Undulose extinction

As the microscope stage is rotated, individual mineral grains appear black when the polarization due to the mineral prevents any light from passing through.

If a mineral is deformed plastically by dislocation processes without recovery, strain builds up within the crystal lattice causing it to warp.

The presence of undulose extinction may help to infer that a crystal grew before a deformation event.

However, some minerals acquire undulose extinction easily and even under the effect of minor or local deformations.

You can help Wikipedia by expanding it.This optics-related article is a stub.