Perhaps the most enigmatic element of the clothing of the High Priest was the breastplate, known in Hebrew as the choshen, which was attached to the ephod, or apron, of the High Priest.
Made of cloth (wool dyed blue, purple, and crimson) that was woven with linen and gold thread, the choshen was two zerets long (half a cubit, or a total of 19 inches) and one zeret (9.5 inches) wide, folded over to create a square. It was adorned with 12 stones which were mounted in gold and inscribed with the names of the tribes, arranged in four rows of three columns. Additionally, the breastplate contained the names of Abraham, Isaac and Jacob, and the words shivtei Yeshurun (“tribes of Jeshurun”). According to the Talmud, wearing the choshen atoned for the sin of errors in judgment on the part of the Children of Israel.
There is much debate concerning the precise identity of the stones and how they correlated to the tribes. According to one opinion, the first row held Bareket (Carbuncle) representing Levi, Pitdah (Prase) for Shimon, and Odem (Ruby) for Reuben. The second row held Yahalom (Pearl) for Zebulun, Sappir (Sapphire) for Issachar, and Nofech (Emerald) for Judah. The third row held Achlamah (Crystal) for Gad, Shevo (Turquoise) for Naphtali, and Leshem (unclear identity) for Dan. The final row contained Yashfeh (Jasper) for Benjamin, Shoham (Onyx) for Joseph, and Tarshish (Chrysolite) for Asher. Two more stones were sewn on the shoulder straps of the ephod (the priestly apron), etched with the names of the Twelve Tribes. According to a rabbinic tradition, the names of the twelve tribes were engraved upon the stones by a mystical creature called a shamir, described as a tiny worm the size of a barley grain that had the remarkable ability to cut stone.
Its very name, chosehn mishpat (the breastplate of judgment), implied that the breastplate was not a passive piece of clothing. The Torah states that inside the breastplate should be placed the Urim and Thummim, words that have no precedent in the Bible. A hint at the purpose of the choshen is given in Numbers:
But he shall present himself to Elazar the Kohen, who shall on his behalf seek the decision of the Urim before Hashem. By such instruction they shall go out and by such instruction they shall come in, he and all the Israelites, the whole community.” Numbers 27:21
From this verse we see that questions were posed to God through the Urim and Thummim. Rashi explains that inside the choshen was an inscription of the name of God which was placed between the folds of the breastplate through which it made its statements clear; Urim derived from the word for light and Thummim derived from Tam, or ‘true’.
Inquiries were made of the Urim and Thummim while the Kohen was wearing the choshen. The Talmud explains that these questions were posed quietly and not in a loud voice. One tradition teaches that the stones lit up, using the inscribed names of the tribes to spell out the answer.
The ability to receive Divine answers via the breastplate ceased with the destruction of the First Temple. There was a breastplate in the Second Temple as it is forbidden for the priests to perform the Temple service without all of the eight Biblically mandated clothes. However, it is believed that in the Second Temple the choshen of the High Priest lacked the name of God and therefore did not have the ability to reveal answers.