[1] For the story see Herodotos 3.41–2 (web address 1). The present allusion to it is unattributable. (Adler suggests Aelian.) The writer's assertion that the recovery of the emerald was a proof of divine favour seems to misunderstand the point of Hdt.'s story, since it was in fact a warning of inevitable destruction -- unless we are meant to understand the warning as a boon.
[2] Adler reports smaragdeion metallon ("emerald mine") in the Ambrosian Lexicon (595), which seems preferable both in itself and given the fact that the plural phrase "emerald mines" occurs five times in Heliodorus.