In Exodus the Lord tells the prophet Moses, "And thou shalt put in the breastplate of judgment the Urim and Thummim, and they shall be upon Aaron's heart, when he goeth in before the Lord" (Exodus 28:30). The Jewish Targum Pseudo-Jonathan has the following for this passage:
And you shall put the breastplate the Urim, which illuminates their words and make manifest the hidden things of the House of Israel, and Tumim which perfect their deeds, for the High Priest who seeks instruction from the Lord through them.
In the Book of Mormon, Ammon tells King Limhi about a similar class of objects. One who possessed certain stones called "interpreters" by the Nephites was known as a "seer."
By them shall all things be revealed, or rather, shall secret things be made manifest, and hidden things shall come to light, and things which are not known shall be made known by them, and also things shall be made known by them which otherwise could not be known (Mosiah 8:17).
The prophet Alma, who was also the Nephite High priest over the Church described how by means of the interpreters, secret things can be revealed and receive illumination.
behold, the Lord saw that his people began to work in darkness . . . .
And the Lord said: I will prepare unto my servant Gazalem, a stone,
which shall shine forth in darkness unto light, that I may
discover unto my people who serve me, that I may discover unto them the
works of their brethren, yea their secret works, their works of
darkness, and their wickedness and abominations (Alma 37:22-23).