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.
In polarized light, quartz crystals can either become uniformly aligned or display a wave-like pattern, known as monocrystalline and polycrystalline quartz, respectively. The latter is marked by undulose extinction, which signifies the presence of dislocation walls within mineral grains. Dislocations are formed through the aggregation of deformation within planes, resulting in the formation of dislocation walls.