John Calvin Commentary Exodus 28:30

"And thou shalt put in the breastplate of judgment the Urim and the Thummim; and they shall be upon Aaron`s heart, when he goeth in before Jehovah: and Aaron shall bear the judgment of the children of Israel upon his heart before Jehovah continually." — Exodus 28:30 (ASV)

And you shall put in the breastplate. From these words, some infer that the Urim and Thummim were distinct from the whole work, which was previously described; others think that they were the twelve stones, because no mention will be made of them when Moses relates that the whole was completed.

But nothing is more probable, as I have already said, than that on the breastplate itself some representation was given of light in doctrine, and of entire uprightness of life. Therefore, after Moses has called it “the breastplate of judgment,” he also speaks of it as “the judgment of the children of Israel.” By this expression, he means a certain and defined system, or an absolutely perfect rule, to which the children of Israel ought to direct and conform themselves.