The perception of a diamond as indestructible stems from its status as the hardest naturally occurring material. This quality often leads to a misunderstanding of its overall durability, creating a paradox when a diamond chips or breaks. Determining if a diamond is fragile requires examining the material science that governs its structural integrity. A diamond’s longevity depends on the interplay between two distinct physical properties: its hardness and its comparative lack of toughness.
The Difference Between Hardness and Toughness
The diamond’s reputation as the hardest material on Earth is based on its resistance to scratching and abrasion. This characteristic is measured on the Mohs scale of mineral hardness, where the diamond earns the maximum rating of 10. No other naturally occurring substance can scratch a diamond’s surface, allowing it to retain its polish and brilliance even with daily wear. This scratch resistance stems from the incredibly strong covalent bonds between carbon atoms within its crystal lattice.
The term “hardness” is distinct from “toughness,” which refers to a material’s ability to resist breaking, chipping, or fracturing from a sharp impact. Diamonds possess only moderate toughness, making them brittle compared to materials like steel or jade. A helpful analogy is comparing glass to steel: glass is hard and resists scratching, but a sharp blow will shatter it easily. This brittleness means that despite its supreme scratch resistance, a diamond remains vulnerable to a forceful blow if struck at the right angle.
Toughness is determined by how much energy a material can absorb before rupture. Because a diamond cannot absorb and distribute impact energy effectively, it is prone to failure under stress. This brittleness is a direct result of the diamond’s internal atomic arrangement. Therefore, a diamond is simultaneously the hardest substance known and a comparatively brittle one, allowing it to be chipped or cracked.
Why Diamond’s Crystal Structure Causes Cleavage
The mechanism behind a diamond’s fragility lies within its tightly organized internal structure. While the covalent bonds are extremely strong, this ordered arrangement creates specific planes of weakness known as cleavage planes. These planes are analogous to the grain in wood, representing directions where the bonds between layers of atoms are slightly weaker.
A diamond possesses four perfect cleavage directions, which correspond to the octahedral {111} planes of its crystal structure. When a diamond is struck with sufficient force parallel to one of these planes, the impact energy is channeled along this structural weakness. The stone will then split cleanly along the plane, a phenomenon that diamond cutters intentionally exploit to shape rough crystals. This splitting is a uniform separation of the crystal structure, not a random fracture.
The existence of these four cleavage planes means that a diamond is highly directional in its vulnerability. The stone is most susceptible to chipping or breaking when force is applied perpendicular to these octahedral faces. This structural reality explains why a diamond can resist abrasion, yet still be fractured by a sharp, focused strike.
How Cut and Inclusions Influence Vulnerability
While cleavage planes are an inherent property, external factors like the stone’s cut and internal flaws significantly amplify its vulnerability. The way a diamond is cut can either protect or expose its planes of weakness, directly influencing its durability. Cuts with sharp, unprotected points, such as the princess, pear, or marquise shapes, are the most susceptible to damage.
The pointed corners of these cuts are often parallel to a cleavage plane. Because the material is thinnest at these points, they act as mechanical stress concentrators. A minor side impact on a pointed corner is much more likely to initiate a fracture than an impact on the rounded edge of a brilliant-cut diamond. Protective settings like V-shaped prongs are often used to shield these vulnerable tips.
Internal flaws, known as inclusions, also play a role in reducing a diamond’s toughness. Inclusions are tiny crystals or structural imperfections that formed within the diamond during its growth, acting as pre-existing stress points. Surface-reaching inclusions, such as deep cavities or large feathers (internal fractures), are concerning when located near a girdle or an edge. These flaws function as failure initiation sites, making it easier for an impact to compromise the stone’s integrity and propagate a fracture.