Hello. I'm glad you've turned to AaEx.
It looks like a decorative glass globe or a marble. It's not an acorn from an oak so what other kinds of plants grow in the same vicinity?
I look forward to your reply.
Another gardener who was watching for the answer to this question sent one asking if this was a gall. That started me looking for galls of oak and I believe it is a gall that results from the cynapid wasp. The gall-forming insect lays its eggs within the plant tissue of the host plant. Fluid deposited with the egg causes plant cells to begin multiplying differently than they would otherwise. After hatching, the tiny larvae feed on the plant tissue and create a home surrounded by live plant tissue. The developing larva continues to produce plant growth compounds that maintain and control host plant cell division. The larva matures, pupates within the gall, then chews its way out of the gall when mature.
Most galls do not cause severe damage to the tree so control measures are usually not needed. Here is more information about galls in general from the U of MN Extension: https://extension.umn.edu/yard-and-garden-insects/insect-and-mite-galls#cynipid-gall-wasps-1346960
I hope this helps.