All things geek website Geekologie found an array of cute Portal rings from a 3D printing company, shapeways.
Although the company prints designs that you supply them with, and you'll therefore have to fill in the details with painting skills of your own, the results can be quite nice looking.
It's little more than cheap costume jewelry, but they're fun designs. Someone just needs to make a silver edition based off of these. Head over to Geekologie for more pictures, including some bonus Assassin's Creed rings.
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Update: You actually can order silver versions for most of these. So someone do that and send it to me. Thanks!
I Should Wear More Jewelry: Paint-It-Yourself Portal Rings [Geekologie]