I may be slow off the block about this one but I just discovered the mystery of the Oak Gall and its maker, the Gall Wasp! The wasp has developed in sync with the oak and can re-programme the acorn gene to develop these amazing growths, providing its larvae with a ready-made nest.
We humans discovered that they make a incredibly long-lasting ink and most our surviving written/drawn history was done in Oak Gall ink.
I foraged these just outside Rome this weekend and will be attempting to make my own ink with them :)