[Joseph Fielding Smith]
Your letter addressed to the President of the Church of Jesus Christ of Latter-day Saints, was referred to me as the historian for answer.
You desire to know what the Urim and Thummim were, how they were used, and what became of them after the translation of the Book of Mormon.
The Urim and Thummim obtained from Moroni were two transparent stones set in silver bows and these stones fastened to a breastplate constituted what is called the Urim and Thummim. It was by the aid of these and by the power of the Lord that Joseph Smith was able to translate the ancient characters from the record and give to the world the Book of Mormon. In making the translation the breastplate was adjusted over the breast with the two stones attached in such a way that they could be held before the eyes. Jut how the characters appeared or how the translation was received by aid of these stones the Prophet Joseph Smith has not told us. All that we have on record that he did say is that he translated the ancient record by the aid of the Urim and Thummim and by the gift and power of God. When he finished the translation it is understood that he returned the Urim and Thummim with the plats to the angel who placed them in his hands.
The Urim and Thummim and breastplate were in the possession of Aaron, who was called to act as the presiding high priest of the order of Aaron, before the congregations of Israel. These stones with the breastplate were handed down from generation to generation in the lineage of the priests, successors to Aaron, until the time came when, through in and apostasy, they were lost to the people of Israel. For references see Exodus 28:30, Leviticus 8:8, Numbers 27:21; Deuteronomy 33:8, I Samuel 28:5, Ezra 2:63 and Nehemiah 7:65. Revelations were frequently received through the Urim and Thummim in those ancient times.
It was revealed to Joseph Smith, while he was translating the Book of Abraham, that Abraham had the Urim and Thummim by which the heavens were opened to his view and many things were made known in prophetic vision concerning Gospel truth and the destiny of his posterity.
The Urim and Thummim give to Joseph Smith were those which were received by the brother of Jared from the Lord the account of which is recorded in the third chapter of the Book of Ether in the Book of Mormon. After the destruction of the people of Jared these stones, called 'interpreters,' were preserved in the hands of the Nephites, finally being placed in the tone box with the plates from which the Book of Mormon was translated. It was Moroni who hid up these plates and the Urim and Thummim, about 420 A.D., and as the custodian of these sacred things, was sent, after he had received his resurrection from the dead, to Joseph Smith the Prophet and revealed to him where the plates were buried and gave him instruction in relation to the coming forth of the history of the ancient peoples of this western hemisphere.
[Joseph Fielding Smith, Letter to Ellsworth McCarthy, as quoted in Minutes of the Apostles of The Church of Jesus Christ of Latter-day Saints, 1910-1951, Privately Published, Salt Lake City, Utah 2010]