Excerpt from Joseph Fielding Smith’s, Doctrines of Salvation, 3:225: ¹

SEER STONE NOT USED IN BOOK OF MORMON TRANSLATION.
We have been taught since the days of the Prophet that the Urim and Thummim were returned with the plates to the angel. We have no record of the Prophet having the Urim and Thummim after the organization of the Church. Statements of translations by the Urim and Thummim after that date are evidently errors. The statement has been made that the Urim and Thummim was on the altar in the Manti Temple when that building was dedicated. The Urim and Thummim so spoken of, however, was the seer stone which was in the possession of the Prophet Joseph Smith in early days. This seer stone is now in the possession of the Church.

While the statement has been made by some writers that the Prophet Joseph Smith used a seer stone part of the time in his translating of the record, and information points to the fact that he did have in his possession such a stone, yet there is no authentic statement in the history of the Church which states that the use of such a stone was made in that translation. The information is all hearsay, and personally, I do not believe that this stone was used for this purpose. The reason I give for this conclusion is found in the statement of the Lord to the Brother of Jared as recorded in Ether 3:22-24

These stones, the Urim and Thummim which were given to the Brother of Jared, were preserved for this very purpose of translating the record, both of the Jaredites and the Nephites. Then again the Prophet was impressed by Moroni with the fact that these stones were given for that very purpose. It hardly seems reasonable to suppose that the Prophet would substitute something evidently inferior under these circumstances. It may have been so, but it is so easy for a story of this kind to be circulated due to the fact that the Prophet did possess a seer stone, which he may have used for some other purposes.

Excerpt from the LDS Gospel Topic Essay, ‘Book of Mormon Translation’: ²

The other instrument, which Joseph Smith discovered in the ground years before he retrieved the gold plates, was a small oval stone, or “seer stone.” As a young man during the 1820s, Joseph Smith, like others in his day, used a seer stone to look for lost objects and buried treasure. As Joseph grew to understand his prophetic calling, he learned that he could use this stone for the higher purpose of translating scripture.

Apparently for convenience, Joseph often translated with the single seer stone rather than the two stones bound together to form the interpreters. These two instruments—the interpreters and the seer stone—were apparently interchangeable and worked in much the same way such that, in the course of time, Joseph Smith and his associates often used the term “Urim and Thummim” to refer to the single stone as well as the interpreters. In ancient times, Israelite priests used the Urim and Thummim to assist in receiving divine communications. Although commentators differ on the nature of the instrument, several ancient sources state that the instrument involved stones that lit up or were divinely illumined. Latter-day Saints later understood the term “Urim and Thummim” to refer exclusively to the interpreters. Joseph Smith and others, however, seem to have understood the term more as a descriptive category of instruments for obtaining divine revelations and less as the name of a specific instrument.