While there might have been some ancient glass at the time of the Jaredites (see commentary accompanying Ether 2:23-25), no glass is known for Mesoamerica. Therefore, neither Mosiah nor Moroni would have used the term. The closest possibility would likely have been obsidian, given the properties and value of obsidian. We do not know what word was on the plates. Joseph Smith was familiar with seer stones as tools of revelation, and I hypothesize, tried to communicate the spiritual nature of these objects by using language from Revelation 21:21: “And the twelve gates [of the heavenly Jerusalem] were twelve pearls; every several gate was of one pearl: and the street of the city was pure gold, as it were transparent glass.” Of course, gold is no more “transparent glass” than stone is.