About 20% of diamonds exhibit fluorescence. Fluorescence can be seen in darkened rooms with black lights. The effect is like a white shirt in a discotheque.

The most common fluorescent color is blue, and this can vary from weak or faint to very strong. Blue fluorescence can be an advantage, particularly in lower (below I) colored diamonds, because blue is the complimentary color to yellow. Adding blue compensates for yellow and makes the diamond appear whiter in any light that has an ultra violet component (e.g. daylight and halogen lighting).

In general the trade discounts D, E and F colored fluorescent diamonds, but a GIA survey of experts and consumers found a slight preference for fluorescent diamonds. 30 years ago fluorescent diamonds were prized and often given a misleading name; ‘blue-white’.

If a diamond has extremely strong flourescence it can appear oily or cloudy, and this is not good.

Another reason the trade discounts flourescent diamonds is that some laboratories use grading lights that emit a small amount of ultra violet light. This means lower grade blue flourescent diamonds are possibly assigned a better grade than they should.