Triboluminescence
Triboluminescence is an optical phenomenon in which light is generated via the breaking of asymmetrical bonds in a crystal when that material is scratched, crushed, or rubbed. This is a variant of luminescence; the term comes from the Greek tribein (to rub) and the Latin lumin (light).
For example, a diamond may begin to glow while being rubbed. This occasionally happens to diamonds while a facet is being ground or the diamond is being sawn during the cutting process. Diamonds may fluoresce blue or red. Also when sugar crystals are crushed, tiny electrical fields are created, separating positive and negative charges that then create sparks while trying to reunite. WintOGreen Lifesavers work especially well for creating such sparks, due to the fact that wintergreen oil (methyl salicylate) is fluorescent and converts ultraviolet light into blue light.
Mechanism of action
Materials scientists have not yet arrived at a full understanding of the effect but the current theory of triboluminescence, which is supported by crystallographic, spectroscopic, and other experimental evidence, posits that upon fracture of asymmetrical materials, charge is separated and when the charges recombine a flash of light is seen a result of the electric discharge ionizing the surrounding air. Research further suggests that crystals which display triboluminescene must lack symmetry (in order to permit charge separation) and be poor conductors. However, there are substances which break this rule, and which do not possess dissymmetry, yet display triboluminescence anyway. It is thought that these materials contain impurities, which confer properties of dissymmetry to the substance. Much of the work done on triboluminescence has been done by Dr. Linda M. Sweeting who is professor of chemistry at Towson University.