On the south side [shall be] the standard of the camp of Reuben, according to their armies,.... This camp consisted of the tribes of Reuben, Simeon, and Gad, which was divided into lesser bodies, regiments, or companies, called armies; and this was placed to the south of the tabernacle, or to the right hand, as Aben Ezra interprets it; this was the right wing of the whole army.
Reuben's stone, according to the Jewish writers , was the "sardius", and his flag was coloured red, and there were figured upon it mandrakes. Simeon's stone was the topaz, and his flag was coloured green, and on it was figured "shechem".
Gad's stone was the agate, and his flag was coloured neither black nor white, but mixed, and there was figured upon it a camp or host, on account of what is said, Ge 49:19;and the captain of the children of Reuben [shall be] Elizur the son of Shedeur; see Nu 1:5.
Bemidbar Rabba, sect. 2. fol. 178.