The primary difference between diamond and graphite is the way their carbon atoms are arranged to form their physical crystal structures. The arrangement of carbon atoms in diamonds makes them bond together strongly, while graphite atoms are held together with a weaker bond, creating a soft physical substance.
Diamond molecules are shaped like perfect tetrahedra and are held together by single atoms forming strong bonds to four other atoms. Their shape makes diamond extra hard and durable to resist compression and broken bonds.
Graphite molecules consist of a single carbon atom attached to three others and gather together with others to form sheets of molecules. Though individual graphite molecules are strongly bonded, the sheets they form are weak and easily broken apart.