Lucy Mack Smith describes the Urim and Thummim as having two smooth, three cornered diamonds set in silver bows.
Lucy Mack Smith, History, 1844–1845, [7]–[8], bk. 5, The Joseph Smith Papers website, accessed January 3, 2023
Joseph saw this and followed me Mother said he do not be uneasy all is right see here Said he I have got the key I knew not what he meant but took the article in my hands and upon after examing it <*> <(*with no covering but a silk handkerchief)> <found> that it consisted of 2 smooth <3 cornered diamonds set in glass and the glass was set in silver bows stones con[n]ected with each other in the same way that old fashioned spectacles are made