The Urim Was an Illuminator
The Urim was a remarkable device placed beneath the twelve transparent gemstones of the High Priest's breastplate. Each stone was engraved with one of the names of the tribes of Israel.
The Urim, which means "lights" or "illuminator," was able to light up as it contained oil. When the High Priest sought God's direction, the Urim would illuminate specific letters on the gemstones, spelling out God's answer and guidance for Israel.