Twinning is represented only by twinlamellae, which are parallel to the planes m and f and are of secondary origin, having been produced by pressure.
The larger ones polarize light, have angular outlines like those of crystals, and may even show twinning and definite optical properties by which they can be identified as belonging to felspar, augite or some other rock-forming mineral.
Twinning is not uncommon, the twinplanes M (i io) and g (roi) being the same as in marcasite.
This frequent twinning gives rise to characteristic forms, with many re-entrant angles, to which the names "spear pyrites" and "cockscomb pyrites" are applied.
Crystals of blende are of very common occurrence, but owing to twinning and distortion and curvature of the faces, they are often rather complex and difficult to decipher.