very interesting. too big to be a button I would think, plus no sign of a shank ever being there. Plus, on the etched cross side, I see what look to be tiny rim denticles, common on what you would see on an old coin or jetton. I assume it's brass? If so, I would say a 1600s jetton is a strong possibility though normally these aren't bigger in diameter than a half dollar--- though some were (this is assuming the cross was etched in on the worn jetton)! Then again, could be some kind of old decorative broche type thing?