What is the most common habit (crystal form) of a gem diamond?

• A. Tetrahedron
• B. Cube
• C. Octahedron
• D. Dodecahedron

Answer: (C)

Crystals reveal the faces that form most readily during growth, and for diamond the stable growth planes are the {111} planes. The octahedron is built from those exact {111} faces, so natural diamond crystals commonly develop an octahedral outline with eight triangular faces. This makes the octahedral habit the most frequently observed in gem diamonds. The cube would require development of {100} faces, which aren’t the primary growth planes for diamond, so cube shapes are much less common. Tetrahedral and dodecahedral forms can occur only under particular conditions and are rare in gem-quality material.