Green Stone Marcasite Sterling Ring

This is a vintage ring and it features a green Stone which I believe is a green onyx. Or, it could be a chalcedony. It also has marcasite in the middle of the gemstone and it is marked 925. The ring is a size 8. You can probably see from the photo that there is one tiny marcasite missing. But when it's on the hand, you can't notice it at all. So I will reflect this in the price.