This mineral Occurs in thin hexagonal plates with bevilled edges, which were formerly referred to the rhombohedral system, on account of their characteristic triangular striations, which run parallel to alternate edges of the hexagon.

Des Cloizeaux in 1867 (Nouvelles Recherches, p. 85) found the crystals from Freiberg, which are sometimes sufficiently thin to transmit light, to be biaxal, the acute bisectrix being perpendicular to the surface of the plates; the apparent angle between the axes could not be determined accurately, but was found to vary from 63° to 88°. The mineral must consequently be regarded as orthorhombic, with a prism angle very near to 60°.