Diamond is harder than graptite because 

In the crystalline lattice of diamond. each carbon atom is linked to four other carbon atoms tetrahedrally by using ${\mathrm{sp}}^3 -$ hybrid orbitals giving rise to a rigid three dimensional network of carbon atoms. For this reason. diamond is extremely hard. Graphite has a hexagonal layer structure in which each carbon atom is linked to three other carbon atoms by using ${\mathrm{sp}}^2$ hybrid orbitals. The hexagonal layers in graphite are held together by weak vander Waal's forces. For this reason. graphite is soft.