Fluorescence is a physical phenomenon in which a material absorbs energy when exposed to UV light and emits this energy in the form of phosphorescent light. Fluorescence indicates the presence of gases during the formation of the diamond. 

Often, diamonds with strong fluorescence have a slightly flawed bluish tint even under normal daylight, which also contains UV light.
Ideally, a diamond should have no fluorescence. Fluorescence significantly reduces the value of a stone. 

The evaluation is done using the following designations: None, Faint, Medium, Strong, Very Strong.