About a third of diamonds fluoresce, like the fluorescent minerals you have seen in natural history museums, or the novelty shop toys under the black (UV) light. The effect is like a white shirt in a discotheque.
Fluoro can be faint to very strong, and the most common fluorescent colour is blue. As blue is the complimentary colour to yellow, the most common tinted colour in diamonds, blue fluorescence can make yellowish diamonds look white or colourless.
A GIA survey found that fluorescent diamonds were favoured over non Fluoro stones, especially in lower colours, but even in the higher colours (D, E and F) which are often discounted by the trade. Many years ago colourless fluorescent diamonds were highly prized and referred to as "blue-white". But sales people used the term too loosely for any diamond with fluorescence; "blue-white" usage was outlawed by US trade practices laws.
One "for" argument for discounting fluorescent diamonds is because the GIA lab grading lights emit a small amount of ultra violet light; fluorescent diamonds might be assigned a better colour grade. The ‘against' argument, although the GIA Gem Trade Lab has not openly discussed the issue, is that UV light is almost always present in viewing environments, so why not grade colour in realistic lighting?
But the most likely reason for Fluoro diamond discounting is because of the sad fact that many jewellery salespeople are not able to explain complex phenomena like Fluoro; a Fluoro (or any comments) written on a report makes the diamond harder to sell = worth less! Imagine this sales killing explanation: “Fluorescence is visible light emitted by electrons when a diamond is excited by higher energy sources (Ultra Violet light or X-rays).”
Some diamonds have extremely strong fluorescence and appear oily or cloudy. This is BAD. But the GIA study found them to so rare that they were unable to find enough cloudy stones from the 26,010 sample to conduct a study of them.
We advise you not to buy a Very Strong unless you can actually see the diamond side by side with non-fluorescent diamonds in shaded daylight (which has a lot of UV light). My experience from the sales floor confirms the GIA findings; most people would choose a fluorescent diamond over a non stone anyway. The fact it may cost less is a real bonus.
Rarely diamonds fluoresce another color like yellow or orange. Do not buy them unless the diamond concerned is a fancy color of the same hue as the fluorescence (which will make it more intense). White diamonds with yellow or orange Fluoro will appear to be a lower colour when seen in light with a UV component.
When the UV light is turned off, fluorescence ceases instantly, but some stones continue to phosphoresce for a little while.