Some high-quality diamonds display a visible light when they are exposed to ultraviolet light. This is known as fluorescence. Under most lighting conditions, a diamond’s fluorescence is not visible to the naked eye. In the diamond industry, the existence of fluorescence in a diamond (caused by the natural mineral properties of the diamond), has caused many debates and has influenced the pricing of diamonds.
You may find in colorless or near-colorless diamonds that exhibit fluorescence have slightly lower prices than those that do not. Then again, fluorescence makes diamonds with lower colors look whiter and brighter.
Read about diamond color