Fluorescence. A diamond's tendency to glow blue (or, rarely, another color) under ultraviolet light. Strong fluorescence can reduce a diamond's resale value by 5–15%, especially in colorless (D–F) stones, because some buyers perceive it as a haziness in daylight.
Roughly one in three natural diamonds shows some fluorescence. In most stones the effect is invisible to the naked eye and has no functional impact.
In rare cases, very strong fluorescence makes a D–F stone look slightly milky in sunlight. Buyers price defensively against this risk, which is why the discount exists.
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