Diamond is hard because

In the structure of diamond each carbon atom in $\mathrm{sp}^{3}$ hybridised $\&$ is covalently bonded with four other carbon atom held at the corners of a regular tetrahedron by covalent bonds. This results in a very big three dimensional polymeric structure in which $\mathrm{C}$ - C distance is $154 \mathrm{pm}$ and bond angle is $109.5^{\circ}$. Owing to very strong covalent bonds by which atoms are held to gether diamond is the hardest substance known.