King James Bible Dictionary

Heb. nophek (Exodus 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 (Revelation 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."

A precious stone.

Color of the rainbow
Revelation 4:3

Merchandise of, in Tyre
Ezekiel 27:16; Ezekiel 28:13

Set in the breastplate
Exodus 28:18

Symbolic, in the foundation of the holy city
Revelation 21:19

a precious stone of a rich green color, upon which its value chiefly depends. This gem was the first in the second row on the breastplate of the high priest. (Exodus 28:18; 39:11) It was imported to Tyre from Syria, (Ezekiel 27:16) was used as a seal or signet, Ecclus. 32.6, as an ornament of clothing and bedding, (Ezekiel 28:13; Judges 10:21) and is spoken of as one of the foundations of Jerusalem. (Revelation 21:19) Tob. 13.16. The rainbow around the throne is compared to emerald in (Revelation 4:3)

EM'ERALD, noun [Latin smaragdus.] A mineral and a precious stone, whose colors are a pure, lively green, varying to a pale, yellowish, bluish, or grass green. It is always crystallized, and almost always appears in regular, hexahedral prisms, more or less perfect, and sometimes slightly modified by truncations on the edges, or on the solid angles. It is a little harder than quartz, becomes electric by friction, is often transparent, sometimes only translucent, and before the blowpipe is fusible into a whitish enamel or glass. The finest emeralds have been found in Peru.

The subspecies of emerald are the precious emerald and the beryl.