The Targum Pseudo-Jonathan on (Exodus 28:19) lists the third row of the breastplate: ligure, and agate, and amethyst, engraved with Gad, Asher, and Issachar. The tribes of this row were, in Jacob's blessings, the tribes of abundance and depth. Gad would be a warrior, a troop shall overcome him (Genesis 49:19). Asher's blessing was his bread shall be fat, and he shall yield royal dainties (Genesis 49:20). Issachar bent his shoulder to bear and became the tribe of scholars who knew the seasons.
The Sages treated the amethyst, the final stone in this row, as especially suggestive. Its Aramaic name, ein egla, means calf's eye, and some traditions linked it to Issachar, whose sons were said to have calm, clear-sighted minds fit for Torah study. The third row, in other words, became a row of inner life. Warrior, host, and scholar stood together, three different ways of sustaining a people.
The takeaway is that the middle of the breastplate was reserved for the middle work of a nation, the unspectacular labor of feeding, protecting, and studying, without which kings and judges have nothing to govern.