Σμάραγδος (Smaragdos) is supposed to have been named from μαίρω or μαρμαίρω (to twinkle or sparkle), whence the dog-star was called Μαῖρα (Maira).  This beauteous precious stone, bearing the colour of hope, was further recommended to Christians because the rainbow of St. John's vision was "in sight like unto an emerald."  Thus, Smaragdos was one of the early martyrs; and the same occurs occasionally in early times, once as an exarch of Ravenna; but it was never frequent enough to be a recognized name, except in two very remote quarters, namely, as the Spanish Esmeralda and the Cornish Meraud, the last nearly, if not quite, extinct. (History of Christian Names, Yonge, 1884)