Okay, the ring was actually made of gold. Nor did it give Charlie Duke dominion over Middle Earth.
According to the Apollo 16 Flight Journal:
219 25 49 Duke (onboard): Guess what I caught floating out the hatch?
219 25 51 Young (onboard): What?
219 25 52 Mattingly (EVA): What's that -
219 25 53 Duke (onboard): A ring.
219 25 54 Mattingly (EVA): Oh, is that right?
219 25 56 Duke (onboard): Yeah. I think it's yours.
219 25 57 Duke (onboard): (Laughter)
219 25 58 Young (onboard): Yeah, it is.
219 25 59 Duke (onboard): Here, hold it, John.
219 26 00 Young (onboard): That's it, all right. We got it.
What is the backstory behind this ring? Who did it belong to, and why was it floating loose?