The property of some minerals of emitting light during exposure to X-rays, ultraviolet light, or cathode radiation and continuing to do so after the exposure has ceased (if light emission ends when the radiation source is switched off, the property is called ‘fluorescence’). The colour of the emitted light varies with the wavelength of the radiation to which the mineral is exposed, and is thought to be due to the presence of traces of organic material or cations within the atomic structure of the mineral.