Diamond

 It is the most precious crystal used in jewellery and it is an allotropic form of carbon. All carbon atoms in diamond are sp3 hybridised carbon atoms and the bonding continues in all direction to form a big giant network of covalent solid. The strong covalent bonds between sp3 hybridised carbon atoms make diamond very strong and hard solid. It is hardest material with very high boiling point of 3550°C.

Diamond has three dimensional structure. Each carbon atom is bonded to four other carbon atoms which occupy four corners of a tetrahedron and each of these four carbon atoms are bonded to four more carbon atoms. Such a pattern is repeated in the entire crystal lattice resulting in a giant molecule. Since the single unit of diamond is a tetrahedron, it follows that carbon atoms have undergone sp3 hybridization.·

C (6) = 2,4

Each carbon atom is covalently bonded to four other carbon atoms by the overlapping of half filled sp3  hybrid orbitals in the crystal structure of diamond and it is difficult to rupture these bonded. C-C bond length is 154 pm. Diamond has high melting point ( 3873 K ) and it is very hard. Diamond has very high density of 3.51gm cm−3 due to three dimensional giant structure. Since all four valence electrons of carbon are involved in bonding and there is no free electron in the crystal structure of diamond. Diamond is a non-conductor of electricity. 

In diamond, it is very difficult to break extended covalent bonding and therefore, diamond is the hardest substance on the earth. It is used as an abrasive for sharpening hard tools, in making dyes and in the manufacture of tungsten filaments for electric light bulbs. Diamond is used for cutting glass, making bores and for rock drilling.