Onyx stones - We have already met with the stone called שהם shoham, Genesis 2:12, and acknowledged the difficulty of ascertaining what is meant by it. Some think the onyx, some the sardine, and some the emerald, is meant. We cannot say precisely what it was; possibly it might have been that fine pale pebble, called the Egyptian pebble, several specimens of which now lie before me, which were brought from the coast of the Red Sea, and other parts in Egypt, by a particular friend of mine, on purpose to add to my collection of minerals. Stones to be set in the ephod - אבני מלאים abney milluim, stones of filling up. Stones so cut as to be proper to be set in the gold work of the breastplate.
The אפד ephod - It is very difficult to tell what this was, or in what form it was made. It was a garment of some kind peculiar to the priests, and ever considered essential to all the parts of Divine worship, for without it no person attempted to inquire of God. As the word itself comes from the root אפד aphad, he tied or bound close, Calmet supposes that it was a kind of girdle, which, brought from behind the neck and over the shoulders, and so hanging down before, was put cross upon the stomach, and then carried round the waist, and thus made a girdle to the tunic. Where the ephod crossed on the breast there was a square ornament called חשן choshen, the breastplate, in which twelve precious stones were set, each bearing one of the names of the twelve sons of Jacob engraven on it. There were two sorts of ephods, one of plain linen for the priests, the other very much embroidered for the high priest. As there was nothing singular in this common sort, no particular description is given; but that of the high priest is described very much in detail Exodus 28:6-8. It was distinguished from the common ephod by being composed of gold, blue, purple, scarlet, fine twisted linen, and cunning work, i.e., superbly ornamented and embroidered. This ephod was fastened on the shoulders with two precious stones, on which the twelve names of the twelve tribes of Israel were engraved, six names on each stone. These two stones, thus engraved, were different from those on the breastplate, with which they have been confounded. From Calmet's description the ephod seems to have been a series of belts, fastened to a collar, which were intended to keep the garments of the priest closely attached to his body: but there is some reason to believe that it was a sort of garment like that worn by our heralds; it covered the back, breast, and belly, and was open at the sides. A piece of the same kind of stuff with itself united it on the shoulders, where the two stones, already mentioned, were placed, and it was probably without sleeves. See Clarke on Exodus 28:2 (note), etc.

Onyx stones,.... So called from their likeness to the nail of a man's finger: the Targum of Onkelos calls them stones of beryl; and the Targum of Jonathan gems of beryl; and the Septuagint version, stones of sardius; and some take them to be the sardonyx stones, which have a likeness both to the onyx and to the sardius:
and stones to be set in the ephod, and in the breastplate; two onyx stones were set in the ephod, one of the garments of the high priest, and an onyx stone, with eleven other precious stones, were set in the breastplate of the high priest: these stones were doubtless among the jewels set in gold and silver the Israelites had of the Egyptians, and brought with them out of Egypt.

ephod--a square cloak, hanging down from the shoulders, and worn by priests.

Lastly, precious stones, שׁהם אבני probably beryls (see at Genesis 2:12), for the ephod (Exodus 28:9), and מלּאים אבני, lit., stones of filling, i.e., jewels that are set (see Exodus 28:16.). On ephod (אפד), see at Exodus 28:6; and on חשׁן, at Exodus 28:15. The precious stones were presented by the princes of the congregation (Exodus 35:27).