What two stones does the king of Salem give Santiago and what do they represent in The Alchemist?

The king of Salem gives
Santiago a white stone and a black stone to help him read the omens to find his
treasure.

Santiago is a young shepherd boy in search of
treasure.  He had a dream that there was treasure, and a gypsy told him to find it.  On his
quest, he ran into the king of Salem, who told him that he needed to find his Personal Legend
and gave him two stones.  One was white and the other black.

"They are called Urim and Thummim. The black signifies 'yes,' and the white 'no.'
When you are unable to read the omens, they will help you to do so. Always ask an objective
question. (part 1)

The king also tells him that he needs
to make his own decisions.  Even though he gives him the stones to help him read the omens, he
stresses that Santiago needs to decide things for himself, and he alone is responsible for his
fate.

One of the most important philosophies of the book is the concept of
the personal legend.  The king of Salem tells Snatiago that people know their personal legends
when they are young, but forget them as they get older.  He wants Santiago to maintain his
focus.