The Fluorescence of a Diamond

Diamond Fluorescence is a hotly debated topic among the diamond industry, because some feel strongly that it has an impact on a value of the diamond, whereas others feel that it makes very little difference on the appearance of the diamond. What diamond fluorescence refers to is the ability of the diamond to emit a soft colored glow when put under an ultraviolet light, or black light as it may be called.

It is found that if a stone has a grade from J to M a moderate amount of fluorescence actually makes the stone more attractive to people because with these color grades the fluorescence helps to cancel out some of the yellow coloring. This makes it appear more colorless then it actually is. However if you have a stone with a grade from D to F, which means it has a high color, fluorescence is thought to get in the way of the flow of the light. The makes the diamonds appear oily or dirty looking. This may not actually be true, but it is perceived to be true by some of the diamond industry, hence the lower price of the diamond. In some cases D to F stones should not be bought anyhow with any fluorescence just because of the possible discoloration but again, that is still up for debate.
In general, the average fluorescence makes absolutely no difference when the diamond is looked at flat side up. You should not let the amount of fluorescence influence your choice of diamonds.

Source: http://www.thediamondblog.org/diamonds-anatomy/the-fluorescence-of-a-diamond.htm