What kind of fruit is this?
Found this on the ground under a mix of beech, oak, and some other unidentified trees. It's firm but not hard, easy to cut, no husk, a little over an inch in diameter. Others had similar external markings but were closer to brown or green. Thanks!
We recently got another question about this and agree that this looks like a fruit of some sort but it isn't!
It is actually a type of gall that oaks can get. There are many, many types of galls. Galls are harmless abnormal tissue swellings which can be caused by insects, in this case a small cynipid wasp.
Here is a page about galls: https://extension.umd.edu/hgic/shade-tree-galls
This one looks like the aptly named 'plum gall'.
There is nothing you need to do.