The pencil, surely? Although you can get a long way with chalk, slate and charcoal, so perhaps not?

Pencils are not a modern invention [0], which is why it doesn't fit the question. The ballpoint pen, as an example, is a 19th century patent [1], so pens in general might fit.

[0] https://en.wikipedia.org/wiki/Pencil#History [1] https://en.wikipedia.org/wiki/Ballpoint_pen