“[Joseph] possessed a seer stone, by which he was enabled to translate as well as from the Urim and Thummim, and for convenience he then used the seer stone.” — Deseret Evening News, November 30, 18811
Joseph translated the Book of Mormon "by the gift and power of God"2 and used several different tools.
One of these tools was the Urim and Thummim. The Urim and Thummim, with the Hebraic roots meaning "Lights and Perfections," has been referenced in both the Bible (e.g., Abraham, Aaron) and the Book of Mormon (the brother of Jared).3 Joseph described the Urim and Thummim as "two stones in silver bows" fastened to a breastplate and said that "God had prepared them for the purpose of translating the book."4
Joseph also used a seer stone to translate. It was a small oval stone he'd discovered years before, and one he'd also used to search for lost items or buried treasure.1
When translating, Joseph would place either the Urim and Thummim or the seer stone in a hat, press the hat to his face, and read the translation out loud.1