A diamonds response when exposed to ultraviolet (UV) sources such as the sun is reported on an AGS or GIA Diamond Grading Report as fluorescence. Fluorescence is visible light emitted from a diamond when exposed to UV sources. Under normal lighting conditions fluorescence is not detectable. On a GIA or AGS Diamond Report if fluorescence is present, the strength and color will be listed. The strength scale is as follows: None or Negligible with AGSL , Faint, Medium, Strong, Very Strong.
According to GIA about 25% to 35% of diamonds exhibit some degree of fluorescence and more than 95% of the diamonds that exhibit fluorescence, the visible color is blue (GIA). Generally the presence of fluorescence does not have any noticeable effect in the overall appearance of the diamond; though rare instances diamonds with very strong fluorescence may exhibit a cloudy or hazy characteristics.
Though the preference of a diamonds fluorescence for perspective diamond buyers comes down to a personal choice, diamond professionals generally prefer no fluorescence in higher colored diamonds while some fluorescence in lower colors is deemed desirable. The general consensus is that the inclusion of the bluish tint that fluorescence adds fools the eye by making the diamond look whiter. For this reason, fluorescence might be a positive (as long as it does not cause haziness) in a diamond that is approximately H-I color and lower.