I have no idea, I too thought it may be something glued on. However it doesn't seem to be. It looks as though it is one piece. Odd looking lump of metal almost looks like an old, very small snap button. I'm puzzled so thought I'd give this forum a go and see if I could get any answers or if anyone has seen anything like this before.
It seems to resemble a rivet and could have been intended for jewelry , as has been said , or attached to something else. I found this wheat penny that seems to have been used in a similar manner though a screw was used instead of a rivet . 1925 screwed by stoneman227, on Flickr
First, welcome to the neighborhood! One thing you will learn as you spend more time in this hobby is that there are infinitely more people who would rather deface or alter coins than there are those who would try to preserve them. Chris
I found a dime with this exact mark and it’s only on the back as well. Mine is a 2002 version and it is one with the dime and not added on
