Strong’s Greek
G4557 — sardonux
The entry below is Strong’s own definition (1890, public domain), and every verse where the word occurs.
σαρδόνυξ
sardonux — pronounced sar-don'-oox
from the base of 4556 and onux (the nail of a finger; hence the "onyx" stone); a "sardonyx", i.e. the gem so called:-- sardonyx. see GREEK for 4556
1 occurrence in the KJV.
Revelation 21:20
The fifth, sardonyx; the sixth, sardius; the seventh, chrysolyte; the eighth, beryl; the ninth, a topaz; the tenth, a chrysoprasus; the eleventh, a jacinth; the twelfth, an amethyst.