy from a specific type of light, such as ultraviolet (UV) light, and then re-emits that energy as visible light. In this process, electrons within the substance absorb the higher-energy UV photons, moving to a higher energy state. When these excited electrons return to their original, lower energy state, they release the absorbed energy in the form of lower-energy photons, which we perceive as a visible glow. This emission of light stops almost immediately once the UV light source is removed.
What Makes Diamonds Fluoresce
The fluorescence in diamonds is primarily caused by the presence of specific trace elements or structural irregularities within the diamond’s crystal lattice. Nitrogen is the most common impurity responsible for fluorescence, typically leading to a blue glow. Boron is another trace element that can cause fluorescence, often resulting in yellow or green hues. These elements are naturally incorporated into the diamond during its formation.
Common Fluorescence Colors
While blue is the most frequently observed color of fluorescence in diamonds, other colors can also occur. Approximately 95% of diamonds that fluoresce will emit a blue light. Less common fluorescence colors include yellow, green, orange, red, or white. The specific color of the glow depends on the type and concentration of the trace elements or structural defects present in the diamond’s unique atomic structure.
How Fluorescence Affects Diamonds
Fluorescence is an identifying characteristic for diamonds, and its intensity is graded by organizations like the Gemological Institute of America (GIA) as None, Faint, Medium, Strong, or Very Strong. Approximately 25% to 35% of natural diamonds exhibit some level of fluorescence when exposed to long-wave UV light. For most diamonds, fluorescence has little to no noticeable effect on their appearance under normal lighting conditions.
In some instances, strong or very strong blue fluorescence can make a diamond with a faint yellowish tint appear whiter or more colorless. This is because blue is complementary to yellow, effectively neutralizing some of the yellow body color. However, very strong fluorescence in colorless diamonds (D-F color grades) is occasionally associated with a hazy or oily appearance, though this is rare.
The impact of fluorescence on a diamond’s value can vary. For higher color grades (D-H), strong blue fluorescence may lead to a discount as it can be considered less desirable. Conversely, for lower color grades (I-M), medium to strong blue fluorescence can make a diamond appear whiter, potentially offering better value.