From the Wikipedia article on the Urim and Thummim:
In the Latter Day Saint movement, the Urim and Thummim (/ˈjʊərɪm …
Smith said that in 1823 the angel Moroni told him of the existence, with the plates, of “two stones in silver bows” fastened to a breastplate, which the angel called the Urim and Thummim and which he said God had prepared for translating the plates. Smith’s mother, Lucy Mack Smith, described them as crystal-like “two smooth three-cornered diamonds.” Oliver Cowdery said the stones were “transparent”.
As Depicted in Book of Mormon AR
William Smith, brother to Joseph Smith, said the Urim and Thummim were connected to the breastplate with a rod, and that a person wearing the breastplate could lean forward and look through the “interpreters”, as they’re called in the text of the Book of Mormon.