Diamonds are the hardest natural material because their carbon atoms form an unbreakable three-dimensional lattice with no weak points.

Diamonds rank 10 on the Mohs hardness scale, developed by German geologist Friedrich Mohs in 1812. This extreme hardness results from covalent bonding, where each carbon atom shares electrons with four neighbors through incredibly short bonds measuring just 0.154 nanometers. This atomic arrangement contributes significantly to the crystal's overall stability.The tetrahedral structure ensures that pressure applied from any direction distributes evenly across the entire lattice. In contrast, graphite consists of carbon atoms arranged in hexagonal sheets held together by weak van der Waals forces. These weak forces allow the layers to slide past one another easily, explaining graphite's softness.Natural diamonds form deep within the Earth, typically 145 to 240 kilometers below the surface, under extreme conditions. They require temperatures exceeding 1,100°C and pressures around 5 billion pascals. Most natural diamonds found today formed between 1 billion and 3.5 billion years ago.While diamonds are the hardest natural substance, they are not the toughest. Toughness refers to a material's ability to resist fracturing or chipping. Because diamonds have perfect cleavage planes, a well-placed strike can actually shatter one. Researchers at institutions like the Smithsonian continue studying these structures to develop super-hard synthetic materials for industrial cutting tools.