Both diamond and graphite are made of carbon. Carbon can form a maximum of 4 covalent bonds due to carbon having 4 electrons on it's outer shell. In diamond, each carbon atom forms 4 covalent bonds in a tetrahedral structure. Whereas, in graphite the carbon atoms only form 3 covalent bonds, creating hexagonally packed sheets of carbon. The sheets of carbon become bonded by weaker intermolecular forces. It is because of these weak intermolecular forces that the layersof graphite can slide over eachother, making the overall substance a lot weaker than diamond.