Diamond and Diamond Grading Practice Exam 2026 - Free Gemology Practice Questions and Study Guide

Cleavage in a diamond follows planes where the crystal bonds can separate with the least resistance. Diamond's carbon atoms are held in a tetrahedral sp3 network, arranged in a cubic lattice. The planes that allow a clean, flat split align with the octahedral directions of that lattice, which are the {111} planes. These octahedral planes run through the crystal in a way that a layer can detach cleanly, giving smooth cleavage surfaces.

Other planes—cubic, tetragonal, or prismatic—don’t align with the bonding pattern in diamond to produce the same clean break, so they’re not as suitable for cleaving. That makes octahedral planes the best choice for clean cleavage in diamond.