“Sardius” is an older name for a red or reddish-brown form of quartz. The King James Version uses this word in Ezekiel 28:13, 39:10, and Ezekiel 28:13.
The King James Version (and others) also uses “sardius” in Revelation 4:3. Many modern Bible translations use “carnelian” instead. These names may refer to the same or a similar red stone.
See also Carnelian; Precious Stones.