The Friend briefly describes the seer stones and how they were used.

Because of his spiritual nature and his willingness to learn the truth, Joseph Smith was tested and found worthy to be the translator of the Book of Mormon. To help him with the translation, Joseph found with the gold plates “a curious instrument which the ancients called Urim and Thummim, which consisted of two transparent stones set in a rim of a bow fastened to a breastplate.”Joseph also used an egg-shaped, brown rock for translating called a seer stone. The translating was done at Peter Whitmer’s home, a friend of the Prophet’s where Oliver Cowdery, Emma Smith (Joseph’s wife), one of the Whitmers, or Martin Harris wrote down the words spoken by the Prophet as soon as they were made known to him.Martin Harris said that on the seer stone “sentences would appear and were read by the Prophet and written by [the one writing them down] and when finished [that person] would say ‘written;’ and if correctly written, the sentence would disappear and another take its place; but if not written correctly it remained until corrected, so that the translation was just as it was engraven on the plates.”See Full