Triboluminescence

Triboluminescence is a property of crystals that emit light when they are crushed.

Why do they emit light?

It is basically light from friction, as the term comes from the Greek tribein, meaning "to rub," and the Latin prefix lumin, meaning "light". In general, luminescence occurs when energy is input into atoms from heat, friction, electricity, or other sources. The electrons in the atom absorb this energy. When the electrons return to their usual state, the energy is released in the form of light.

Triboluminescence of sugar is another example: it is lightning on a very small scale. When a sugar crystal is stressed, the positive and negative charges in the crystal are separated, generating an electric potential. When enough charge has accumulated, the electrons jump across a fracture in the crystal, colliding with and exciting electrons in the nitrogen molecules. Most of the light emitted by the nitrogen in the air is ultraviolet, but a small fraction is in the visible region. To most people the emission appears bluish-white, although some people discern a blue-green color (human color vision in the dark is not very good).

There are many crystals that exhibit this property, among them is N-acetylanthranilic acid.

Preparation of N-acetylanthranilic acid:

Reflux 10g of anthranilic acid and 30ml of acetic anhydride for 15mins. Cool the solution to room temperature and add 10ml of water through the condenser. Heat the mixture to boiling and allow to cool slowly. Filter the crystals of N-acetylanthranilic acid obtained and wash with a small amount of cold methanol. Avoid excess methanol.

To demonstrate the property of triboluminiscence the crystals should be well formed and completely free of solvent. 
The triboluminiscence is best observed by placing several crystals of the compound between two watch glasses and gently grinding. 
The light emitted is readily observed in a dark. 
The effect has been explained due to excitation of molecules by an electric discharge between the surface of the fractured crystal and subsequent fluorescence.

Ref:J.C.E aug 1975. vol 52
Chemistry and our world. M. Venugopal. Harper and Row. 1975.