What Are the Urim and Thamim?

Since urim is thought to mean lights, it's been proposed that it was a light colored stone. And these may have had the words urim and thumim written on one face. How can we sort through these possibilities? One of the things to consider is what you could call the band width issue. That is, how much data ther im andThumim provided with a single answer? At the low end of the spectrum, the various yes, no, this or that answers could suggest that they conveyed only one bit of information per consultation. But thats just speculation.