Diamond fluorescence. Fluorescence in a diamond refers to the blue glow that comes from the diamond when put under ultraviolet light. Now the diamond industry looks at this as a bad thing and discounts diamonds with strong blue fluorescence as much as 15%. This is not a bad thing fluorescence will make your diamond appear whiter while saving you money. So search out a diamond with strong blue fluorescence and compare the stone to a diamond without fluorescence and judge for yourself.