Fluorescence is a glow the diamond has when exposed to ultraviolet light. This is a natural phenomenon that occurs is about 30% of all diamonds. While diamonds may glow in different colors, the vast majority glow in blue. Normally, fluorescence has no visible effect (unless you walk around with a black light all day). However, there is a market value difference. Diamonds in the D-F range (colorless) are discounted if they have strong fluorescence. All Whiteflash A CUT ABOVE diamonds have negligible fluorescence.