Boyd’s Bible Dictionary:
(little coal). A gem of deep red color (Isa. 54:12). A stone in the high-priest’s breastplate (Ex. 28:17; 39:10).
Concise Bible Dictionary:
Two Hebrew words are so translated.
1. eqdach, a stone of a fiery sparkling nature (Isa. 54:12).
2. bareqeth, a stone of a glittering brightness (Ex. 28:17; Ex. 39:10; Ezek. 28:13).
Cabochon Carbuncle Garnet
Strong’s Dictionary of Hebrew Words:
Meaning:
from 6916; burning, i.e. a carbuncle or other fiery gem
Jackson’s Dictionary of Scripture Proper Names:
I will kindle (only in Isa. 54:12) : lightning stone: (literally she shot forth)