sardonyx

Boyd’s Bible Dictionary:

A precious stone combining the sard and onyx varieties, whence its name
(Rev. 21:20).

Concise Bible Dictionary:

A precious stone associated with one of the foundations of the holy Jerusalem (Rev. 21:20). The word does not occur in the AV of the Old Testament. Aquila, in his Greek version, uses it for the onyx in Genesis 2:12. It is judged to be a variety of chalcedony, or of agate, of various colors, with stripes of a different shade.
Chalcedony Var. Agate

Strong’s Dictionary of Greek Words:

Number:
4557 (find all occurrences in KJV Bible)
Greek:
σαρδόνυξ
Transliteration:
sardonux
Phonic:
sar-don’-oox
Meaning:
from the base of 4556 and ὄνυξ (the nail of a finger; hence the "onyx" stone); a "sardonyx", i.e. the gem so called
KJV Usage:
sardonyx

Jackson’s Dictionary of Scripture Proper Names:

ruddy: (finger) nail colored (?)