Diamonds are the hardest natural substance on Earth and can only be scratched by other diamonds.

Diamonds are composed of pure carbon atoms arranged in a face-centered cubic crystal structure. This unique lattice allows each carbon atom to form four strong covalent bonds with its neighbors at a distance of only 0.154 nanometers. This dense atomic packing is what gives the diamond its unparalleled resistance to deformation and scratching.The Mohs scale of mineral hardness was developed by German geologist Friedrich Mohs in 1812 to categorize minerals based on their scratch resistance. On this scale, diamond is the standard for level 10, which is significantly harder than corundum, the level 9 mineral. Scientific testing using a sclerometer shows that diamonds are actually four times harder than rubies and sapphires.Despite their extreme hardness, diamonds possess a property known as 'perfect cleavage' in four directions. This means that if a diamond is struck with enough force at a specific angle, the atomic bonds can separate along these planes. This vulnerability is why a diamond can be crushed by a common steel hammer even though the steel cannot scratch the diamond's surface.Industrial applications rely heavily on these properties, with approximately 80% of mined diamonds used for cutting, drilling, and grinding. In 2005, researchers at the University of Bayreuth created 'aggregated diamond nanorods,' which are actually 11% harder than natural diamonds. However, in the realm of naturally occurring materials, the diamond remains the undisputed champion of hardness.