What is ultraviolet fluorescence?

Fluorescence is the visible light some gemstones emit when they are exposed to invisible ultraviolet (UV) rays. In natural diamonds, blue is the most common color of fluorescence, but other colors may be visible.
 
On a GIA Diamond Grading Report, fluorescence refers to the strength, or intensity, of the diamond’s reaction to long-wave UV, which is an essential component of daylight. To learn more about diamond fluorescence, download this PDF.