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. Many postal envelopes will produce a blue glow when opened in the dark or in low-light conditions. Also, when sugar crystals are crushed, tiny electrical fields are created, separating positive and negative charges that then create sparks while trying to reunite. Wint-O-Green Life Savers work especially well for creating such sparks, because wintergreen oil (methyl salicylate) is fluorescent and converts ultraviolet light into blue light. Some Band Aid wrappers also glow bluish-green when unwrapped swiftly.
The discovery of triboluminescence was accidental. In the late 1790's sugar production began to produce more refined pure sugar crystals. These crystals were formed into a large solid cone for transport and sale. This solid cone of sugar had to be broken into usable chunks using a device known as a sugar nip. People began to notice that as sugar was "nipped" in low light, tiny bursts of light were visible. The first recorded observation however, occurred even earlier and is attributed to English scholar Francis Bacon when he wrote in his 1620 "Novum Organum" that "It is well known that all sugar, whether candied or plain, if it be hard, will sparkle when broken or scraped in the dark.". The scientist Robert Boyle also reported on some of his work on triboluminescence in 1663.
Fractoluminescence is the emission of light from the fracture of a crystal. Depending upon the atomic and molecular composition of the crystal, when the crystal fractures a charge separation can occur making one side of the fractured crystal positively charged and the other side negatively charged. If the charge separation results in a large enough electric potential, a discharge across the gap and through the bath gas between the interfaces can occur. The potential at which this occurs depends upon the dielectric properties of the bath gas.
This phenomenon can be demonstrated by removing ice from a freezer in a darkened room, under conditions in which the ice makes cracking sounds from sudden thermal expansion. If the ambient light is dim enough, flashes of white light from the cracking ice can be observed.
Fractoluminescence is a subset of the more broad category of mechanoluminescence which is light emission resulting from any mechanical action on a solid. Also, fractoluminescence is often mistaken for triboluminescence which, strictly speaking, is light emission caused by the rubbing together of two surfaces, resulting from charge build-up on the surfaces.