While speaking with Melchizedek, the King of Salem, Santiago remembers that his grandfather had told him that butterflies, crickets, and four-leaf clovers are good omens. Melchizedek validates that memory; however, he also advises him to become more aware of, and to follow through with, any omens that he encounters on his journey to find his treasure. The premise behind omens is that they communicate what the universe, or God, wants us to know in order to fulfill our Personal Legends. Melchizedek advises Santiago to notice the omens, interpret their meaning, and follow through by making the best decisions he can. To help the boy fine tune his skills for interpreting omens, Melchizedek gives Urim and Thummim to Santiago. These two stones can be used to interpret omens by asking "yes" or "no" questions. Santiago uses these stones only once in the story.  

Santiago uses these stones to help him after he is robbed and left with no money to help him in his travels. He asks if the King of Salem's blessing is still with him, and the answer is "yes." He next asks if he will find his treasure, but the stones fall through his pocket to the ground and don't provide a clear answer to his question. From that point on, Santiago decides that he won't use the Urim and Thummim to guide his journey for his treasure because the king's blessing comforts him. Also, he is determined to notice, interpret, and follow the omens by himself. This decision helps him to sharpen his skills during his journey across the desert, in the oasis, and finally at the pyramids in Egypt.

Therefore, the King of Salem teaches Santiago to follow omens on his journey; but he also gives him the Urim and Thummim to help him if he ever feels stuck. Santiago only uses the interpretive stones once. From that time forward, the boy does his best to interpret omens using his own intellect and feelings. He becomes superb at interpreting omens, too. Santiago sharpens his interpretive skills so well that he saves the oasis from an attack, learns how to speak with the natural elements, and saves himself and the Alchemist from dying at the hands of warlords. 

The king opens Santiago's eyes to the possibility that there are signs/omens all around him and that he needs to be on the look out for these as he goes about his life. Sort of like when you first learn a new vocabulary word. Did you ever notice that once you learn a new word, you start noticing that word more? The thing is, that new word you're starting to notice isn't just appearing for the first time around you. It's been there before. You just hadn't noticed it before. Once your eyes have been "opened," you begin to see things that have always been there but you weren't "open" to seeing. And such is life.

Without the king pointing this reality out to Santiago, he might have continued on his journey with no success.