In diamond, each carbon atom is covalently linked to four other carbon atoms lying at the four corners of a regular tetrahedron. This gives rise to a rigid three-dimensional network of carbon atoms. This is also the reason why diamond is the hardest substance known, with high density and melting point.
Fig: 6.15 - Structure of diamondSilicon carbide has also three-dimensional structure as of diamond, with each silicon atom surrounded by four carbon atoms and each carbon atom surrounded by four silicon atoms.
Graphite has carbon atoms arranged in hexagonal parallel layers. This results in a double bond characteristic of graphite; each carbon atom is bonded to three others by a sp2 covalent bond as well being bonded among the adjacent layers by Van der Waal's forces.
Fig: 6.16 - Structure of graphite