Which is the softest crystal direction in a diamond?

Which is the softest crystal direction in a diamond?

• A. Dodecahedral
• B. Tetragonal
• C. Cubic
• D. Octahedral

Answer: (A)

Diamond’s hardness isn’t the same in every direction because the strong covalent network lines up with specific crystal planes, so some directions split or wear more easily than others. The easiest cleavage occurs along the {111} planes, which form the faces of an octahedron. Those planes offer the least resistance to breaking, so stress tends to cause separation most readily in that direction. Crystals that show a dodecahedral habit grow with faces that align with directions incorporating these weak planes, so the crystal behaves as if that direction is the softest. The other forms mentioned describe overall symmetry or different shapes of the crystal rather than the direction of least resistance, so they don’t indicate the softest direction as effectively as the dodecahedral form.

Diamond’s hardness isn’t the same in every direction because the strong covalent network lines up with specific crystal planes, so some directions split or wear more easily than others. The easiest cleavage occurs along the {111} planes, which form the faces of an octahedron. Those planes offer the least resistance to breaking, so stress tends to cause separation most readily in that direction. Crystals that show a dodecahedral habit grow with faces that align with directions incorporating these weak planes, so the crystal behaves as if that direction is the softest. The other forms mentioned describe overall symmetry or different shapes of the crystal rather than the direction of least resistance, so they don’t indicate the softest direction as effectively as the dodecahedral form.