A seer, also known as a prophet, is the only person authorized to use these special interpreters. The Urim and Thummim and a breastplate are mentioned occasionally in the Old Testament, but it is evident that they were lost or taken from the Jews before the time of Christ. Some of the prophets of the Nephites in the Book of Mormon are also mentioned as having seer stones.
When Joseph Smith was given the responsibility to translate the Book of Mormon he used the Urim and Thummim. He gave a brief description of them in Joseph Smith History 1:35: 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. Joseph Smith does not give any accounts of the exact process used to translate with the Urim and Thummim, but instead stresses the fact that his ability to translate was a gift from God, and through the Spirit.
Joseph Smith also later told Elder Orson Pratt that the seer stones were not essential to translation or revelation, and that they were provided for him when he was inexperienced in receiving revelation. As Joseph Smith continued to translate and become better acquainted with revelation he no longer needed them.
Richard L. Evans, a professor at Brigham Young University, elaborated on the use of seer stones in the translation process in an article in the Ensign Magazine