What is the silvery substance at the center of pencils comprised of?
The "lead" of modern pencils is actually not made of lead. Instead, the "lead" of the pencil is actually a mix of finely ground graphite and clay powders. Most pencil cores are made of graphite mixed with a clay binder which leaves grey or black marks that can be easily erased. Graphite pencils are used for both writing and drawing and result in durable markings: though writing is easily removable with an eraser, it is otherwise resistant to moisture, most chemicals, ultraviolet radiation, and natural aging.
Charcoal pencils do exist, but are less common, and are mostly used by artists.