Diamond

Diamond is a transparent mineral and the hardest known substance. It was not until the end of the 18th century that Lavoisier and Tennant identified it as a crystalline form of carbon.

Each atom is bound to four neighbouring atoms at the vertices of a regular tetrahedron. These chemical bonds between the atoms are strong and have a length of 0.136 nm between atoms. Their tetrahedral symmetry results in a dense anisotropic solid.