What is the hardest natural substance on Earth?

Diamond is the hardest natural substance on Earth, measured by its exceptional resistance to scratching. On the Mohs mineral-hardness scale, diamond receives the top rating of 10.

Diamond’s hardness comes from its crystal structure. Each carbon atom is strongly bonded to four neighboring carbon atoms, creating a rigid three-dimensional network. This makes diamond extremely effective for cutting, drilling, grinding, and polishing, so industrial diamonds are used in tools as well as jewelry.

Hardness does not mean that diamond is impossible to break. Diamond can chip or fracture because it is relatively brittle, especially along particular crystal planes. This is why a diamond can scratch glass yet still be damaged by a sharp impact. The Mohs scale measures scratch resistance, not overall strength, toughness, or resistance to crushing.

Natural diamonds form deep in Earth’s mantle under intense pressure and heat, then reach the surface through unusual volcanic rocks such as kimberlite. Synthetic diamonds can be made in laboratories, but the familiar natural mineral remains the standard answer to this question.