A word in the Gnomish Lexicon of the 1910s for “oak, oaktree”, also usable for “oakwood (mat)” (GL/42). Its derivation is unclear since it has no obvious Early Qenya cognate, but Tolkien did compare it to G. {gofron >>} govron “acorn” so it might have originally been intended to be a reduction of that word, but Tolkien deleted the entry for govron (GL/42). Given all the elaborations of grôn “oak”, I think it likely that it is an adjective “*oaken” rather than a noun so that (for example) grondrui “oakwood” = “*oaken wood”.