Fluorescence is the colored glow that only a small percentage of diamonds in the world emit when exposed to ultraviolet light. The Gemological Institute of America (GIA) grades diamond fluorescence on a scale of "None" to "Very Strong." This grading has to do with how intense the glow is under a UV light. So, a diamond with no fluorescence will not glow at all, whereas a diamond with very strong fluorescence will glow intensely. Unlike the Four C’s — the main criteria on which diamonds are graded — fluorescence is a supplemental characteristic of diamonds.