Each C-atom in diamond is - hybridized
Have you seen a diamond?
Can you predict its structure?
Each C-atom is sp3 hybridsed. C-C bond distance in diamond is 154 pm, which is a normal distance of C-C covalent bond. As diamond consists of a number of tetrahedral units, it is a giant molecule. Also since each C-atom form four covalent bonds, it produces a repetitive three-dimensional structure using only covalent bonds.