Fractoluminescence is often used as a synonym for triboluminescence.  It is the emission of light from the fracture (rather than rubbing) of a crystal, but fracturing often occurs with rubbing.  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. Like in triboluminescence, 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.