From the Wikipedia article on Joseph Smith’s seer stone:
According to Latter Day Saint theology, seer stones were believed to have been used by Joseph Smith, as well as ancient prophets, to receive revelations from God. Members of The Church of Jesus Christ of Latter-day Saints believe that Joseph Smith used a seer stone in the Book of Mormon translation effort.[1][2]
Seer stones are mentioned in the Book of Mormon in the Book of Mosiah, where they are also called “interpreters” and described as being used by seers to translate and receive revelations. The term “Urim and Thummim” is usually used by Latter Day Saints members to refer to the “interpreters” mentioned in the Book of Mormon. Some Latter Day Saints use the term Urim and Thummim and seer stones interchangeably.[3]
The Actual Seer Stone
In 2015, the Church of Jesus Christ of Latter-day Saints published pictures of the seer stone that Joseph Smith used to translate the Book of Mormon.