In "The Alchemist," what is the purpose of the stone that old man gives him?

The old man gave Santiago stones from a jeweled breastplate which he called Urim and Thummim.  These were considered to be tools of divination (providing divine answers to questions).  While the history of how Urim and Thummim were used differs, in the novel, Santaigo was able to cast the stones to divine what he should do.  In this way, it would be attributed to God's will for Santiago whatever the stones showed, and he was to follow the "advice" of the stones to determine his higher purpose.