Number 2 Tatoo
The blue-gray mark left perminantly in your outer layer of skin (epidermis) after getting stabbed or stabbing yourself with with a pencil. (commonly a number 2 pencil) It can either contain the lead (actually graphite) still inside or can just be particles that are visible through the skin. There is currently no way to remove it.
Remember when you stabbed me with a pencil last year? I still have a number 2 tatoo.