Diamonds are celebrated as the hardest naturally occurring material on Earth, a status earned from their unique internal composition. This structure is a dense, repeating crystalline lattice of carbon atoms linked by incredibly strong covalent bonds. While this network solid is exceptionally difficult to scratch, the common misconception is that a diamond is unbreakable. The surprising truth is that despite unparalleled resistance to scratching, diamonds have a structural vulnerability that makes them susceptible to fracture from a well-placed, focused impact.
The Critical Distinction Between Hardness and Toughness
The misunderstanding about a diamond’s indestructibility stems from confusing the scientific properties of hardness and toughness. Hardness is a material’s resistance to scratching, measured by the Mohs scale. Diamond registers a perfect score of 10, meaning no other naturally occurring substance can scratch its surface. This quality makes diamond powder an effective abrasive for cutting other materials.
Toughness, or tenacity, is a separate property describing a material’s ability to resist breaking or fracturing when subjected to impact. While diamond’s hardness is supreme, its toughness is only rated as moderate on the mineral scale. This lower toughness means diamonds are surprisingly brittle compared to materials like steel or industrial ceramics. A sharp blow can cause a diamond to cleave or shatter, even though it easily resists scratching.
Toughness relates to the energy a material absorbs before rupturing. For instance, substances like jadeite score lower on the Mohs scale but possess exceptional toughness, making them much less prone to chipping than a diamond. The diamond’s brittleness is a direct consequence of its highly ordered internal structure, which introduces specific planes of weakness.
Diamond’s Structural Weakness: Cleavage Planes
The crystalline structure that grants diamond its incredible hardness is also the source of its fragility. Diamonds are formed in the cubic crystal system, resulting in four perfect cleavage planes. These planes are specific directions within the crystal lattice where the atomic bonds are slightly weaker and fewer in number.
These internal planes of weakness run parallel to the faces of the octahedron shape, common in rough diamond crystals. When force is applied precisely parallel to one of these four directions, the weaker bonds break cleanly and smoothly. This action causes a split, or cleavage, rather than an irregular fracture.
The presence of these perfect cleavage planes means that breaking a diamond is directional, not merely a matter of brute force. A sharp, localized impact aligned with a cleavage plane will cause the stone to split with far less force than required to crush it. This directional vulnerability allows a diamond to withstand immense pressure from one angle, yet shatter easily from a light strike applied from another.
Quantifying the Force Needed for Fracture
The pressure required to break a diamond depends entirely on the type and direction of the applied force relative to the crystal structure. To crush a diamond uniformly, massive static pressure must be applied across the entire surface area. The compressive yield strength—the pressure needed to cause it to deform under uniform force—is estimated to be between 130 and 140 Gigapascals (GPa).
To put this crushing pressure into perspective, 130 GPa is over 1.2 million times the atmospheric pressure at sea level. This immense figure shows that if a diamond were squeezed evenly from all sides, it could withstand forces equivalent to those found deep within the Earth’s mantle. Diamonds are often used as anvils in scientific instruments to generate and contain pressures up to 600 GPa, showcasing their incredible resistance to uniform compression.
Impact Force vs. Crushing Force
The force needed to break a diamond through impact is dramatically lower because it exploits the cleavage planes. Jewelers and gemologists know that a sharp, precise blow aligned with a cleavage plane is sufficient to cause a split. This localized impact force is applied over a very tiny area, creating a high concentration of stress that the weaker bonds cannot sustain. Even just a few pounds of force, delivered with a sharp point against a cleavage plane, can initiate a clean split, which is a minuscule fraction of the GPa required for crushing.
How Jewelers Exploit Diamond’s Vulnerability
The weakness inherent in the diamond’s crystal structure is actively utilized in the cutting and shaping process. Highly skilled artisans, known as diamond cleavers, examine the rough stone to identify the location of its cleavage planes. They use “cleaving” to intentionally split the diamond along these pre-existing lines of weakness.
To cleave a stone, the cutter first notches a shallow groove into the surface precisely on the cleavage plane, often using a laser or another diamond. A steel blade is then placed into the groove, and the cutter delivers a quick, light tap to the blade with a small mallet. This minimal force is perfectly directed to propagate a fracture along the weak plane, cleanly splitting the rough stone.
This method is far more efficient than sawing, which requires the tedious process of grinding away material using diamond dust. By exploiting the cleavage plane, the cutter can quickly separate flawed sections or divide a rough stone into smaller, manageable pieces for faceting. This controlled vulnerability allows the world’s hardest material to be transformed into finished gems.