text: There remain a few ambiguities about the effect. The current theory of triboluminescence—based upon crystallographic, spectroscopic, and other experimental evidence—is that upon fracture of asymmetrical materials, charge is separated. When the charges recombine, the electrical discharge ionizes the surrounding air, causing a flash of light. Research further suggests that crystals that display triboluminescence often lack symmetry and are poor conductors.  However, there are substances which break this rule, and which do not possess asymmetry, yet display triboluminescence, such as hexakis(antipyrine)terbium iodide.  It is thought that these materials contain impurities, which make the substance locally asymmetric. Further information on some of the possible processes involved can be found in the page on the triboelectric effect.
  The biological phenomenon of triboluminescence is thought to be controlled by recombination of free radicals during mechanical activation.