Diamond is celebrated as the hardest natural material on Earth, scoring a 10 on the Mohs scale for its resistance to scratching. However, a diamond’s durability is better defined by its toughness, which is its ability to resist breaking or chipping from impact. Diamonds possess only moderate toughness because their internal atomic structure contains inherent planes of weakness, making them susceptible to damage under certain mechanical and thermal conditions.
Inherent Weaknesses: Cleavage Planes and Inclusions
Within the tightly packed carbon lattice, there are four perfect cleavage planes, which are directions where the atomic bonds are significantly weaker. When a diamond receives a sharp blow precisely parallel to one of these planes, the force can easily split the stone.
Internal imperfections, known as inclusions, can also create stress points that compromise the diamond’s structural integrity. Surface-reaching flaws like “feathers,” which are tiny, crack-like fractures, can act as starting points for a chip. If a feather is located near the girdle or a pointed corner and receives an impact, the pre-existing flaw makes chipping much more likely. Large cavities or knots that break the diamond’s surface similarly create vulnerable areas.
Design Flaws: The Vulnerability of Cut and Shape
The cutting process itself can introduce significant susceptibility to chipping by creating sharp, unprotected edges. The girdle, the thin perimeter separating the top and bottom of the diamond, is the most common area where chipping occurs. An “extremely thin” or “knife-edge” girdle is particularly fragile because it is a razor-thin junction of facets with minimal supporting material. This mechanical weakness makes the stone highly vulnerable to damage during setting or from a side impact.
Certain diamond shapes feature points and sharp corners that inherently concentrate stress and align closely with the natural cleavage planes. Cuts such as the Princess, Marquise, Pear, and Trillion are at a higher risk of corner damage compared to the more rounded brilliant or cushion shapes. For example, a hard impact to the corner of a Princess cut is more likely to cause a chip than a similar impact to the rounded edge of a Round Brilliant.
The culet, the small facet or point at the bottom of the diamond, can also be a weak point. While a traditional culet is a tiny, protective facet, many modern cuts end in a sharp point with no facet, which is more susceptible to chipping when unset. Though the culet is typically protected once mounted, an exposed point or one chipped during setting can compromise the stone’s overall durability.
External Forces: Setting Security and Physical Stress
A diamond’s security within its jewelry setting is a major factor in its resistance to external chipping. If the prongs holding the diamond become bent, worn, or loose, the stone can shift slightly, exposing its delicate girdle or corners to impact. A loose diamond rattles in its mounting, allowing a vulnerable edge to repeatedly strike the metal or an external object during daily wear. This constant movement can exploit an existing weakness and lead to damage.
The type of external force that causes a chip is typically a sharp, sudden blow against a hard surface, such as hitting a ring against a metal countertop. This instantaneous impact, especially if directed at a pointed feature or a thin section of the girdle, provides the necessary energy to overcome the diamond’s limited toughness. The risk is compounded when this force is aligned with one of the diamond’s internal cleavage planes.
Thermal shock can also contribute to chipping, although it is less common than physical impact. This phenomenon occurs when a diamond is exposed to a rapid and drastic change in temperature, such as moving from extreme cold to intense heat. The sudden expansion or contraction creates internal stress that can initiate or extend a fracture, particularly in stones that already contain inclusions. Extreme heat, such as that from a jeweler’s torch, can also cause surface damage like oxidation, which begins around 700°C.