Fluorescence is the tendency of a diamond to emit a (soft) glow when exposed to ultraviolet light (UV light). The fluorescence effect is present in over 30% of diamonds and is an important consideration when buying a loose diamond.

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When UV light strikes a diamond with fluorescent properties, the stone emits a glow. Most often blue, it can also be shades of green, yellow, white or red. Fluorescence can occur in different intensities. Gemological laboratories like the GIA rate diamond fluorescence on a scale from "None" to "Very Strong". (Want to speak with a diamond expert? Call 866-737-0754 for expert advice on fluorescence.)

None, Nil, Negligible
No fluorescence, no influence on color.
Faint, Slight
Weak fluorescence, not a significant influence on color (barely perceptible).
Medium
Average fluorescence, small influence.
Strong, Extremely Strong
Strong fluorescence, substantial color influence.