It is not known for certain exactly what the Urim and Thummim really were, but it is thought that they may have been two precious stones, possibly gems, which were identical in shape. One or the other could be drawn from the pouch in order to provide a yes or no answer in seeking the Lord for guidance.

Exod 28:30 "And you shall put in the breastplate of judgment the Urim and the Thummim, and they shall be over Aaron's heart when he goes in before the LORD. So Aaron shall bear the judgment of the children of Israel over his heart before the LORD continually."

Since Scripture explicitly states that the Urim and Thummim were placed in the breastplate, it would seem that they were separate from the twelve stones mounted on the outside. The name Urim means "lights, " while Thummim means "perfections;" and these meanings have led some to speculate that perhaps the stones flashed in a particular way to indicate "yes" or "no. "

"We can draw no other conclusion than that the Urim and Thummim are to be regarded as a certain medium, given by the Lord to His people, through which, whenever the congregation required divine illumination to guide its actions, that illumination was guaranteed. When God was displeased with His people in later history, He refused to permit the Urim and Thummim to function as a means of guidance. Apparently in a day when man lacked most of the the revelation of the Word of God, he required some other source of information of divine will." [Keil and Delitzsch Commentary on the Old Testament]