Diamond is a closely packed together lattice of carbon, due to the way in which diamond is formed, through extreme pressure and temperatures, unlike other carbon structures. The carbons in the lattice are linked together, with multiple covalent bonds to other carbons in the structure, closely together, so the intermolecular bonds are much stronger, the closer they are together, the stronger the bond and the more likely that the carbon is bonded to more than 2 other carbons. In Graphite, the carbons are usually bonded to 2 other carbon atoms, in diamond, the carbons are bonded to 4 other carbons (usually), this forms a tetrahedral arrangement of carbons. Sapphire (or aluminum oxide) is very strong also due to its compact structure also, but the bonds aren't as strong.