Here are a couple of comparison images. These were made for the new movies. Your ring, however, could be anywhere from 1955 when the books were first published to the present.
that ring is from the Lord of the Rings movies. I found one identical to it last summer in a local park. Your's looks darker than mine does though, mine has a hint of gold on it so mine is modern, your's could be older than that. Congrats on the find though!
I have no clue to what the ring is but that is a sweet find. I would love to find a ring like that wether it is gold or not. Like the other guy said put it on and see if you disapear!! If so you can go hunting and every one will leave you alone. People will just think the gofors are out digging in their yards. You could hit some great spots that way. Best of luck and hope you find more.