About 50% of all diamonds exude a light blue or in some cases a light green, yellow or reddish light. A light emission that persists is called phosphorescence.

In most diamonds, the fluorescence is very slight and can only be brought about by direct radiation. Whether or not fluorescence should affect the value of the diamond is the subject of debate, therefore be wary of dealers who claim that more or less fluorescence should affect the price of the diamond. Be advised that for diamonds with colour grades of J to M, slight fluorescence may actually assist in fading some of the yellow hue, therefore making the diamond appear whiter. For a diamond with high colour grading, no or slight fluorescence is best, like in the case of D to F grades. In the case of the mid range grades like G, H and I, a moderate presence of fluorescence may improve the colour effect of the diamond.

Fluorescence can be seen clearly using a long-wave ultra-violet lamp. This examination is carried out in a darkened room and the diamonds are placed on the glass surface of the UV lamp. It is interesting to know that diamonds that have a strong fluorescence show this quality in UV enriched daylight. In daylight it may be thought that the diamond is a rare white, however under examination with the ultra-violet lamp, the diamond may be slightly tinted white.

As mentioned, the influence of fluorescence on price may vary, however to ensure the correct identification of fluorescence, a long-wave ultra-violet light examination at 366nm needs to be carried out.