Diamonds are widely recognized for their exceptional beauty and enduring sparkle. Beyond their allure as gemstones, diamonds possess extreme hardness, a unique physical property that makes them incredibly resistant to scratching. Understanding what materials can or cannot scratch a diamond reveals much about material strength.
Understanding Diamond Hardness
Material hardness describes a substance’s resistance to permanent deformation, particularly scratching or indentation. The Mohs scale of mineral hardness provides a relative measure for this property, ranking materials from 1 (softest) to 10 (hardest) based on their ability to scratch one another. A mineral can scratch any material lower on the scale and be scratched by any material higher up. Diamond occupies the very top of this scale, registering a hardness of 10.
The scale is not linear; for instance, diamond at 10 is significantly harder than corundum (sapphire and ruby) at 9, by a factor of several times, not just a single increment. Diamond’s extreme hardness stems from its atomic structure: each carbon atom forms strong covalent bonds with four neighboring carbon atoms in a tightly packed, three-dimensional crystalline lattice. This robust arrangement requires substantial energy to disrupt, making it highly resistant to scratching.
Materials That Can Scratch a Diamond
Given its position at the pinnacle of the Mohs scale, very few materials can scratch a diamond. The most common and effective material capable of scratching a diamond is another diamond. This property is utilized in the diamond industry, where diamonds are cut and polished using diamond-tipped tools or diamond dust.
Beyond natural diamonds, certain synthetic superhard materials approach or even surpass diamond’s hardness. Cubic boron nitride (cBN) is one such synthetic material, recognized as the second hardest substance after diamond, and is used in industrial applications where extreme hardness is required. Ultrahard fullerite, a form of carbon with a unique three-dimensional polymer structure, can scratch diamond in controlled laboratory tests, demonstrating hardness values that can exceed those of natural diamond. Lonsdaleite, a carbon-based material found in meteorites, is theoretically predicted to be harder than conventional diamond due to its distinct hexagonal crystal structure.
Materials That Cannot Scratch a Diamond
Many materials commonly perceived as hard are considerably softer than diamond and cannot scratch it. Steel, a widely used metal, typically has a Mohs hardness of around 5.5 to 6.5, making it incapable of scratching a diamond. Common glass, rating around 5.5 on the Mohs scale, will also be scratched by a diamond.
Quartz, a prevalent mineral found in sand, has a Mohs hardness of 7. While quartz can scratch many everyday materials, it cannot scratch a diamond. Even sandpaper, which contains quartz or other abrasive minerals, will not scratch a diamond. Corundum, including sapphire and ruby, is rated 9 on the Mohs scale. Despite being the second hardest natural mineral, corundum is significantly softer than diamond and cannot scratch it.
Practical Applications of Diamond Hardness
The extreme hardness of diamonds makes them invaluable across a wide array of industrial and scientific applications. In manufacturing, diamonds are integrated into cutting, grinding, and drilling tools designed for hard materials like concrete, stone, and metals. Diamond-tipped drill bits bore through tough surfaces, while diamond abrasives, often fine dust, are used for precision grinding and polishing.
Diamonds also play a role in specialized scientific instruments. Their hardness allows them to be used in high-pressure environments, such as diamond anvil cells, which subject materials to immense pressures for research. Diamond properties are utilized in microscopy probes and high-performance bearings. In gemology, diamond’s unparalleled hardness is a key factor in distinguishing genuine diamonds from imitations through scratch tests.