I have not seen any thimbles, even ones many centuries old with that shape, so it is most likely a button, and might even be a German or French button at that........
Here is a good thimble site to look at and judge for yourself.
The last time I seen something like that there were two fo them,,, sitting on the front of Madona...
Oops sorry,,, I don't have a clue. Interesting though...
Good luck, & Happy hunting~