Fluorescence causes some diamonds to emit a visible glow when exposed to invisible ultraviolet (UV) light. This phenomenon is a form of photoluminescence, where high-energy UV radiation is absorbed and re-emitted as lower-energy visible light. Although not one of the traditional “Four Cs” of diamond grading, this characteristic is present in approximately 25% to 35% of all natural diamonds. Understanding this reaction is important because the presence or absence of fluorescence can influence a diamond’s visual appearance and its market value.
The Trace Element Cause of Diamond Fluorescence
Fluorescence in a diamond is caused by tiny structural defects or impurities incorporated into the crystal lattice during the stone’s formation deep within the Earth. These defects are often related to trace elements such as nitrogen, boron, or aluminum. Nitrogen is the most common impurity found in diamonds and is responsible for the majority of observed fluorescence.
The scientific mechanism involves these impurities acting as luminescence centers that interact with UV light. When an electron absorbs a high-energy photon from the UV source, it jumps to a temporary, higher-energy excited state. The electron immediately relaxes back to its stable ground state, releasing the excess energy as a photon of visible light (a glow). The most common configuration causing this effect is the N3 center, which consists of a cluster of three nitrogen atoms surrounding a vacancy. This structure is responsible for the characteristic blue color seen in most natural diamonds.
Standardized Measurement of Fluorescence Intensity and Color
The jewelry industry uses a standardized system to evaluate and report the intensity of a diamond’s reaction to UV light. This assessment is typically performed under controlled conditions using long-wave ultraviolet light, which mimics the UV component found in natural daylight. The resulting intensity of the glow is assigned one of five descriptive grades by major gemological laboratories.
The scale ranges from “None” or “Inert” (meaning no reaction is observed), through “Faint,” “Medium,” “Strong,” and finally “Very Strong.” For diamonds graded as Medium, Strong, or Very Strong, the report also notes the color of the emitted light. Blue is the most common color, accounting for over 95% of all fluorescent diamonds. While blue is dominant, other colors such as yellow, green, orange, or white are occasionally observed, linked to different types of nitrogen-related defects or other trace elements. The standardized intensity grade provides a consistent metric for comparing this characteristic across different stones.
Practical Impact on Diamond Appearance and Pricing
The presence of fluorescence can have two primary practical consequences for a diamond’s visual appearance, depending on its intensity and the original color of the stone. For diamonds that possess a slight yellowish body color, typically those graded I-J or lower on the color scale, a faint to medium blue fluorescence can sometimes be beneficial. Blue is the complementary color to yellow, meaning the emitted blue light can counteract the faint yellow tint, making the diamond appear whiter or more colorless to the eye.
Conversely, the market’s main concern arises with diamonds that exhibit Strong or Very Strong fluorescence, particularly in the higher color grades (D-F). In a small percentage of these cases, the intense reaction can cause the diamond to display a hazy, milky, or oily appearance, even in normal lighting conditions. This cloudiness is detrimental to the stone’s brilliance and is the reason for market apprehension.
Consequently, fluorescence influences market pricing, often leading to a discount for stones that exhibit Medium, Strong, or Very Strong fluorescence, especially those in the D-H color range. Diamonds with a “None” or “Faint” grade are generally preferred by the market and do not carry a price penalty. However, for buyers focusing on lower color grades, selecting a diamond with a Medium blue fluorescence can be a strategic choice to achieve a whiter-looking stone at a comparatively lower cost.