http://sidneyrigdon.com/dbroadhu/IL/sain1872.htm
Edward Stevenson (1881), “One of the Three Witnesses,” reprinted from Deseret News, 30 Nov. 1881 in Millennial Star, 44 (6 Feb. 1882): 86-87.
Martin Harris, also one of the Three Witnesses to the Book of Mormon, provided this information to his friend Edward Stevenson, who would later become part of the LDS First Council of Seventy. Martin Harris related an incident that occurred during the time that he wrote that portion of the translation of the Book of Mormon which he was favored to write direct from the mouth of the Prophet Joseph Smith. He said that the Prophet possessed a seer stone, by which he was enabled to translate as well as from the Urim and Thummim, and for convenience he then used the seer stone, Martin explained the translation as follows: By aid of the seer stone, sentences would appear and were read by the Prophet and written by Martin and when finished he would say “Written,” and if correctly written that sentence would disappear and another appear in its place, but if not written correctly it remained until corrected, so that the translation was just as it was engraven on the plates, precisely in the language then used.
Description of the spectacles:
“The two stones set in a bow of silver were about two inches in diameter, perfectly round, and about five-eighths of an inch thick at the centre; but not so thick at the edges where they came into the bow. They were joined by a round bar of silver, about three-eighths of an inch in diameter, and about four inches long, which, with the two stones, would make eight inches.”
http://www.olivercowdery.com/smithhome/1850s/1859Tiff.htm#pg163a
Emma:
“Now the first that my husband translated, was translated by the use of the Urim and Thummim, and that was the part that Martin Harris lost, and that he used a small stone, not exactly black, but was rather a dark color…” (Letter dated March 27, 1876 from Emma to Mrs. Pilgrim, now in the library of the Reorganized Church).