Buckminsterfullerene ($C_{60}$) has a higher melting point than diamond because its spherical molecules are held together by a dense network of covalent bonds.

In silicon dioxide (${SiO}_2$), each silicon atom is bonded to two oxygen atoms by double covalent bonds, forming a simple molecular structure with low electrical conductivity.

Graphene is a single layer of graphite in which each carbon atom forms three $\sigma$-bonds in a trigonal planar arrangement, leaving a delocalised $\pi$-system that allows electrical conduction.

Silicon has a giant metallic structure similar to graphite, allowing it to conduct electricity efficiently at room temperature.