Parley Pratt described Joseph translating the BOM with "two transparent stones, clear as chrystal."
Parley P. Pratt, "Discovery of an Ancient Record in America," Millennial Star 1 no. 2 (June 1840): 30
With this record was found a large breastplate, apparently of copper, such as had been worn for defence; and also a curious instrument, called by the ancients the Urim and Thummim, which consisted of two transparent stones, clear as chrystal, set in the rims of a bow of silver. This was in use, in ancient times, by persons called seers; it was an instrument by the use of which they received revelation of things distant, or of things past or future. By this means, and by this gift of the Almighty, Mr. Smith was enabled to translate the record into the English language, and it was first printed and published in 1830.