σαρδόνυξ (L, σαρδιόνυξ), -υχος, ὁ (< σάρδιον, ὄνυξ), sardonyx, a stone marked by the red of the sard and the white of the onyx: Rev.21:20.† (AS)
Thayer:
1) a sardonyx, a precious stone marked by the red colours of the carnelian (sard) and the white of the onyx
σαρδόνυξ
sardonux
sar-don'-oox
From the base of G4556 and ὄνυξ onux (the nail of a finger; hence the “onyx” stone); a “sardonyx”, that is, the gem so called