Twinning

Crystal Structure

Definition

A phenomenon in which two or more crystals of the same species grow together in a symmetrical, intergrowth pattern defined by a common twin plane or axis.

Detailed Explanation

Twinning can be simple (two crystals joined at a plane) or repeated (polysynthetic twinning, producing striations visible on crystal faces). In rubies, polysynthetic twinning planes appear as fine parallel lines visible under magnification and are used as an identification feature. Spinel commonly forms octahedral twins called spinel law twins. Chrysoberyl forms characteristic heart-shaped or trilateral cyclic twins. Understanding twinning is important because twin planes can act like cleavage during cutting, causing unexpected breakage.

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