An intriguing phenomenon of some diamonds is that they appear to glow blue under ultraviolet light. This glow is called fluorescence, and lasts only as long as the diamond is exposed to the ultraviolet light.\
Fluorescence is caused by the presence of nitrogen as a trace element in your diamond. Three nitrogen atoms in lattice position in the crystal structure will commonly cause blue fluorescence.
Sometimes, the blue is strong and sometimes the blue is faint. In larger diamonds, fluorescence takes away from the value of the diamond, but in smaller diamonds it has virtually no effect on the price of the diamond. In larger diamonds, sometimes the fact that a diamond is fluorescent is hidden from the buyer, therefore adding to the profit margin of the retailer.
Personally, I like the look of a fluorescent diamond. Under high intensity sodium lighting, like at the ball park, your diamond may take on a subtle shade of blue. If you are selling a larger diamond, most buyers will use a small Ultraviolet flashlight to determine if diamond is fluorescent or not.
GIA reports on their website that 25-35% of diamonds submitted to them for grading exhibit some degree of fluorescence. Of those, only 10% show strengths of fluorescence in the medium, strong, or very strong categories. The presence of nitrogen in the diamond crystal structure does not have any influence on the hardness or durability of a diamond.