: carbon of a soft black lustrous form that conducts electricity and is used in lead pencils and electrolytic anodes, as a lubricant, and as a moderator in nuclear reactors—called also plumbago
Mineral allotrope of carbon. It is dark gray to black, opaque, and very soft. Its layered structure, with rings of six atoms arranged in widely spaced parallel sheets, gives it its slippery quality. It occurs in nature and is used (mixed with clay) as the lead in pencils. It is also used in lubricants, crucibles, polishes, arc lamps, batteries, brushes for electric motors, and nuclear reactor cores.