In crossed polarized light, the quartz grain can go extinct all at once, called monocrystalline quartz, or in waves, called polycrystalline quartz. The extinction in waves is called undulose extinction and indicates dislocation walls in mineral grains. Dislocation walls are where dislocations, intracrystalline deformation via movement of a dislocation front within a plane, organize themselves into planes of sufficient quantity.
When examining quartz crystals under polarized light, they can either become uniformly aligned or display a wavelike pattern, referred to as monocrystalline and polycrystalline quartz. The latter is characterized by undulose extinction, which indicates the presence of dislocation walls within mineral grains. Dislocations are formed through the accumulation of deformation within planes, leading to the creation of dislocation walls.