King Mosiah possessed ‘two stones which were fastened into the two rims of a bow, ’ called by the NephitesInterpreters, with which he translated the Jaredite record, and these were handed down from generation to generation for the purposes of interpreting languages. How Mosiah came into possession of these two stones or Urim and Thummim the record does not tell us, more than to say that it was a ‘gift from God.’ [ Mosiah 21:28.] Mosiah had this gift or Urim and Thummim before the people of Limhi discovered the record of Ether. They may have been received when the ‘large stone’ was brought to Mosiah with engravings upon it, which he interpreted by the ‘gift and power of God.’ [ Omni 1:20–21.] They may have been given to him, or to some other prophet before his day, just as the Brother of Jared received them—from the Lord.
That the Urim and Thummim, or two stones, given to the Brother of Jared were those in the possession of Mosiah appears evident from Book of Mormon teachings. The Brother of Jared was commanded to seal up his writings of the vision he had when Christ appeared to him, so that they could not be read by his people. This vision was recorded in a language which was confounded, for it was not to go forth until after the resurrection of Christ. The Urim and Thummim were also sealed up so that they could not be used for the purpose of interpreting those sacred writings of this vision, until such time as the Lord should grant to man to interpret them. When they were to be revealed, they were to be interpreted by the aid of the same Urim and Thummim. [ Ether 3:21–28.]
(Joseph Fielding Smith, Doctrines of Salvation, 3:223–24)