The church believes that this is the stone that Joseph used to translate parts of the Book of Mormon. This would be the stone that he put into a hat, then immersed his face into the hat and would then dictate what he saw to his various scribes. Many account support this form of translation. Joseph found this stone in the ground “years before he retrieved the gold plates.” This is a different “instrument” than the Urim and Thummim, though some accounts may have used the term Urim and Thummim when referring to the single stone.
Here are some links on the rocks!