Characteristic of Nicol and Wallaston prisms, its paragon material is rhombohedral calcite and the Kerr Effect describes its development in an electric field.  
Generally the development of this material property corresponds to a real-symmetric rank-2 dielectric tensor whose eigenvalues correspond to the three orthogonal principle axes of polarization.  Characteristic of Nicol and Wallaston prisms, its paragon material is rhombohedral calcite and the Kerr Effect describes its development in an electric field.  Referring to the division of light into ordinary and extraordinary components, FTP, name this physical property of materials that can be classified by two distinct anisotropic indices of refraction.
Generally the development of this material property corresponds to a real-symmetric rank-2 dielectric tensor whose eigenvalues correspond to the three orthogonal principle axes of polarization.  
birefringence or double refraction (accept equivalents)