This is a term used by many experts to describe gems that appear to glow in the dark. The effect is similar to the luminous hands on a watch that help you tell the time when you wake in the middle of the night.
Phosphorescence is a process in which the energy absorbed by certain gems from ultra violet light is slowly released in the form of light from within the gem. For example, Kunzite, Diamond and Tourmaline are sometimes phosphorescent. Kunzite is possibly the best gem in which to observe phosphorescence and most high-quality Kunzite will appear to glow for several minutes after the ultra violet light source is removed. A famous example of a phosphorescent gem is the infamous Hope Diamond: it is said that after shining an ultraviolet light on it, it will glow red-orange for about five minutes when the light source has been removed.
