Diamond crystal, molecular model. Diamond is a form of the element carbon. Each carbon atom (black) in a diamond molecule is positioned at the centre of a tetrahedron, where it is linked to four other atoms by strong covalent bonds. This repeating tetrahedral arrangement allows no rotation about the bonds, and so the structure is completely rigid, making diamond the hardest known naturally-occurring material. It is used in industry to tip heavy-duty cutting and drilling equipment and is also a precious gemstone. This ball and spoke model is to the scale of 1 centimetre = 0.1 nanometres.