Any emission of light by a mineral during exposure to ultraviolet light is fluorescence. This phenomenon is usually observed in minerals containing impurity ions called activators. Most fluorescence is faint and can be seen only in the dark. If the light emission continues after the exciting rays are cut off, the mineral is said to be phosphorescent. Fluorescence is an unpredictable property, for some specimens of a mineral show it, whereas other apparently similar specimens, even from the same locality, do not. Enter the mine tunnel and look for the fluorescent minerals to the right.