An emerald, the size of three fists, and burning with an inner light. A most worthy prize for any adventuring party.
You find a book in the library, then when you start to read it, seems to change from pages of words to pages of images that seem to live and move. If you touch the page after the images appear, you travel into the story and now live in that world.
There is a book on that world, that leads back to this one.