What is the hardest known natural substance?

Hardness in minerals means resistance to being scratched, and diamond sits at the top of the Mohs scale at 10. What makes it so hard is not the element but the arrangement. Diamond is pure carbon in which every atom is bonded to four neighbours in a rigid three-dimensional tetrahedral framework, so scratching it would mean breaking strong covalent bonds in every direction at once. Graphite is also pure carbon, yet it is soft enough to write with, because its atoms are bonded into flat sheets that slide easily over one another. The same element gives you the hardest common material and one of the slipperiest, purely through geometry. Natural diamonds form deep in the mantle under enormous pressure and are carried up by rare volcanic eruptions. Because hardness is only about scratching, diamond is not unbreakable: it has cleavage planes and a sharp hammer blow can split one, which is exactly how gem cutters work.

Some laboratory-made materials rival or edge past diamond in hardness, but none of them occur naturally.