These are Oak Marble Galls. They aren't naturally occurring. They are actually chemically induced to form by the Gall Wasp for the rearing of a single wasp larva. Eventually the young wasp wakes up, breakfasts on the inner lining (very nutritious), chews a hole in the gall and emerges. If you look very closely at the bottom gall in the picture below you can just about see the exit hole.
They used to make inks and dyes from Tannic Acid which can be extracted from the galls. Also it is said that ground up galls mixed with hog's lard is a great cure for piles. I'll leave the testing of this remedy to someone else.
I’m kind of liking this theme. - Those are the infamous koi. Maddy is the white one. Ari the gold. Byakuya is the black one. Koi’s faces are a little strange when they lengthen their faces...
10 hours ago