Boyd’s Bible Dictionary:
(stone of Sardis). The sard or carnelian, a blood-red or flesh-colored stone, first in first row of high priest’s breastplate (Ex. 28:17; Rev. 4:3).
Concise Bible Dictionary:
One of the precious stones in the breastplate (Ex. 28:17; Ex. 39:10). It also embellished one of the foundations of the holy Jerusalem (Rev. 21:20). It was one of the stones that covered the king of Tyrus—doubtless portraying Satan before his fall (Ezek. 28:13). In heaven One who sat upon the throne was “to look upon like a jasper and a sardine stone” (Rev. 4:3). It is supposed to be the sard, a superior variety of agate, of various colors, some blood-red, and others of a golden hue.
Strong’s Dictionary of Greek Words:
Meaning:
properly, an adjective from an uncertain base; sardian (3037 being implied), i.e. (as noun) the gem so called
Jackson’s Dictionary of Scripture Proper Names:
ruddiness