What is the hardest mineral known on Earth called?

The hardest mineral known on Earth is diamond. Its exceptional hardness comes from the way carbon atoms are joined in a rigid three-dimensional crystal structure, with each atom covalently bonded to four neighbors.

Diamond sits at 10 on the Mohs scale of mineral hardness, the highest value on the familiar scratch-test scale created by Friedrich Mohs in 1812. The scale is ordinal rather than evenly spaced: diamond is not merely slightly harder than corundum at 9. On an absolute hardness comparison, diamond is about four times harder than corundum.

Hardness is not the same as toughness. Diamond resists scratching extremely well, but it can still chip or fracture along certain crystal planes. That distinction explains why diamond-tipped tools are excellent for cutting and abrasion, yet a gemstone can be damaged by a sharp impact.

The word diamond comes from the Greek adamas, meaning unconquerable or unbreakable. Synthetic materials and laboratory-made carbon structures may challenge diamond in specialized tests, but diamond remains the standard answer for the hardest naturally occurring mineral.