What were the Urim and Thummim and how did the priest use them?
The Urim and Thummim were evidently two little gemstones placed in a pouch behind the ephod, though Scripture does not define them. When a yes-or-no decision was needed, the priest drew them out, perhaps cast them like dice, and their outcome was God's answer.
Les explains that the Urim and Thummim—which he calls "umen and thumen"—were evidently two little gemstones kept in a pouch behind the ephod. Scripture does not define them. When a yes-or-no decision was required, the priest would pull them out and perhaps throw them on a table like dice; however they ended up, that was God's answer. He connects this to the choosing of Matthias, where the disciples cast lots and the lot fell on Matthias, who was numbered with the eleven as God's choice.