Oak gall from a California live oak (Q. agrifolia) in the hills above my house.
I'm going to try my hand at making oak gall ink soon.
This site has some interesting factoids, and some advice that might foil my attempts: It says be sure to use galls that the wasps have not yet left (as evidenced by the exit holes) because the tannin content is higher. I'll still give it a try.
Oak gall ink was the favored ink of Leonardo and Bach. Unfornately, "corrosive to cellulose" is probably not a great property for ink to have. At least not if you are the paper.