The conversion of graphite to diamond is an endothermic process, meaning that it requires an input of energy in order to occur. This is because the bonds between the carbon atoms in graphite are stronger than the bonds between the carbon atoms in diamond. In order to break the bonds in graphite and rearrange the atoms into the diamond structure, energy must be added to the system. This energy can be provided by heating the graphite to a high temperature or by subjecting it to high pressure.