Diamond is formed from carbon atoms arranged in a strong three-dimensional tetrahedral lattice. This highly ordered crystal structure is the source of its status as the hardest known natural substance, achieving a perfect 10 on the Mohs scale of mineral hardness. Because only another diamond can scratch its surface, many believe the gemstone is indestructible. However, despite this reputation for durability, the diamond crystal possesses specific structural and chemical vulnerabilities that allow it to be broken, fractured, or chemically altered under the right conditions.
The Critical Distinction Between Hardness and Toughness
To understand how a diamond can be broken, it is necessary to distinguish between two separate material properties: hardness and toughness. Hardness is a measure of a material’s resistance to scratching and abrasion, which is where the diamond excels with its Mohs rating of 10. This quality ensures that a polished diamond maintains its brilliance because it is resistant to wear from everyday dust, which often contains quartz.
Toughness, however, is a material’s ability to resist fracture, impact, or chipping when subjected to a sudden, sharp blow. Here, the diamond’s performance is surprisingly modest compared to many other engineering materials. The diamond’s rigid, brittle structure means that while it resists scratching, a well-placed mechanical impact can easily cause it to chip or break completely.
The Structural Weak Point: Cleavage Planes
The vulnerability to impact force is directly related to the diamond’s crystalline structure, which contains specific planes of weakness called cleavage planes. These planes are directions within the lattice where the covalent bonds between carbon atoms are fewer or weaker than in other directions. The diamond exhibits four directions of perfect cleavage, which align with the faces of its inherent octahedral crystal shape.
When a diamond receives a sharp, forceful impact, the energy will preferentially travel along these cleavage planes rather than fracturing the stronger bulk of the crystal. If the force is delivered precisely parallel to one of these planes, the diamond can split cleanly. This controlled splitting demonstrates that the material is structurally susceptible to a targeted mechanical action. A fracture along a cleavage plane results in a smooth, flat surface, which is distinctly different from the irregular surface produced by a random fracture.
Fracture Caused by Extreme Temperature Change
A diamond can be broken without any physical strike through thermal shock. This occurs when the gemstone is subjected to a rapid and extreme shift in temperature, such as moving it quickly from a hot environment to a cold one. Diamonds are excellent thermal conductors, but this rapid temperature change causes the outer layers of the stone to expand or contract much faster than the inner core.
This differential expansion creates immense internal stress that the material’s moderate toughness cannot withstand. The resulting tension can exceed the cohesive strength of the carbon lattice, leading to internal fractures that radiate outward. In extreme cases, this stress causes the diamond to shatter, often producing cracks across its surface.
Chemical Alteration and Oxidation at High Heat
Beyond physical breakage, a diamond can be chemically destroyed through exposure to high heat and oxygen. Since diamonds are composed of pure carbon, they are susceptible to combustion, or oxidation, when heated in the presence of air. The ignition point for a diamond ranges between approximately 700°C and 840°C (1,292°F and 1,544°F) when exposed to oxygen.
When this temperature threshold is reached, the carbon atoms react with atmospheric oxygen, converting the solid diamond into carbon dioxide gas. Even below the burning point, a diamond can undergo a phase transition where its sp3-bonded carbon atoms convert to sp2-bonded carbon, essentially turning the surface into graphite. This chemical degradation becomes significant above 700°C.