Their differences are due to the fact that graphite contains impurities that weaken its structure, while diamond is always a pure form of carbon. 

Their differences are due to the environments they form in: graphite forms from organic material, whereas diamond forms from volcanic gases, resulting in different strengths. 

Their differences are due to how the carbon atom bonds and atomic arrangement;  diamond’s carbon atoms have all covalent bonds while graphite's carbon atoms form strong sheets which are held together by weak intermolecular forces.