Exodus 28:30, Urim and Thummim

Also put the Urim and the Thummim 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. 

//Urim and Thummim were objects used as an ancient Israelite means of divination. Ever wonder what these objects looked like? We have no idea. They trace back to at least the 8th century BC, referenced in the book of Hosea. Their use seems to have disappeared prior to Babylonian captivity. The Talmud explains that they were lost when Jerusalem was sacked by Babylon.

1 Samuel chapter 14 finds God giving Saul the silent treatment, and Saul finally decides God must be miffed about a sin committed by Israel. He gathers the leaders of his army and stands them together, while he and his son Jonathan stand apart. Then he inquires of God "by lot" to determine the innocent party. He and Jonathan are selected. He inquires again between he and Jonathan, and finds his son to be guilty. (Jonathan was supposed to be fasting, and snuck a taste of honey).

We assume this “casting of lots” referred again to Urim and Thummim. They appear to have been small objects belonging to the high priest, worn on the breastplate, or perhaps in a pouch or pocket inside the breastplate. Some picture them to have been tiny tablets of bone or wood. Textual scholars believe the name Thummim derives from the root word meaning innocent, while Urim derives from a root meaning cursed. Most likely, the high priest put his hand into the pocket, swirled it around a bit, and randomly chose one of the two, determining the party's innocence or guilt. To determine a sinner from a group of people, the priest could divide the crowd in two, use his U&T to determine the side with the guilty party, and repeat the process until God had singled out one person.