The urim and thummim were used to determine God’s guidance on specific questions. For example, Saul used the urim and thummim to determine who was guilty when he thought someone had sinned.
Therefore Saul said, “O Lord God of Israel, why have you not answered your servant this day? If this guilt is in me or in Jonathan my son, O Lord, God of Israel, give Urim. But if this guilt is in your people Israel, give Thummim.” And Jonathan and Saul were taken, but the people escaped. Then Saul said, “Cast the lot between me and my son Jonathan.” And Jonathan was taken.
This incident shows that the use of urim and thummim was similar to casting lots.
Some people translate the words urim and thummin to mean “curse” and “blessing”, others claim they mean “dark” and “light”. However, the literal meaning seems to be “light” and “perfections”.
The Bible never explains exactly what “urim and thummim” were, but they were attached to the high priest’s breastplate (Exodus 28:30; Leviticus 8:8). It has been speculated that they were black and white stones, but they could also have been made of wood or bone or any number of other materials.
Examples of the use of urim and thummim are found in Numbers 27:21; Deuteronomy 33:8; 1 Samuel 14:41; 28:6; Ezra 2:63 and Nehemiah 7:65. Other scriptures probably also speak of the urim and thummim, although they are not mentioned by name. For example, Joshua 7:14-18 and 2 Samuel 21:1.
They appear to have been lost by the time of the New Testament.