Diamond is the natural mineral that ranks highest on the Mohs hardness scale.
Friedrich Mohs placed diamond at 10 when he introduced his relative hardness scale in 1812. It can scratch every other mineral in the standard scale because its carbon atoms are linked in a very strong three-dimensional crystal lattice.
Diamond is not the same as the hardest material in every engineering test. Synthetic substances such as aggregated diamond nanorod materials can outperform ordinary diamond in some measurements. Also, diamond is hard but not unbreakable: its cleavage directions can allow it to split under a sharp blow.
The gem variety is usually valued for transparency and brilliance, while industrial diamond is used in cutting, drilling, grinding, and polishing.