What is Crystal twinning

Crystal twinning occurs when two divide crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations. A twin boundary or composition surface separates the two crystals. Crystallographers classify twinned crystals by a number of twin laws. These twin laws are exact to the  crystal system. The type of twinning can be a diagnostic tool in mineral identification. Simple twinned crystals might be contact twins or penetration twins.