When subjected to ultraviolet light, diamonds can show fluorescence. About 20% of diamonds are fluorescent under ultraviolet light with at 366 nm wavelength.
The evaluation of this phenomenon can be useful in recognising a gemstone or for commercial evaluation of the same.
In the United States a diamond with a blue fluorescence in colours from D to G, is popular because the white colour is enhanced.
NB: A recent study by HRD on the diamonds certification confirmed this thesis and showed that even strong fluorescence does not negatively impact a diamond’s appearance.
Link to the article: “Scientific analyses of grading (near-) colourless fluorescent diamonds”
The fluorescence therefore has a negative impact only on the price, decreasing slightly the price, especially if it is a diamond of investment. This is perhaps due to the fact that investment gemstones must be the rarest and taking into account only gems with no or very weak fluorescence, the specimens available are greatly reduced.