Each carbon atom in the crystal forms a covalent bond with four other adjacent carbon atoms in sp3 hybrid orbitals, and each four adjacent carbon atoms form a regular tetrahedron. The crystal type is a diamond structure, so the crystal shape without polishing is often an octahedron. The C-C bond in diamond is very strong, and all valence electrons participate in the formation of covalent bonds. There are no free electrons, so diamond is very hard, non-conductive, and has a melting point of 3815°C. The ignition point of diamond is 720-800℃ in pure oxygen and 850-1,000℃ in air. Industrially, diamond is mainly used to make drilling probes and grinding tools. Diamonds with complete shapes are polished and are called diamonds, which can be used to make high-end jewelry and are very expensive.