"And thou shalt put in the breastplate of judgment the Urim and the Thummim; and they shall be upon Aaron's heart, when he goeth in before the LORD: and Aaron shall bear the judgment of the children of Israel upon his heart before the LORD continually."
.............................Exodus 28:30

...........................The breastplate was made of the same materials as the ephod and it was doubled or folded to form a bag to hold the Urim and Thummim.

..........................................The Urim and Thummim were precious stones bearing significant names which no one has ever known.