Which crystal planes are cleavage planes on a diamond?

• A. Cubic planes
• B. Dodecahedral planes
• C. Octahedral planes
• D. Trigonal planes

Answer: (C)

In diamond, cleavage occurs along planes where the crystal lattice has a natural, repeating weakness. These are the octahedral planes, which in a cubic crystal correspond to the {111} family. Geometrically, these planes form the faces of a regular octahedron that fits inside the cube, so you can visualize flat, mirror-like facets appearing along those directions when the crystal splits. Because the bond arrangement is most easily separated along these {111} planes, diamond splits or cleaves most cleanly there.

Planes aligned with the cube’s faces, or other crystal systems’ planes, don’t provide the same easy, flat split for diamond, so they aren’t considered the cleavage planes. The term octahedral planes specifically describes the directions along which diamond cleaves.