Diamond structure. Molecular model of diamond, a form of the element carbon (C). Carbon atoms are shown as spheres (black) linked by covalent bonds (grey). Each carbon atom is positioned at the centre of a tetrahedron, linked to four others at the corners. This repeating rigid structure does not allow rotation about the bonds, making diamond the hardest known naturally-occurring material. Diamond is used as a precious gemstone, and in industry to tip heavy-duty cutting and drilling equipment.