Modern gemology generally identifies sardius with sard, a deep brownish-red variety of chalcedony, itself closely related to and sometimes not sharply distinguished from carnelian, the more orange-red variety that appears as its own separate entry in this atlas. The distinction between sard and carnelian is a matter of shade and depth of color rather than a difference in mineral structure, and ancient writers did not always draw the line where modern gemologists now do, which is part of why "sardius," "sard" and "carnelian" overlap so heavily in older translations and commentaries.
The King James Bible's choice of "sardius" for the Hebrew odem preserves an older English name for what would, in a modern gem catalogue, most likely simply be called carnelian or sard, making the distinction between this entry and this atlas's separate Carnelian entry primarily one of historical, biblical naming rather than of the stone itself.
Whatever it is called, the stone has one of the longest continuous records of use of any material in this atlas. Carnelian beads were traded out of the Indus valley into Mesopotamia in the third millennium BC, including the distinctive etched beads produced by treating the surface with an alkaline paste before heating, examples of which were excavated from the royal graves at Ur.
Egyptian use is comparably early and better documented in a religious setting: carnelian appears constantly in amulets and inlay, and the red of the stone was one of the working colours of Egyptian funerary craft rather than an occasional choice. A material that available, and that widely worked, was always going to end up in a priestly vestment somewhere.