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. Ordinary friction tape (the cloth type -- not the shiny electrician's tape) displays a glowing line where the end of the tape is being pulled away from the roll. 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.