This is one of those small but precious possessions. It's a Roman ring key. Not a ring for keeping keys on, but a key that is worn as a ring. Probably for a jewelry box or small casket.
Tumbler locks have been around since something like 1000BCE, and were quite common throughout much of the ancient world. In Babylon and Greece, they were long bronze rods, and women would carry them resting on their shoulders. Rome refined keys and locks considerably, making them both smaller and more beautiful. They became status symbols for people with property to protect. Locks for small boxes, not only doors, became possible, and resulted in pieces like my own ring.
I, of course, bought it as a symbol of Hekate. If you, too, are a follower of hers, you can find Roman keys relatively inexpensively -- often $50 or less -- on eBay. (I know, that's not actually inexpensive, but for a two-thousand-year-old object it is!)
You can see the way the teeth of the key are shaped, much more intricately than the earlier keys shown at some of those links.
The key itself actually stands quite a ways off from the finger. I can't help but think that it must have caught on things fairly often, so it may not have been worn as jewelry, but been hidden away someplace. And, as you can see, the ring itself is quite small. I can only fit it on my pinkie finger.