If I recall correctly, that's the one you get from the Hermit in the forest. He has a book, a helm, and the acorn and will trade you for them.
If that's the right one, I've never found a use for it but I also have been hanging on to it and would like to know. Normally, you can't sell quest items but you can sell things like the dragon scale which can be used to make armor so I'm not sure if this is a keeper or not.
--- Apathy: It's not just a way of life. It's.... uhhhh... ah, who cares....
I've always just assumed it'll be used for something somewhere, but then again, I also assumed that my story-loving bard from Orlais would like a story book about a queen from native country, but both of these things are apparently wrong.