Approximately 30% of all diamonds show a fluorescent response under UV. In most cases, the fluorescence is brightest under long wave (365nm). The fluorescent response is activated by a complex interplay between various types of defects in the crystal structure of the diamond, the two most notable defects being carbon vacancies (carbon atoms missing from the crystal lattice) and substitution defects where elements such as nitrogen and boron take the place of the normally occurring carbon atoms in the crystal structure. Depending on the number and type of defects in the diamond crystal, and the proximity of these defects to one another, the result can be fluorescence of any one of a number of different colors, with blue/white being the most common, but green, yellow and red fluorescence can occur also. In some cases the fluorescence is accompanied by phosphorescence, or afterglow, after the UV source is turned off.
The following series of photographs show examples of fluorescent and non-fluorescent diamonds.
Although none of the diamond crystals in the photograph above are suitable for faceting, they provide excellent examples of both fluorescent and non-fluorescent diamonds. This picture was taken under visible light and the diamonds range in color from colorless to yellow.
The above photograph shows the same group of diamonds seen under long wave UV. Two of the 4 diamonds show a strong blue/white fluorescent response.
After the long wave UV source is turned off, the two fluorescent diamonds show yellow phosphorescence or afterglow for several seconds.