Which term describes a diamond that is a flat, triangular twinned crystal?

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

Answer: (B)

A macle describes a diamond that is a flat, triangular twin crystal. Twinning in diamonds occurs when a second crystal forms in alignment with a twin plane, creating two intergrown halves that share a boundary. The result is a flat, triangular appearance—often two pyramidal halves joined along a common plane—giving a lozenge-like shape rather than a single, regular crystal form. This specific flat, triangular twin form is exactly what the term macle denotes.

An octahedron, while a common natural diamond crystal form, is a single-crystal shape and not a flat triangular twin. Graining refers to internal growth lines within the crystal, and growth marks are surface features from growth disturbances; neither describes the flat triangular twin configuration.