Which term describes a diamond with a flat, triangular shape resulting from twinning?

• A. Graining (Grain Lines)
• B. Habit
• C. Macle
• D. Octahedron

Answer: (C)

In diamonds, twinning can produce a macle—a diamond that shows a flat, triangular face created by a twin plane. This specific habit occurs when two or more crystal blocks align along a twinning plane, resulting in a distinct triangular facet that looks flat and plate-like. Because macles are defined by that characteristic flat triangle produced specifically by twinning, it’s the best way to describe a diamond with that shape. Graining refers to internal growth lines, which are not about a triangular twin face. Habit is the general term for a crystal’s overall shape, and an octahedron is a common, regular diamond form not defined by twinning.