Question: What is the history of the Urim and Thummim in the Americas?
Answer: When the brother of Jared was commanded to seal up a record of his vision of Christ, he sealed up two stones with the record.
King Mosiah had in his possession two stones–the Urim and Thummin–before the people of Limhi discovered the Jaredite record. It is possible that the stones were given to Mosiah at the time a large stone containing engravings was brought to Mosiah (see Omni 1:20-21).
Moroni is commanded to seal up the writings of the brother of Jared: “Behold, I have written upon these plates the very things which the brother of Jared saw; and there never were greater things made manifest than those which were made manifest unto the brother of Jared.
“Wherefore the Lord hath commanded me to write them; and I have written them. And he commanded me that I should seal them up; and he also hath commanded that I should seal up the interpretation thereof; wherefore I have sealed up the interpreters, according to the commandment of the Lord” (Ether 4:4-5).
According to D&C 17:1, the Urim and Thummin possessed by the brother of Jared was given to Joseph Smith on September 22, 1827. The stones were framed in silver bows and fastened to a breastplate.
The three witnesses to the Book of Mormon were to see the plates and other sacred items, including: “...the Urim and Thummim, which were given to the brother of Jared upon the mount, when he talked with the Lord face to face...” (D&C17:1).
Source: 400 Questions and Answers About the Book of Mormon by Susan Easton Black, p. 204.