Fluorescence means that a diamond will glow usually in blue under UV light when illuminated. The effect is like a white shirt at a disco party, it glows. Fluorescence can range from faint to very strong and can make a yellowish diamond look white or colourless. When the diamond is not under UV light the fluorescence stops immediately, but some stones continue to phosphoresce for a little while. Fluorescence results due to visible light being emitted by electrons, occurring when a diamond is excited by a higher energy source, for example UV light. Some diamonds which have strong fluorescence appear oily or cloudy, which is not good. However a study performed by GIA showed that these diamonds are very rare; they were unable to find enough cloudy stones from the 26,010 sample they used.
Diamond Fluorescence is a hotly debated topic in the diamond industry, but a 1997 study conducted by GIA suggests fluorescence makes very little difference to the appearance of the diamond. The GIA study selected a group of similarly graded diamonds with varying degrees of fluorescence to be analysed and commented on by the study participants. Participants in the study included: trained diamond graders, trade professionals (e.g. jewellers) and average observers (general buying public). All these diamond experts concluded that whether a diamond has fluorescence or not should not greatly impact the quality of the diamond. Thus appearance of fluorescence or not in a diamond should not influence your purchasing decision.