Why is diamond a good electrical insulator despite being made of carbon atoms?

Diamond is an excellent electrical insulator because of its specific crystal structure and large band gap.12

In a diamond crystal, every carbon atom is covalently bonded to four neighboring carbon atoms in a rigid, three-dimensional tetrahedral arrangement. This structure results in all valence electrons being tightly bound within these strong covalent bonds, leaving no free electrons available to conduct electricity.32

From the perspective of band theory, diamond possesses a very large energy gap—approximately 5.4 to 5.5 eV—between its valence band and its conduction band. Because this gap is so wide, electrons cannot easily gain enough energy to jump from the filled valence band to the empty conduction band to facilitate current flow. While materials with smaller band gaps are classified as semiconductors, diamond’s exceptionally large gap renders it an effective insulator.12