Mishnayot Tamid 7:1 says that if the Kohen Gadol entered the Heichal to prostrate himself during the daily service, an honor guard of three kohan would assist him:
בִּזְמַן שֶׁכֹּהֵן גָּדוֹל נִכְנָס לְהִשְׁתַּחֲווֹת, שְׁלשָׁה אוֹחֲזִין בּוֹ, אֶחָד בִּימִינוֹ, וְאֶחָד בִּשְׂמֹאלוֹ, וְאֶחָד בַּאֲבָנִים טוֹבוֹת
When the Kohen Gadol entered to bow, three would hold him, one on his right side, one on his left side and one held on to the precious stones.
R' Ovadia miBartenura explains what "precious stones" we're talking about:
שעל כתפות האפוד
That are on the shoulders of the Apron
The Kohen Gadol's apron had an onyx stone fastened on each shoulder. You can see an artist's rendition of them in this Chabad.org illustration.
It seems to me that it would be quite awkward to help the Kohen Gadol walk by holding these stones.
Why specifically would the other kohen help by supporting these stones? Is there symbolic significance?
How would the other kohen grasp the stones?
Would he pinch-grip them from above? That sounds difficult.
Would he slip his hands under them, between them and the Kohen Gadol's shoulders? I expect that would create an improper separation between the Kohen Gadol and his holy vestements.