carbuncle (n.)

early 13c., "fiery jewel, gem of a deep red color, ruby," also the name of a semi-mythical gem from the East Indies formerly believed to be capable of shining in the dark, 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 used of red, eruptive subcutaneous inflammations and tumors from late 14c. Also "red spot on the nose or face caused by intemperance" (1680s).