A friend found this ring in their garden the other day, searched the interned for days in order to find analogies, but nothing turned up. Even asked some friends who have more experience with such kind of finds: one of them said that it could be bronze age/Roman, the other said that it's from the 19th/early 20th century.
Some other additional information:
Material: the metal was oxidized black, so I suppose it is made of copper.
Location: Eastern Europe, the region was not part of the Roman Empire, there are mostly Bronze Age and Migration Period (mostly East Germanic tribes) archaeological sites around the area.