Should we assume the other side is "blank" since you didn't show it? My first thought is button-- it looks like a button. What's the size?
I'm "torn" here. It's either late Byzantine emperor Buttonicus or early English King George the Buttonister Earl of plaid shirtland!
It looks like a coin of Mary Queen of Scots. Notice you can read MARIA DEI https://www.the-saleroom.com/en-us/...0026/lot-c46db74d-0743-46d8-83ff-a5090119e40d
@Orfew is right, it is a plack of Mary, Queen of Scots. It's a shame the date's not there, but it's interesting that it's not countermarked.