Figure 5 shows part of the structure and bonding in diamond - AQA - GCSE Chemistry Combined Science - Question 5 - 2018 - Paper 1

Diamond has a high melting point due to the presence of strong covalent bonds that hold the carbon atoms together in a three-dimensional tetrahedral structure. Each carbon atom forms four covalent bonds with other carbon atoms, creating a giant covalent structure. This extensive bonding network requires a significant amount of energy to break apart, resulting in a high melting point.