Quick Definition
σάρδιον, ου, τό n.n. carnelian
Strong's Definition
sardian (3037 being implied), i.e. (as noun) the gem so called
Derivation: properly, an adjective from an uncertain base;
KJV Usage: sardius
Thayer's Greek Lexicon
σάρδιον, -ov, τό, [neut. of σάρδιος, see below], sard, sardius, a precious stone, of which there are two kinds, concerning which Theophr. de lapid. 16, 5, § 30 ed. Schneid. says, τοῦ yap σαρδίου τὸ μὲν διαφανὲς ἐρυθρότερον δὲ καλεῖται θῆλυ, τὸ δὲ διαφανὲς μὲν μελάντερον δὲ καὶ ἄρσεν, the former of which is called carnelian (because flesh-colored; Hebr. 078, Sept. σάρδιον, Ex. xxviii. 17; XXXVi. 17 (xxxix. 10); Ezek. xxviii. 13; αἱματόεντα σάρδια, Orph. de lapid. 16, 5), the latter sard: Rev. iv. 3 (Ree. σαρδίνῳ); xxl. 20GLTTr WH. Hence the adj. σάρδιος, -a, -ov, [fr. Σάρδεις, cf. Plin. ἢ. π. 37, 7] sardine se. λίθος (the full phrase occurs Ex. xxxv. 8 [var.]): Rev. xxi. 20 Ree. [Β. Ὁ. 58. vv. Sardine, Sardius.]
Mounce Concise Greek Dictionary
σάρδιον sardion 2x
carnelian, a reddish precious stone, Rev. 4:3; 21:20*
Abbott-Smith Greek Lexicon
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.†
Moulton & Milligan — Vocabulary of the Greek NT
Liddell-Scott-Jones — Greek-English Lexicon
σάρδιον (sardion) — gem. 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+]