Some of you may remember a bizarre experience like this: when peeling an adhesive tape or opening a band-aid in a pitch dark environment, one can see an eerie glow from where the separation occurs.
The explanation behind this? Triboluminescence. "Tribo" is the prefix that related to friction, combined with "luminescence", the word describes a phenomenon in which visible light is generated two materials (usually in direct contact) rubbed against each other or pulled apart.
Scientists have not come to a consensus what causes triboluminescence. It is very likely to happen upon fracture of asymmetrical materials, during which chemical bonds are broken, and charges are released. When the charges reform bonds, the electric discharge ionizes the surrounding air, causing a flash of light. When occurring in the vacuum, where there's no gas molecule nearby, a beam of x-ray can be generated due to the high-velocity travel of the electrons.
Source: Physics Girl via Youtube