Diamond

One of the naturally occurring forms of carbon found in meteorites. Each C atom is bonded through covalent sp3 hydrid orbitals to four others. The strength of the C-C bonds makes diamond the hardest naturally occurring substance (according to the Mohs scale) in terms of resistance to scratching. There are several distinct polymorphs, including cubic diamonds, hexagonal diamonds, and n-diamonds. Note that lonsdaleite, a rare mineral found in some carbon-bearing meteorites, is 58 % harder than diamonds.