What is the origin of lead in pencils?

Lead has never been used in pencils. The 'lead' in pencils is graphite, a form of carbon, mixed with clay.

Modern pencils are the descendants of ancient writing instruments. In ancient Rome, scribes wrote on papyrus (an early form of paper) with a thin metal rod called a stylus, which left a light but readable mark. The ones made of lead worked best.

When pieces of rock from a mineral deposit in England were found to be good for marking sheep, the people presumed it was a lead deposit. Then the pieces were made into the first pencils. Chemists said the deposit was actually graphite, a form of carbon, but 'lead pencils' were so popular that the name is still used to this day.