The breastpiece (hō’šěn) is described in Exodus 28:15. It was roughly a 25 cm square, made of similar material to the ephod. It was also studded with twelve jewels which symbolized the twelve tribes of Israel. It also included pouches for the Urim and Thummim.
8 And he placed the breastpiece on him, and in the breastpiece he put the Urim and the Thummim.