Exodus 25:7

gems of beryl that are gems of perfection, for engraving and insertion in the ephoda and in the breastplate.

AVNEI SHOHAM’ (ONYX STONES) ‘V’AVNEI MILU’IM’ (AND STONES FOR SETTING). “Because they used to make a setting for the stone in gold — a kind of indentation — and they placed the stone there to fill the indentation, they are called avnei milu’im (stones for filling-in), and the place of the indentation is called mishbetzet (setting). FOR THE EPHOD AND THE BREASTPLATE. The onyx stones were for the ephod, and the stones for setting were for the breastplate.” This is Rashi’s language. Similarly Rashi explained: “They shall be inclosed in gold in their settings, — surrounded by gold settings of such a depth that each setting shall be filled by the thickness of the stone. This is the meaning of the word milu’otham (their filling), and this is the meaning of every expression of milui (filling) mentioned in this connection.”
But it does not appear to me to be at all correct to say that Scripture calls them already now avnei milu’im, because in some time as yet in the future He was to command that the indentations made for them were to be filled with these stones! Moreover, the onyx stones were also enclosed in settings of gold,, Verse 11 — in the case of the ephod. and yet He did not call them milu’im. Again, our Rabbis have already said in the Gemara: “These stones are not incised with a chisel, for it is said b’milu’otham (in their filling).” Now if the explanation b’milu’otham would be, [as Rashi has it], that they fill the indentations with the stones, there would have been no proof at all from this verse that they should not incise the names [of the tribes] on them with a chisel. Besides, the manner of the setting of the stones which the Rabbi [Rashi] mentioned, namely that it was made in a kind of indentation, is not correct. Instead, it is as Onkelos rendered it: [meshubatzim zahav — inclosed in gold] meramtzan didhav, which means that they made at the bottom a gold setting according to the measure of the stone, and from it they projected a fork-like shape of three prongs which would hold the stone. [The word meramtzan mentioned by Onkelos] is similar to the expressions of the Sages: “and they take it out beramtza (with a pointed tool) made of iron;” “as when he bored a hole in it beramtza (with a pick) made of iron.” This is how they also do it today when they set precious stones in rings, in order that they be seen from all angles and …

מלואים, for the gemstone to fill the recessed area intended for it to be set in, similar to the rings used as signet rings.

אבני שהם, “onyx stones;” these, and the jewels which follow, were required for the breastplate and the epaulets of the garments of the High Priest. How precisely did this work? The two onyx stones were affixed each to one of the shoulder pieces of the ephod. (Exodus 28,9 as well as 28,12) The stones collectively described as אבני מלואים, “jewels to be placed within the twelve settings provided for them on the breastplate of the High priest.” (Exodus 28,9). These jewels are referred to euphemistically on numerous occasions in the Bible, perhaps the best known example being Psalms 113,5-6: המגביהי לשבת המשפילי לראות בשמים ובארץ, “Who, though enthroned high, sees what is below, and Who sees what is in heaven and what is on earth.” You find an onyx stone mentioned also in Exodus 28,20, where it is one of the jewels on the breastplate of the High Priest.