Graphite

Most drawing pencils and graphite sticks are made with compressed mixtures of graphite and clay which vary in hardness according to manipulations of the mixture. The softest pencils have little or no clay allowing more graphite particles to adhere to the paper's surface. Pencils are marked with a number using the H and B systems to indicate the lead's hardness. Medium to hard pencils are marked with levels of the H system. 9H is the hardest and the lead's softness increases as the numbers decrease to 2H and H. Within the B system, the leads increase in softness as the numbers increase. In the middle of the range of pencils are the F and HB pencils. The HB pencil is the equivalent of a no. 2 office pencil.

A full range of graphite grades looks like this:
Softest 9B 8B 7B 6B 5B 4B 3B 2B B HB F H 2H 3H 4H 5H 6H 7H 8H 9H Hardest