Ezra Booth says that Joseph used the same dark glass to translate the BOM as he did for treasure digging.
Ezra Booth, "Mormonism no. III," Ohio Star (Ravenna, Ohio), October 27, 1831
[I]n translating.—The subject stands before his eyes in print, but it matters not whether his eyes are open or shut; he can see as well one way as the other. You have probably read the testimony of the three witnesses appended to the Book of Mormon. These witnesses testify, that an angel appeared to them, and presented them the golden plates, and the voice of God declared it to be a Divine Record. To this they frequently testify, in the presence of large congregations. . . .These [buried] treasures were discovered several years since, by means of the dark glass, the same with which Smith says he translated most of the Book of Mormon.