Pencil erasers are made from synthetic rubber and pumice, a volcanic glass, plus a soybean-based filler.
It's not the rubber that erases errors, as is commonly thought, but pumice that actually scratches out mistakes.
Pencil-makers also note that the "lead" in a pencil is not lead at all. It is a mixture of graphite and clay. As for why most pencils are yellow, no one seems to know.