What is the hardest naturally occurring carbon allotrope?

The hardest naturally occurring carbon allotrope is diamond. Its exceptional hardness comes from a three-dimensional crystal structure in which every carbon atom forms strong covalent bonds with four neighboring carbon atoms.

Those bonds create a rigid diamond-cubic lattice that resists scratching and deformation. Diamond defines 10 on the Mohs scale of mineral hardness, the highest value on that scale. This makes diamond useful not only in jewelry but also in cutting tools, drilling equipment, polishing compounds, and industrial abrasives.

Diamond and graphite are both made entirely of carbon, yet their properties differ dramatically because their atoms are arranged differently. Graphite is built from layers that can slide over one another, making it soft and useful as a lubricant. Graphene, a single layer of graphite, is exceptionally strong for its thickness but is not normally described as the hardest bulk natural mineral.

The phrase naturally occurring matters: some engineered materials and specialized forms of diamond may rival or exceed conventional diamond in particular tests. Under ordinary mineralogical usage, however, natural diamond remains the standard answer.