Which term describes a crystal shape with eight equal triangular faces?

• A. Graining
• B. Macle
• C. Point Defect
• D. Octahedron

Answer: (D)

The concept being tested is crystal habit—the external shape a crystal tends to form. An octahedron is a regular polyhedron with eight faces, and all eight faces are equilateral triangles. So when a crystal shape has eight equal triangular faces, it is described as an octahedron. In diamonds, octahedral forms are a classic habit.

Graining refers to internal growth lines or zones within the crystal, not its external geometry. Macle (macle) describes a twinned crystal habit where two or more crystals are intergrown along a plane, not a uniform eight-triangular-face shape. Point defect is a lattice imperfection such as a vacancy or impurity, which again is not a crystal form.