Take these," said the old man, holding out a white stone and a black stone that had been embedded at the center of the breastplate. "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.
The Alchemist - 10th Anniversary Edition (ed. HarperSanFrancisco, 1998)