Triboluminescence is a phenomenon in which light is generated when a material is mechanically pulled apart, ripped, scratched, crushed, or rubbed. The phenomenon is not fully understood, but appears to be caused by the separation and reunification of static electrical charges. Triboluminescence can be observed when breaking sugar crystals and peeling adhesive tapes.
(from Wikipedia)
In most cases the light emitted is very faint and you need to have your eyes properly dark-adapted but this demonstration produces crystals that will emit light that is clearly visible – in subdued lighting at least.
