Many diamonds emit a visible light called fluorescence when they're exposed to ultraviolet(UV) radiation. Under the right conditions, you can see fluorescence in about 35 percent of gem diamonds.
Blue is the most common fluorescent color in gem-quality diamonds.
GIA studies show that, for overwhelming majority of diamonds, the strength of fluorescence has no widely noticeable effect on appearance. In many instances, observers prefer the appearance of diamonds that have medium to strong fluorescence.
The Strength of fluorescence is classfied into 5 stages; None, Faint/Slight, Medium, Strong, Very Strong.