Lexical Definition

‎σάρδιος‎ sárdios (sar'-dee-os) gem ^G:N-N
properly, an adjective from an uncertain base

sardius.

• sardian (3037 being implied), i.e. (as noun) the gem so called

Abbott-Smith

σάρδιον, -ον, τό [in LXX for אֹדֶם, Ex 28:17, al.; also for שֹׁהַם ;]
the sardian stone, sard (of which carnelian is one variety): Re 4:3 21:20.†

Abbott-Smith References

Liddell-Scott-Jones

σάρδιον, τό,

the Sardian stone, [NT+5th c.BC+]; as a seal, [Refs 4th c.BC+]; σάρδια, of female ornaments, [Refs 5th c.BC+]—This stone was of two kinds, the transparent-red or female being our carnelian, the transparent-brown or male our sardine, [Refs 4th c.BC+]:—later λίθος σάρδιος, [Refs 6th c.AD+] in “de An.” 321.10 (plural); σάρδινος λ., [Refs]; σαρδόνιον, [Refs 5th c.AD+]

_{LSJ Scripture Refs: Rev.4.3}