Which one of the following statements is NOT correct?

In diamond, each carbon atom is $sp^3$ hybridized and covalently bonded to four other carbon atoms, forming a rigid three-dimensional network. Since all four valence electrons of each carbon atom are localized in these strong covalent bonds, there are no free electrons available to conduct electricity. Thus, diamond is an insulator, not a good conductor of electricity.

The other statements are correct:
Diamond is extremely hard due to its extended three-dimensional covalent bonding.
Fullerenes (like $C_{60}$) are composed of both five-membered and six-membered carbon rings.
Graphite has a layered structure with weak van der Waals forces between the layers, allowing them to slide over one another, making it an excellent solid lubricant.

Answer: Diamond is a good conductor of electricity