Stress. An applied stress (differential stress, to be precise) deforms the quartz. To accommodate the deformation, dislocations, a type of crystalline defect, are generated within the quartz lattice. If many dislocations are generated and distributed relatively evenly, the crystal will be warped. The undulatory extinction is the optical manifestation of the crystals warped state. If the dislocations arrange themselves into networks, the networks can become new grain boundaries. Generating many new grain boundaries out of dislocations is one type of dynamic recrystallization.
