Exodus 28:15-30

The breastplate] This is really a pouch, one span, or half a cubit, square, made of the same material as the ephod, and ornamented on the outside with twelve jewels set in four rows, each stone being engraved with the name of a tribe. The pouch is intended to hold the Urim and Thummim, by means of which God's judgments are declared (see on Exodus 28:30), and is therefore here called the breast-plate of judgment.