For thousands of years, people have used graphite to exercise creativity and bring ideas to life. Graphite is the key component of modern day pencil lead—in fact, lead is a misnomer as it’s actually made up of a mixture of graphite and clay. The formulation of this mixture determines its lead grade. The higher the proportion of graphite content relative to clay the lead has, the softer and darker the lead will be, and vice versa.
Lead grade is an important consideration for both the artist and writer. The hardness of a lead affects its strength, smoothness, smudge resistance, and pigmentation. Writers can fine tune the darkness of their lines by moving a step up or down in lead hardness, while an artist can employ the full range of lead grades to achieve different effects.-Miriam. “The Best Lead Grade For Every Application.” JetPens.com. August 27, 2019.
Probably more than you want to know on the topic of selecting a pencil.