Although both are... 1. Alltotropes of carbon, i.e. both are composed of carbon. 2. Giant covalent structures. Which are composed of non-metal aroms, held adjacently by covalent bonds. These atoms are arranged in a regular repeating strucutre known as a lattice. The high quanity of bonds : surface area ratio makes both carbon and diamond very strong. Differences 1. Diamond: each carbon atom bonds to 4 other carbon atoms, WHILST, Graphite: each carbon atom bonds to 3 other carbon atoms. Thus, diamond bears more of a tetrahedral structure, whereas graphite takes the form of layers. The presence of layers means that atoms can slide over each other easily. Thus, graphite's use as a lubricant. The 4th electron isn't involved in bonding and thus is free to move in between layers and is able to carry electricity. Hence, graphite is a weak conductor of electricity. 3. Diamond has stronger intramolecular bonds than graphite. As a result more energy is required to overcome the bonds existing in diamond. Therefore, diamond has a higher boiling point. Also, diamond is used for drilling, cutting, etc which is indicative of it's strength.