Pencil and eraser rings
Shown at the Holon Institute of Technology industrial design department's "Next Exit" exhibition in Tel Aviv, these "Pencil Case Rings" replace the standard jewels with tiny pencil and eraser nubbins. Looks like a great DIY project for someone with some casting skills.
It might be awkward to use, but if someone made a ring with a smaller writing tip — a gel pen, perhaps — I could see myself replacing the pen I always keep clipped to my shirt with something similar.
Pencil case rings [Flickr via Coolest Gadgets]

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I made one of these over a year ago, using Friendly Plastic and pencil lead.