Twinning: in mineralogy, a phenomenon of co-growth of crystals of the same species, gathered in such a way that one covers the other by rotation around an axis, by reflection with respect to a plane or for both of these conditions. Differently described, it is the result of a variation of the Crystallization Direction during the crystal growth process and manifests itself (for example in Diamond) in the form of thin parallel lines or striae emerging also on the surface (see also: Twinned Crystal).
