or, Peau d'Ane (Po' Dan).
I ain't read no Balzac (yet), but the featured article on Wikipedia today tells me that he's got a book about "a magic piece of shagreen that fulfills [one's] every desire. For each wish granted, however, the skin shrinks and consumes a portion of [one's] physical energy."
I can't imagine this hasn't been used in our game before, and it's gonna be used again. By me. What I dig is, as Wikipedia also tells me, that shagreen (chagrin in French, how's that for a suggestive name for a magic item?) is leather from a horse's back or an onager--a wild ass. And nowadays from shark or ray skin. It keeps getting AWESOMER.
That reminds me of the Donkey Skin story, about the princess who, to avoid marrying the king her father (what the fuck kind of fairy tale is this!), insisted that he first make her a dress the color of the weather--and the dress of blue had cumulus clouds drifting across it--then the color of moonlight--and the dress shone silver--then the color of the sun--and the dress made everything around it shine. I don't remember how it ends. I hope it's with the old bastard in the dungeon.
I guess there should be a donkey skin in the story somewhere. I forgot that bit. Anyone remember? I know. Wikipedia.