Reference: Sardine, Sardius
Morish
odem, ???????. One of the precious stones in the breastplate. Ex 28:17; 39:10. It also embellished one of the foundations of the holy Jerusalem. Re 21:20. It was one of the stones that covered the king of Tyrus (doubtless portraying Satan before his fall). Eze 28:13. In heaven One who sat upon the throne was "to look upon like a jasper and a sardine stone." Re 4:3. It is supposed to be the sard, a superior variety of agate, of various colours, some blood-red, and others of a golden hue.
See Verses Found in Dictionary
Smith
(red) (Heb. odem) the stone which occupied the first place in the first row of the high priest's breastplate.
The sard, which is probably the stone denoted by odem, is a superior variety of agate, sometimes called camelian, and has long been a favorite stone for the engraver's art. Sardis differ in color: there is a bright-red variety, and perhaps the Hebrew odem from a root means "to be red," points to this kind.