graphite

Concise Oxford English Dictionary © 2008 Oxford University Press:
graphite/ˈɡrafʌɪt/

▶noun a grey, crystalline, electrically conducting form of carbon which is used as a solid lubricant, as pencil lead, and as a moderator in nuclear reactors.

– derivatives
graphitic adjective,
graphitization (or graphitisation) noun,
graphitize (or graphitise) verb.

– origin C18: from Ger. Graphit, from Gk graphein ‘write’ (because of its use in pencils).