Plastic deformation of germanium crystals by compression along the ⟨111⟩ symmetry direction results in birefringence for light propagating in any direction. The crystal in this case breaks up into regions several millimeters across, with varying orientation of the index ellipsoid and the birefringence.
© 2008 Optical Society of America
E. N. Roshak, P. S. Shirshnev, and A. K. Przhevuskiĭ, "Anisotropy of the optical properties and inhomogeneity of plastically deformed germanium crystals," J. Opt. Technol. 75, 599-602 (2008)