I went for a walk in the woods today, and on a tiny oak tree, I found this wonderful cluster of, so called, Oak Apples. Though of course they are not fruit, but galls, a sort of tumor formed when the tree is infected by a parasitic gall wasp in the family Cynipidae. The true acorns, (Seeds ) inset, are nearly gone now.
The Galls were much harvested in the past, because they are exceptionally high in tannin, presumably the trees attempt to fight of the wasps. The tannin content was valued because when mixed with iron sulfate it formed a strong dark blue/black dye, which in ancient times formed the basis of one of the best and most long lasting inks. They do look very like fruit though.