What is the hardest known naturally occurring material?

The hardest known naturally occurring material is diamond. Its hardness comes from a tightly bonded crystal structure in which each carbon atom is covalently connected to four neighboring carbon atoms in a three-dimensional lattice.

Diamond defines 10 on the Mohs scale of mineral hardness, which ranks materials by their ability to scratch one another. This scale is ordinal rather than linear: diamond is not merely one step harder than corundum at 9; its measured resistance to scratching is several times greater. Diamond tools therefore cut, grind, and polish many other materials.

Hardness should not be confused with toughness. Diamond resists scratching exceptionally well, but it can still chip or fracture because its crystal structure includes cleavage planes. Graphite is also made of carbon, yet its atoms form weakly connected layers, making graphite soft enough to leave marks on paper.

Synthetic ultrahard materials and specialized diamond forms can complicate claims about the “hardest” material under particular laboratory tests. For a standard general-knowledge question asking about a naturally occurring material and scratch hardness, however, diamond is the canonical answer.