What is the X in X-mas?
early 13c., "fiery jewel," from Old North French carbuncle (Old French charbocle, charboncle) "carbuncle-stone," also "carbuncle, boil," from Latin carbunculus "red gem," also "red, inflamed spot," literally "a little coal," from carbo (genitive carbonis) "coal" (see carbon). Originally of rubies, garnets, and other red jewels; in English the word was applied to tumors from late 14c.
carbuncle car·bun·cle (kär'bŭng'kəl)
A deep-seated pyogenic infection of several contiguous hair follicles, with formation of connecting sinuses, often preceded or accompanied by fever, malaise, and prostration.
(Ex. 28:17; 39:10; Ezek. 28:13). Heb. barkath; LXX. smaragdos; Vulgate, smaragdus; Revised Version, marg., "emerald." The Hebrew word is from a root meaning "to glitter," "lighten," "flash." When held up to the sun, this gem shines like a burning coal, a dark-red glowing coal, and hence is called "carbunculus", i.e., a little coal. It was one of the jewels in the first row of the high priest's breastplate. It has been conjectured by some that the garnet is meant. In Isa. 54:12 the Hebrew word is _'ekdah_, used in the prophetic description of the glory and beauty of the mansions above. Next to the diamond it is the hardest and most costly of all precious stones.