Mosiah was a seer and used this power to translate the record of the Jaredites. What is interesting is that Mosiah acted as seer and not Alma. It is apparent that Mosiah had the power of seership well before Alma showed up in Zarahemla (Mosiah 8:13-14). However, when Alma arrived, Mosiah apparently recognized his capacity for spiritual leadership and gave him power over the churches of Zarahemla (Mosiah 25:19). He was given stewardship over all church administrative matters. This is especially evident in the incident of the sinners of the church found in Mosiah 26. Mosiah, as king, declined judgment to Alma who was apparently the presiding authority for the churches (Mosiah 26:12). In this situation, we see Alma inquiring for divine communication on church government and receiving an answer directly from the Lord. Yet in Mosiah 28, we see the king and seer, Mosiah, inquiring of the Lord regarding the missionary requests of his sons. Mosiah also received an answer directly from the Lord. He also translated the record of the Jaredites as a seer, which we know is a calling greater than that of a prophet. Thus, Mosiah was the king, prophet, and seer, and Alma was the prophet to whom was given administrative stewardship over the church. Their roles were different but equally necessary.
“Two Stones Which Were Fastened into the Two Rims of a Bow”
Here, we get a rare description of the Urim and Thummim (literally “lights and perfections”). This must have been the Urim and Thummim given to the brother of Jared (Ether 3:23-4). The Urim and Thummim consisted of a breastplate to which were attached some sort of metal pieces, called silver bows, which positioned the stones before the face of the interpreter. The stones spoken of became like translucent lenses through which the interpreter looked in order to read and translate other languages. The best physical description of an Urim and Thummim is as follows: “two transparent stones set in the rim of a [silver] bow fastened to a breast plate” (HC 4:537; JS-H 1:35).
Joseph Fielding Smith
"The Lord gave to the brother of Jared the Urim and Thummim which he brought with him to this continent. These were separate and distinct from the Urim and Thummim had by Abraham and in Israel in the days of Aaron. …
"…King Mosiah possessed ’… two stones which were fastened into the two rims of a bow‘ called by the Nephites ’Interpreters,’ with which he translated the Jaredite record, and these were handed down from generation to generation for the purpose 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 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)’ 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.
"BROTHER OF JARED AND MOSIAH HAD SAME STONES
“That the Urim and Thummim, or two stones, given to the brother of Jared were those in the possession of Mosiah appears evident…” (Answers to Gospel Questions, vol 1, pp. 159-161)