Holiday reading is a great source of interesting vocabulary. This turned up in S M Stirling’s time-travel story, Island in the Sea of Time, in which Nantucket Island is thrown back 3000 years through an unspecified event: “You could see smudges of black woodsmoke drifting out over the smaragdine brightness of the harbor.” Poul Anderson’s There Will be Time, another time-travel story, also includes it: “On a transverse axis, vision reached from glittering blue across the Sea of Marmora to a mast-crowded Golden Horn and the rich suburbs and smaragdine heights beyond.”
Fittingly, the term is ancient. It means an emerald-green colour. It probably derives from Sanskrit marataka for an emerald (though a Semitic source has also been suggested). This was taken into Greek as smaragdos, then to Latin smaragdus and thence into English as smaragd as a name for the same stone. Smaragdine came from Latin smaragdinus, relating to the emerald. Emerald also comes from the same source, via Old French e(s)meraud.
Its most famous appearance in literature is the Smaragdine Table of Hermes Trismegistus, an emerald-green tablet supposedly found by Alexander the Great in the tomb of Hermes, though it was actually a late medieval forgery. On it was said to be written, in Phoenician characters, the 13 precepts underlying alchemy, including (in this translation from Latin by Sir Isaac Newton), “That which is below is like that which is above and that which is above is like that which is below to do the miracles of one only thing.”