Some diamonds exhibit the property of fluorescence, properly called photo-luminescence, when exposed to ultraviolet light. The fluorescence is caused by trace amounts of Boron in the diamond. Little to moderate amounts of fluorescence do not affect the color of the diamond noticeably, but diamonds with a high degree of fluorescence may appear hazy and are therefore less desirable.

Although the most common color is blue, diamonds also fluoresce in other, less desirable colors such as yellow and green. A "perfect" diamond is what jewelers and gemologists call a blue-white diamond. The blue refers to the fluorescence and the white to the color of the diamond's body. In diamonds of H color or lower, fluorescence is normally a plus, and will increase the diamonds' beauty and desirability.

The seven grades of fluorescence as described by the Gemological Institute of America are None, Faint, Medium, Medium blue, Strong, Strong blue and Intense Blue.