Heb. nophek (Ex. 28:18; 39:11); i.e., the “glowing stone”, probably the carbuncle, a precious stone in the breastplate of the high priest. It is mentioned (Rev. 21:19) as one of the foundations of the New Jerusalem. The name given to this stone in the New Testament Greek is smaragdos, which means “live coal.”
Dictionary entry
Emerald
Easton's Bible Dictionary