Why is graphite, a non-metal, a good conductor of electricity?

Graphite is an allotrope of carbon, a non-metal. Its unique structure consists of layers of carbon atoms arranged in hexagonal rings. Within each layer, carbon atoms are covalently bonded, but each carbon atom is bonded to only three other carbon atoms, leaving one valence electron free. These delocalized electrons between the layers are free to move, making graphite an excellent conductor of electricity, unlike most other non-metals.