  We study the linear polarization properties of the photoluminescence of the
neutral and negatively charged nitrogen vacancies and neutral vacancies in
diamond crystals as function of their symmetry and their response to strong
external magnetic fields. The linear polarization degree, which exceeds 10% at
room temperature, and rotation of the polarization plane of their zero-phonon
lines significantly depend on the crystal rotation around specific axes
demonstrating anisotropic angular evolutions. The sign of the polarization
plane rotation is changed periodically through the crystal rotation, which
indicates a switching between electron excited states of orthogonal linear
polarizations. At external magnetic fields of up to 10 T, the angular
dependences of the linear polarization degree experience a remarkable phase
shift. Moreover, the rotation of the linear polarization plane increases
linearly with rising magnetic field at 6 K and room temperature, for the
negatively charged nitrogen vacancies, which is attributed to magneto-optical
Faraday rotation.
