An intriguing phenomenon of some diamonds is that they appear to glow blue, and less often green, yellow, orange, or a combination of these colours when exposed to invisible ultraviolet rays. This glow is called fluorescence, and lasts only as long as the diamond is exposed to the ultraviolet rays.
In most cases, fluorescence is caused by the presence of nitrogen as a trace element impurity in diamond. In addition to colour, fluorescence also varies by strength - from none, faint, medium, strong, and very strong (the most common terms in grading reports). Studies have shown that the strength of fluorescence has no widely noticeable effect on a colourless or near-colourless diamond's appearance. In fact, many prefer the appearance of diamonds that have medium to strong fluorescence. In very rare cases, some diamonds with fluorescence may appear hazy, milky or oily.