Fluorescence, which is a phenomenon visible in 25 percent to 35 percent of diamonds, is a glow that emanates from the stone when it exposed to ultraviolet (UV) light, be it from the sun or from a fluorescent lamp. It is generally bluish and less often yellow or orange, and once the UV light source is removed it disappears.

There is debate in the trade whether fluorescence improves or detracts from a diamond’s appearance. A GIA  study, which polled the opinion of non-professional observers, suggested that there was no systematic effects of blue fluorescence on the face-up appearance of diamonds, with even experienced observers failing to agree consistently on the its effects from one stone to the next.

But the feeling among many in the trade is that, in the higher color grades, fluorescence is negative factor, particularly when it is intense. This is because bluish glow may cause diamonds in the D to H color range to appear hazy or oily. In the lower color ranges, however, from I to M, the general opinion is that bluish fluorescence enhances a diamond’s appearance, making the faintly yellow stones  appear more colorless in natural daylight.

GIA diamond grading reports identify fluorescence, when visible, and describe its intensity as Faint, Medium, Strong or Very Strong. When the fluorescence is Medium, Strong, or Very Strong, the color of the fluorescence is noted.