The high priest Aaron and his successors wore an ornate item called a breastplate (“breastpiece” in some translations). Worn on the chest, it was a square of fine fabric, attached to the priest’s clothes with gold chains.
Mounted in the breastplate were twelve gems, each representing one of the tribes of Israel and engraved with its name. Thus when the priest entered the Holy of Holies, he would bear the names of the Israelites over his heart. The piece is described in Exodus 28:15–30.