This is the Brick Ring sold by Shapeways user COVO. It's available in a variety of metals (starting at $41 for steel) and was designed to attach LEGO blocks to. That way you can customize your fiance's engagement ring with all bricks she desires. I'm going propose with an entire LEGO pirate ship set attached to it -- there's no way she'll be able to resist!
UPDATE: She said no. She also said I've got a minifig sized penis even though she's told me before it's at least the size of a G.I. Joe.
Keep going for a video in case you're having a hard time imagining what a ring with a bunch of different LEGO bricks stuck to it might look like.
Thanks to Mitch, who plans on buying one for every finger and building an entire medieval castle on his knuckles.