Probably not the most practical every day wear, but it's fun.
I made this customizable using ID in mm. I didn't bother setting up a table of ring sizes, so you can use something like this ring size table to convert the sizes. I found I needed to add about 0.3mm to get it to fit comfortably, but YMMV.
Monoprice Maker Select Mini
I've done a couple prints of this. The one with the open part down makes a nicer diamond, but a weaker ring. Doing it sideways makes a slightly messier diamond, but a stronger ring. This seems like a good compromise.
I made a little diamond shaped planter and it was received with "this would make a nice ring!" She liked it, so I put a ring on it.