Easton's Bible Dictionary

Onyx - a hail; claw; hoof, (Heb. shoham), a precious stone adorning the breast-plate of the high priest and the shoulders of the ephod (Ex. 28:9-12, 20; 35:27; Job 28:16; Ezek. 28:13). It was found in the land of Havilah (Gen. 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).