Diamond fluorescence is the tendency of a diamond to emit a colored glow when exposed to ultraviolet light (UV light) . This phenomenon occurs in over 30% of diamonds and is an important consideration when buying a loose diamond. Fluorescence can occur in different intensities, and gemological laboratories like the GIA rate diamond fluorescence on a scale from "None" to "Very Strong". The most common color for diamond fluorescence is blue, but it can also be shades of green, yellow, white, or red.

Not all diamonds fluoresce, and only about 25% to 35% of diamonds exhibit some degree of fluorescence in reaction to long-wave UV light. More than 95% of these diamonds fluoresce blue, while a rare few fluoresce other colors such as yellow or green. The degree of fluorescence can vary, and diamonds with none or faint fluorescence are rarer than those with strong or very strong fluorescence.

Fluorescence can sometimes increase or decrease the value of a diamond, and it can affect diamond appearance. In some instances, diamonds with strong fluorescence may look milky and lifeless in daylight, but in most cases, the appearance of a diamond with strong fluorescence is rarely affected. The bluish tint from fluorescence can improve a lower-color diamond (J-M color rated) by canceling out the faint yellow, resulting in a colorless appearance. However, in a very high-color diamond (D-F color rated), fluorescence may have the opposite effect.

Overall, diamond fluorescence should not be a major factor in the purchase of a diamond since its effects are negligible, if not slightly positive. The exception would be to exercise caution in purchasing a diamond with strong fluorescence in D-F color diamonds or very strong fluorescence in G-H color diamonds (which do not possess enough body color to offset the degree of fluorescence) .