Which statement about the crystal planes present in diamonds is true?

• A. They contain cubic planes only.
• B. They contain tetragonal planes.
• C. They contain cubic, octahedral, and dodecahedral planes.
• D. They do not contain any octahedral planes.

Answer: (C)

Diamond has a cubic lattice (the diamond cubic structure), so its atoms are arranged in a way that supports three main families of crystal planes: cubic planes, octahedral planes, and dodecahedral planes. Cubic planes run perpendicular to the cube axes and can form cube-like facets. Octahedral planes align with directions that produce octahedron-like geometry, which is why octahedral shapes and facets are commonly seen in diamond crystals. Dodecahedral planes come from a related set of symmetry directions in the same cubic system, allowing twelve-faced forms to occur. Because this lattice is not tetragonal, planes of that type don’t form in diamonds, and asserting there are no octahedral planes would contradict the crystal’s symmetry. Therefore the statement that diamonds contain cubic, octahedral, and dodecahedral planes accurately reflects their crystal plane families.