From the base of sardios and onux (the nail of a finger; hence the "onyx" stone); a "sardonyx", i.e. The gem so called -- sardonyx.
see GREEK sardios
STRONGS NT 4557: σαρδόνυξσαρδόνυξ (Lachmann σαρδιονυξ), σαρδονυχος, ὁ, (σάρδιον and ὄνυξ), sardonyx, a precious stone marked by the red colors of the carnelian (sard) and the white of the onyx (B. D. under the word; Riehm, HWB, under the word Edelsteine 12): Revelation 21:20. (Josephus, Plutarch, Ptolemy, others; (Genesis 2:12 Aq. (Montf.)).)