
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.[2] Smith’s mother, Lucy Mack Smith, described them as crystal-like “two smooth three-cornered diamonds.”[3] Oliver Cowdery said the stones were “transparent”.[4]
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.