Question:
Diamond and graphite are both made from carbon atoms. What is it about the way these atoms are bonded that makes diamond so much harder than graphite?

Answer:
In diamond, the carbon atoms are all bonded to each other with covalent bonds, which involve the sharing of electrons and are very strong bonds. With graphite, the carbon atoms are bonded covalently only in sheets, but the sheets are weakly bonded together using electrostatic van der Waals bonds, and therefore slide easily off of each other. The greasy feel of graphite is a result of the sheets of graphite sliding past each other.