A seer stone is a rock used for divination. Joseph Smith Jr. owned a minimum of two seer stones. He began using a seer stone as early as at least 1819. It was common at that time to use seer stones in attempts to gain revelations from God.
They were also used in attempts to find buried treasure.
Both Joseph Smith Sr. and Jr. were known to use seer stones and had reputations for doing so. Some say they did so successfully, others claim they were conning people out of money. Joseph Smith Jr. would place a seer stone in a white stovepipe hat, then put his face in the hat to block the light to “see” or divine information as a form of revelation from God.
Seer stones were used for “scrying,” a form of divination.
Scrying was used by Biblical figures such as Joseph of Egypt (Genesis 44:1-5), Miriam (see Sefer HaAggadah), the sister of Moses, and the High Priests that wore the Urim and Thummim.
See Urim and Thummim.