The Two Sardonyx Stones
Two sardonyx stones were fixed in settings of gold on the High Priest’s shoulders; one on the right, and one on the left. The names of the tribes of Israel were engraved upon these two stones, according to the instructions of the verse: “And you shall take two sardonyx stones, and engrave upon them the names of the children of Israel; there shall be six names on one stone, and six names on the second stone in the order of their birth.” (Ex. 28:9-10)
The Bible calls these two stones “remembrance stones,” as it is written, “And you shall place the two stones on the two shoulder pieces of the ephod as remembrance stones for the children of Israel. And Aaron shall carry their names before G-d on his two shoulders as a remembrance.” (Ibid. v. 12)
The sages explain the meaning of this expression: when the High Priest entered into the holy place dressed in the ephod, the Holy One saw all the tribes of Israel inscribed before Him and He was moved to have mercy on His people.
Another two square gold settings were fixed on the High Priest’s shoulders, directly under the sardonyx stones. Golden chains extended from these settings to the golden hooks in the rings of the breastplate, in order to fix the breastplate to the ephod.