Fluorescence

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Fluorescence is not directly related to a diamond's color. This separate characteristic refers to whether or not a stone will produce a color reaction when exposed to long-wave ultraviolet (UV) light (such as the light that comes from black light). A stone that has fluorescence will give off a distinctive glowing blue coloration when exposed to UV light. Under most other lighting conditions, this fluorescence is not detectable to the eye. 

The fluorescence of a diamond is defined by its intensity as either None, Faint, Medium, Strong, or Very Strong. While some say a fluorescence grade of strong or very strong will cause a diamond to appear oily or murky, others claim with great conviction that it actually enhances the diamond's overall appearance, especially in lower color grade from J and lower.  My opinion after seeing thousands of diamonds is that anything with medium fluorescence or better will not have any impact on a diamond.  When a diamond exhibits strong or very strong fluorescence, it may or may not cause the diamond to have a slight haze;  It is a case base case basis.  A study put forth by the Gemological Institute of America (GIA) concluded that fluorescence in diamonds is just a matter of aesthetics and should not be a major concern when selecting a diamond.