What is a crystal consisting of two or more parts formed in a symmetrical manner with shared crystal planes?

• A. A crystal cut with symmetry.
• B. A polycrystalline aggregate without alignment.
• C. Crystal consisting of two or more parts formed in a symmetrical manner with shared crystal planes.
• D. A crystal formed by twinning.

Answer: (C)

Twinning in crystals is when two or more parts of a crystal grow together in a symmetrical orientation, sharing a specific crystalline plane. The parts are intergrown and related by a definite symmetry across a boundary called the twin plane, so the overall crystal has a mirrored or otherwise symmetry-related relationship between its domains. This description—two or more parts formed symmetrically with shared crystal planes—captures exactly what a twinned crystal is.

This is different from a polycrystalline aggregate, which consists of many crystals with random orientations and no single, shared plane tying the parts together. It’s also different from a crystal cut with symmetry, which refers to how a gemstone is shaped rather than how its internal structure formed. The focused idea here is the intergrowth of two or more crystal parts in a symmetrical relationship along a plane, i.e., twinning.