A diamond is a solid form of crystallized carbon, created deep within the Earth’s mantle under intense heat and pressure. The ideal diamond is considered colorless, measured by the standardized D-Z color grading system. This system measures the degree of yellow or brown tint, with D being colorless and Z showing an obvious hue. However, trace elements or structural features can introduce a blue hue or tint. This blue can manifest in two ways: as a temporary glow or as a permanent body color.
The Primary Source of Blue: Fluorescence
The most common way a diamond exhibits a blue tint is through fluorescence, a temporary light emission phenomenon. This effect occurs when the diamond is exposed to ultraviolet (UV) light, such as that found in natural sunlight or some indoor sources. The diamond absorbs the UV radiation and then emits it as visible light, most often appearing as a soft blue glow.
This blue fluorescence is typically caused by minute traces of nitrogen impurities within the diamond’s crystal structure. The nitrogen atoms absorb the UV energy, and upon returning to a stable state, they release a photon of visible blue light. Grading laboratories categorize the intensity of this reaction into five levels: None, Faint, Medium, Strong, and Very Strong.
Fluorescence is a temporary reaction that ceases immediately when the UV light source is removed, unlike permanent body color. Only 25 to 35 percent of natural diamonds display some level of fluorescence, and over 95 percent of those emit blue light. The presence and intensity of this blue glow are noted on a diamond’s grading report, separate from the primary color grade.
Blue Color vs. Blue Tint: Boron and Natural Blue Diamonds
It is important to distinguish the temporary blue glow of fluorescence from a diamond that possesses a true, permanent blue body color. Rare and highly prized natural blue diamonds fall outside the standard D-Z color grading scale and are classified as “fancy color” diamonds. These stones owe their fixed blue hue to the presence of the trace element boron.
Boron atoms replace a small number of carbon atoms in the diamond’s crystal structure, creating Type IIb diamonds. These diamonds are extremely rare, making up less than 0.1 percent of all natural diamonds. The concentration of boron is typically only a few parts per million, which is sufficient to absorb light in the red end of the visible spectrum, causing the stone to appear blue.
The Hope Diamond is a well-known example, possessing a deeply saturated blue color directly attributable to its boron content. The permanent, intrinsic color of these fancy blue diamonds is graded based on the saturation and purity of the hue, not the absence of color like the D-Z scale.
How the Blue Tint Influences Appearance and Value
The blue fluorescence observed in many diamonds has a complex impact on both the stone’s appearance and its market price. For diamonds in the lower color grades (I through M color), the blue fluorescence can be beneficial. Since blue is the complementary color to yellow, the blue glow effectively neutralizes the yellowish undertone under daylight conditions, making the diamond appear whiter and closer to a higher color grade.
This whitening effect can lead to diamonds in the I-M color range with strong blue fluorescence trading at a slight premium. However, the effect is often viewed negatively for diamonds in the highest color grades, such as D, E, or F. In these colorless stones, strong or very strong blue fluorescence can be associated with a hazy, milky, or oily appearance, reducing the stone’s transparency and brilliance.
While strong fluorescence itself does not cause haziness, it can intensify existing atomic-scale defects within the diamond structure, manifesting as a cloudy look. Consequently, high-color diamonds with strong to very strong fluorescence often trade at a discount, sometimes up to 20 to 30 percent lower than identical stones without the fluorescence. Whether the blue tint is perceived as positive or negative depends heavily on the diamond’s original body color and the specific intensity of the fluorescence.