If a crystal receives a physical blow, is rubbed or twisted, it builds up a tiny electric charge across some of the planes within the crystal. The opposite is also true, and if a crystal is placed in an electric field, it will bend and flex in a minute way. This is known as the piezoelectric effect used in timepieces and chronometers.
The more a crystal is worked and handled, the greater the electric charge will be, although still incredibly small. It is enough to make a difference and cause a tingling effect when handled by sensitive people like crystal healers.