What is the hardest natural substance on Earth?

The resistance a material offers to scratching or abrasion is known as its hardness, and when it comes to natural substances, one material stands supremely above the rest. This remarkable substance is a form of pure carbon, forged deep within the Earth's mantle under immense heat and pressure. Its unparalleled ability to resist being scratched places it at the very top of the Mohs scale of mineral hardness, with a defining value of 10.

The secret to this exceptional hardness lies in its atomic structure. Each carbon atom within the substance is covalently bonded to four other carbon atoms, forming a robust, three-dimensional tetrahedral lattice. These powerful covalent bonds are among the strongest known in nature, and their rigid, dense arrangement makes the entire crystal incredibly stable and difficult to break apart or disrupt. This tightly packed, interlocking structure is what gives it its extraordinary resistance to scratching.

Formed over billions of years, roughly 87 to 120 miles beneath the Earth's surface, these crystals endure temperatures between 1300 and 9000 degrees Celsius and pressures equivalent to millions of kilograms pressing down. This extreme genesis contributes to its unique properties. Beyond its beauty as a gemstone, its extreme hardness makes it invaluable in various industrial applications, such as cutting, drilling, and polishing tools, where it can shape materials that would wear down ordinary tools. While it is the hardest natural material, meaning it resists scratching better than anything else, it is important to note that it can still be cleaved or shattered with a sharp blow due to its internal crystal planes, illustrating a difference between hardness and toughness.