This is the Mare in the Moon ring I won at the charity auction at Canterlot Gardens for $420. I know it's a bit glarey, but it's better then using my phone or 3DS.
In full, it's a sterling silver ring and band, with a clear quartz stone. Inside the stone you can see the lunar image of Nightmare Moon, and on the side of the ring is NMM's image. It's open ended which means it can fit anyone with a little work.
I met the maker of this ring and actually roomed with him at CG, so he showed me it beforehand, and when I saw it, I wanted it! >:3 You can see him here at his DA page:
http://chaoticbrony.deviantart.com/