Fluorescence (diamond)

Diamond fluorescence is the glow a diamond emits when exposed to ultraviolet light, most commonly longwave UV at 365 nm (the wavelength used by GIA grading). Roughly 35% of natural diamonds fluoresce; the remaining 65% are graded None. Of fluorescent stones, over 95% glow blue under longwave UV — caused by trace nitrogen aggregates (N3 centers) in Type Ia diamond. Yellow, orange, white, and green fluorescence exist but are rare.

GIA grades fluorescence on a five-step scale: None, Faint, Medium, Strong, Very Strong. The grade reflects the intensity of the glow under a standard GIA longwave UV lamp, compared against a graded master set. The grade does not predict whether the fluorescence helps or hurts the stone — that depends on body color.

In I to M body colors, blue fluorescence helps. The blue glow masks the underlying yellow body color enough to lift face-up appearance by roughly half a grade (Medium) or a full grade (Strong). A Strong Blue J can read as warm white face-up while a None Blue J reads as visibly tinted. The trade prices fluorescence at I-M as a small premium of 5% to 10% for Medium and 0% to 5% for Strong.

In D to G body colors, blue fluorescence hurts. The fluorescence does not need to mask body color (there isn't any), and a small percentage of fluorescent stones — roughly 3% of Strong and Very Strong — face up "milky" or "oily" or "hazy" in UV-rich light such as daylight near a window. The trade discounts Medium Blue D-G by 5% to 15% and Strong Blue D-G by 10% to 25% on milky-risk worry. Very Strong Blue D-G discounts run 15% to 35%.

The milky risk is the load-bearing question. Not every Strong-fluorescence diamond is milky; the risk increases with fluorescence intensity but is not deterministic. The only reliable test is examination in daylight (which contains UV) or under a fluorescence lamp comparable to GIA's. A buyer offered a Strong Blue stone at a 15% discount should verify it is not milky before accepting; an online buyer should require a daylight video before purchase. GIA notes "graded under daylight" on the report when the stone has been examined in daylight, but this is not a guarantee.

Faint Blue does not affect appearance or value in either direction. Faint Blue trades at the same price as None Blue at every body color and grade.