Diamond is a form of carbon. In diamond, each carbon atom forms four covalent bonds with four other carbon atoms. This pattern repeats in every direction, so the whole crystal is one giant covalent structure: a huge network of atoms joined by strong covalent bonds, not a collection of separate small molecules.
The properties of diamond follow from this structure. It is very hard, because every atom is held firmly in place by four strong covalent bonds, so the network is rigid. It has a very high melting point, because a large amount of energy is needed to break the many strong covalent bonds.
Diamond does not conduct electricity. All four outer electrons of each carbon atom are used in bonding, so there are no free electrons or ions that can move and carry charge. Its hardness makes diamond useful for the tips of cutting tools and drills.
