Sardius takes its name from Sardis, the wealthy capital of ancient Lydia in Asia Minor, a city ancient sources associate with an early and substantial supply of the deep red-orange stone reaching Mediterranean markets. The naming pattern, a gem called after the trading city most associated with it rather than after its actual place of geological origin, recurs elsewhere in ancient mineral names and reflects how such stones typically reached buyers: through a market's reputation rather than through direct knowledge of where in the earth the stone was actually found.
As the first stone named in Exodus 28's list of twelve, assigned to Reuben, Jacob's firstborn son, sardius holds the opening position in the breastplate sequence, a placement several commentators have read, like jasper's closing one, as meaningfully paired with the birth order of the tribe it represents.
The Hebrew name works on a different principle from the Greek one. The word is odem, built on the root meaning red, the same root behind adom for the colour and behind the names Adam and adamah, the ground. Where the Greek and Latin traditions named the stone after the market it came through, the Hebrew names it after the one property a viewer cannot mistake, which makes odem one of the more secure readings in the breastplate list even where the exact mineral remains open.
That security is relative rather than absolute. Naming a stone by its colour narrows the field to the red chalcedonies and rules out very little within it, so the Hebrew tells us the stone was red without telling us which red stone it was.