Some diamonds phosphoresce, that is, if they are brightly illuminated either by daylight or artificial light and are then taken into a darkened room, they continue to give out a glow which gradually fades away. Such phosphorescence is not a special property of diamond alone for it is also shown by many other crystals. It is closely connected with impurities in the crystals and is not a property of very highly purified materials. . . .

Some diamonds when rubbed in the dark will emit a glow, and this, too is very closely related to phosphorescence. It is typical also of many other crystals carrying impurities. . . .

Many diamonds when illuminated with invisible ultra-violet light give out a visible glow. This is called fluorescence and it leads to the emission of a blue or a yellow-green light.

—S. Tolansky, from The History and Use of Diamond, 1962.