Charles Ellicott Commentary Exodus 35:27

"And the rulers brought the onyx stones, and the stones to be set, for the ephod, and for the breastplate;" — Exodus 35:27 (ASV)

The rulers brought onyx stones, and stones to be set. The "rulers" intended here are probably the "princes of the tribes" of Israel (Numbers 1:16; Numbers 3:3; Numbers 3:5, and other similar passages). The twelve stones required for the breastplate would naturally have been contributed by the twelve chiefs of the tribes whose names they were to bear (Exodus 28:21). The two onyx stones for the ephod (Exodus 28:9–12) may have been an additional gift from two of their number, who happened to possess stones of the large size required.