Fluorescence is a naturally occurring phenomenon that appears in certain minerals and gems. Some quality diamonds display a visible light when they are exposed to ultraviolet light. This light is known as fluorescence. Under most lighting conditions, a diamond's fluorescence is not visible to the naked eye, although the diamond will exhibit a soft colored glow if held under an ultraviolet lamp.
Not all diamond fluoresce. If a specific diamond does not fluoresce, the grading report will list the diamond's fluorescence as either inert or none.
The degree of fluorescence varies from faint to medium to strong to very strong. Faint means that the stone has a slight glow that is difficult to see under ultraviolet light. Very strong means that the diamond emits a deep glow that is very clear under ultraviolet light. The color of the fluorescence can also vary, although blue is the most common color. Yellow, green and white are other colors that a fluorescent diamond may exhibit.
Fluorescence usually has no effect on a diamond's appearance in regular lighting conditions. In some cases, however, a strong blue fluorescence can make a yellow colored diamond appear whiter. In rare cases, it can or cause a stone to appear milky or oily.