Why do diamonds have these properties ?
A carbon atom can form four covalent bonds. This lets the carbon atom form many organic compounds (e.g. Diamond).
Diamond has a covalent structure in which:
1) Each carbon atom is joined to four other carbon atoms by covalent bonds.
2) The carbon atoms have a regular lattice arrangement.
3) It doesn't have free electrons
The strong structure, held together by strong covalent bonds, makes diamond very hard.