graphite

Blackish-grey, soft, flaky, crystalline form of carbon. It is used as a lubricant and as the active component of pencil lead.

Graphite, like diamond and fullerene, is an allotrope of carbon. The carbon atoms are strongly bonded together in sheets, but the bonds between the sheets are weak, allowing the layers to slide over one another. Graphite has a very high melting point (3,500°C/6,332°F), and is a good conductor of heat and electricity. It absorbs neutrons and is therefore used to moderate the chain reaction in nuclear reactors.