The crystal structure of diamond is a face-centered cubic FCC lattice, specifically, it has a diamond cubic structure. Each carbon atom in the diamond is covalently bonded to four other carbon atoms in a tetrahedral arrangement. This strong and highly symmetrical arrangement of carbon atoms contributes to the unique physical properties of diamond.The high hardness of diamond is a result of the strong covalent bonds between carbon atoms. These bonds require a significant amount of energy to break, making diamond one of the hardest known materials. The tetrahedral arrangement of carbon atoms also leads to a highly rigid and incompressible structure, which further contributes to its hardness.The high thermal conductivity of diamond is due to the strong covalent bonds and the diamond cubic structure. The lattice vibrations, known as phonons, can travel through the crystal lattice with minimal scattering, allowing for efficient heat transfer. Diamond has one of the highest thermal conductivities of any material, making it an excellent heat sink for various applications.In summary, the diamond cubic structure, characterized by a face-centered cubic lattice and strong covalent bonds between carbon atoms, is responsible for the high hardness and thermal conductivity of diamond. These properties make diamond an exceptional material for various industrial and technological applications.