Here’s probably the best word on the Urim and Thummim. It’s a couple of paragraphs from Dr. Bruce Waltke’s excellent book (which I highly recommend), entitled, “Finding the Will of God” (pp. 62-64):
“The priest could use the urim and thummin to determine God’s will in a particular situation. We are not exactly sure what the urim and thummin were, but the priest carried in his breastplate perhaps two sticks or stones, one white and the other black, that would give a yes or no answer to a specific question. Should Israel be preparing for battle, they would somehow shake or toss the sticks. If they turned up black the Israelites would not go to battle, and if they turned up white they would proceed into battle with the knowledge that they were in the will of God. That is one form of divination that God allowed in the Old Testament. We read in Exodus 28:30, “Also put the Urim and the Thummin in the breastpiece, so they may be over Aaron’s heart whenever he enters the presence of the Lord. Thus Aaron will always bear the means of making decisions for the Israelites over his heart before the Lord.”
Much has been made of the urim and thummin by modern mystics, who want to find in them the key to the divine mind. All sorts of fanciful explanations have been put forward, including that the items may have glowed, that they had secret words engraved on them, or that they were ancient artifacts with magical powers. However, it should be noted that 1 Samuel 28:6 makes clear a definite answer was not always obtainable, so it may not have been as simple as tossing two stones on ground. Moses never used them; they were given for the high priest in aiding those who could not find God’s guidance any other way.
Some translate the words urim and thummin to mean “curse” and “blessing,” others simply “dark” and “light,” although the literal translation seems to be “light” and “perfections.” There is no proof that there were only two items; some early rabbis believed that the urim and thummin were a series of stones with Hebraic characters on them by which the Lord could spell out a message for the high priest. However, most scholars believe them to be two sticks or stones, perhaps precious stones, that God used in a miraculous way to reveal His will. They were used for national decisions like going to war, and for priestly matters.
The Old Testament seems to indicate that the urim and thummin faded from use during the early days of Israel’s monarchy, and are only referred to once after the Babylonian exile. This may be so because the institution of monarchy God inaugurated the office of prophet. The prophets now participated in God’s heavenly court and communicated God’s messages to the courts in Jerusalem and Samaria. Apparently prophets who revealed God’s word to the king replaced the urim and thummin, through which He revealed His mind to the priest. Nevertheless, we still find Ezra using this device to determine the ancestry of the priests who returned from the exile in Ezra 2:63. After this the Bible never mentions the urim and thummin again. God did not preserve it for His people. They are one more allowance from God to assist His people at a certain point in history.”