What is the X in X-mas?
c.1300, from Latin calcedonius, in Vulgate translating Greek khalkedon in Rev. xxi:19, found nowhere else. Connection with Chalcedon in Asia Minor "is very doubtful" [OED]. The city name is from Phoenician and means "new town."
Mentioned only in Rev. 21:19, as one of the precious stones in the foundation of the New Jerusalem. The name of this stone is derived from Chalcedon, where it is said to have been first discovered. In modern mineralogy this is the name of an agate-like quartz of a bluish colour. Pliny so names the Indian ruby. The mineral intended in Revelation is probably the Hebrew _nophekh_, translated "emerald" (Ex. 28:18; 39:11; Ezek. 27:16; 28:13). It is rendered "anthrax" in the LXX., and "carbunculus" in the Vulgate. (See CARBUNCLE.)