Seeing as the two onyx-stones have not been mentioned until now, why does the Torah add the word "es"?
Oznayim la'Torah: Because in spite of the rarity of a precious stone large enough on which to engrave the names of six tribes, everyone in the camp knew of the existence of the two stones.
Where exactly, were the two (onyx) stones placed?
Sotah, 36a: On the Kohen Gadol's shoulders (in the golden settings at the ends of the shoulder-straps). 1
See Torah Temimah.
Here it says that the names were in the stones. Pasuk 11 says that the stones were on the names!
Da'as Zekenim: The letters were sunken in. The names were written on (engraved into) the stones, and the stones were raised above the names.