King James Bible Dictionary

A hail; claw; hoof, (Heb. shoham), a precious stone adorning the breast-plate of the high priest and the shoulders of the ephod (Exodus 28:9-12, 20; 35:27; Job 28:16; Ezekiel 28:13). It was found in the land of Havilah (Genesis 2:12). The LXX. translates the Hebrew word by smaragdos, an emerald. Some think that the sardonyx is meant. But the onyx differs from the sardonyx in this, that while the latter has two layers (black and white) the former has three (black, white, and red).

Precious stone
Job 28:16; Ezekiel 28:13

Used in erecting the temple
1 Chronicles 29:2

Seen in the foundations of the city of the New Jerusalem in John's apocalyptic vision
Revelation 21:20

Exported from Havilah
Genesis 2:12

Used in the breastplate
Exodus 28:9-12; Exodus 28:20; Exodus 39:6; Exodus 39:13

Contributed by Israelites for the priests' garments
Exodus 25:7; Exodus 35:9

(a nail) is the translation of the Hebrew shoham ; but there is some doubt as to its signification. Some writers believe that the "beryl" is intended; but the balance of authority is in favor of some variety of the onyx. ("The onyx is not a transparent stone, but as the color of the flesh appears through the nail (Greek onyx) on the human body, so the reddish mass which is below shines delicately through the whitish surface of the onyx. There are several varieties. White and reddish stripes alternating form the sardonyx; white and reddish gray, the chalcedony. When polished it has a fine lustre, and is easily wrought into a gem of great beauty."-Rosenmiller.

ON'YX, noun [Gr. a nail. Latin onyx ] A semi-pellucid gem with variously colored zones or veins, a variety of chalcedony.