Need Chronicle help/ideas about Lesser Ring (long)

I wish to introduce a Lesser Ring into my late 3rd Age chronicle. It takes place 10 years before the War of the Ring. The PC found a "dwarven puzzle box" long ago but couldn't decipher it. After chasing down a group of Greenway Bandits that stole from Bree's mayor, she found the box in their stash. Basically it was a steel box decorated in dwarven runes that acts as a Tolkienesque rubik's cube. It requires a Games TN of 20 to open. I put some backround into it, saying they were treasured pieces of artwork and were only given to the most wealthy and powerful lords of the dwarves. Once the puzzle is solved, a gift of rare beauty and wonder lay inside as a present to the dwarf in question.
Eventually she will run into a dwarf who has the skill to open the box, but when she opens it she'll find a simple mithril band, decorated in elvish runes. A lesser ring.
I wanted the lesser ring to have the powers of being able to detect precious metals such as gold, silver, and mithril if they existed within 100' of the wearer of the ring. Is this too powerful for a lesser ring? It's history would simply be as an elvish token of friendship to the dwarves, given to them to help in their mining. The Elves never knew that Sauron's corrupting influence was upon it of course. It would be it's only power, but the longer one wore it and used it for this purpose the more corruption they'd gain, along with the Grasping flaw. The adventure will start when her and her companions are traveling through Mirkwood and a Nazgul begins to hunt them down, drawn to the ring. They might seek to return the ring to it's dwarven owner, but the dwarves are already looking for it-and when they realise what it is they will become greedily drawn to it. Is this ring's magic appropriate for Tolkien's world or is it too powerful an item? That's my main concerns, questions/critiques welcome. Thanks!

That doesn't seem too far-fetched or too powerful to me. I'd be sure to not make the ring's abilities too "D&D-like" - by that I mean you might want to figure out what happens when one puts it on, and how one "knows where to dig" all of a sudden. Is it only usable by dwarves? You don't want a +5 Ring of Mining or whatever. It should be subtle and...persuasive. I imagine it might create or make use of the users desire for riches.

I used a lesser ring of my own devising in a Middle-earth game I ran a few years back. It was a ring Sauron had made to summon and communicate with Thuringwethil...