The Book of Mormon is a translated document, originally written in a language which Joseph Smith Jr. described as Hebrew, written in “Reformed Egyptian” characters. As Joseph Smith was not well educated, even in English, how was he able to translate it? The Lord provided him with seer stones, also called the Urim and Thummim.
They are described as “instruments prepared by God to assist man in obtaining revelation and in translating languages. In the Hebrew language the words [Urim and Thummim] mean ‘lights and perfections.’ The Urim and Thummim consist of two stones set in silver bows and sometimes found set into a breastplate.
In Mormon belief, a seer is also a prophet, and a seer is the only person able and authorized to use these seer stones. The Urim and Thummim are mentioned in the Old Testament, at first with Aaron, acting as a Levite Priest, and mentioned occasionally after, also linked with Levite priests. Mormons believe that Abraham received revelation through the Urim and Thummim, and the Book of Mormon mentions some Nephite prophets also in possession of Urim and Thummim.
When Joseph Smith, by the direction of the angel Moroni, found the golden plates (which would be translated into the Book of Mormon) in the Hill Cumorah, he found the Urim and Thummim with them. In Joseph Smith History 1:35, he describes them as follows,
“…there were two stones in silver bows—and these stones, fastened to a breastplate, constituted what is called the Urim and Thummim—deposited with the plates; and the possession and use of these stones were what constituted ‘seers’ in ancient or former times; and that God had prepared them for the purpose of translating the book.”
The golden plates and the Urim and Thummim were sacred. Joseph Smith was to keep them safe until the angel Moroni should call for them. Indeed, although sacred, both the Urim and Thummim and the gold plates were physical objects that could be stolen or lost. The gold plates in particular, when rumor got around of their existence, became a target for would-be thieves and those who wished to discredit the Mormon faith.
Beyond keeping them safe, Joseph Smith was not to show either the Urim and Thummim or the golden plates to anyone, save on the express direction of the Lord. The Three Witnesses and the Eight Witnesses of the Book of Mormon were those permitted to see the sacred work in its golden, untranslated form.
Joseph Smith doesn’t recount the exact process of translation with the Urim and Thumim, but he emphasizes that the ability to translate was a spiritual gift from God. The Urim and Thummim were sacred tools, but intended to be used with the Spirit and God’s direction.
Later, Joseph Smith told Elder Orson Pratt that seer stones were not essential to receive translation or revelation. Rather, they were given to him because he was inexperienced, when he was inexperienced. As Joseph Smith continued to translate and become better acquainted with revelation he no longer needed the seer stones.