The exact same thing happened to me in second grade.In Jr. high school I dropped a graphite pencil on the floor. As reached down to grab it, a classmate tried to kick it closer to me. The result was the pencil "lead" broke off in my hand in the webbing between 2 fingers. I pulled the "lead" out, but you could see the pencil mark inside my skin for decades. At some point it finally faded away.
I should amend my post..... I put on some glasses and saw that the tattoo is still there but very faint. WHo knew this happened to so many people. I thought it was unique event.....The exact same thing happened to me in second grade.
60+ years later I can still see it.