pencil lead

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The material (usually black or dark gray) which allows a pencil to work. Usually a mixture of graphite and smaller amounts of other material, but not actual lead.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun mixture of graphite with clay in different degrees of hardness; the marking substance in a pencil