Charles Ellicott Commentary Exodus 25:7

"onyx stones, and stones to be set, for the ephod, and for the breastplate." — Exodus 25:7 (ASV)

Onyx stones. —The Hebrew shoham is rendered here by “sard” (Septuagint), “sardonyx” (Vulgate and Josephus), and “beryl” (Rosenmüller and others). In Job 28:16, the same word is rendered by the Septuagint “onyx.” There is thus considerable doubt what stone is meant. Only three such stones seem to have been required as offerings: one for the high priest’s breastplate (Exodus 28:20), and two for the shoulder pieces of the ephod (Exodus 28:9–12).

Stones to be set in the ephod and in the breastplate. —Hebrew, stones of insertion for the ephod and for the breastplate. The stones of the ephod were only two, both probably either onyx or sardonyx; those of the breastplate were twelve in number, all different (Exodus 28:17–20).