Carbuncle

Carbuncle \KAHR-buhng-kuhl\ , noun;
1. Pathology: A painful circumscribed inflammation of the subcutaneous tissue, resulting in suppuration and sloughing, and having a tendency to spread somewhat like a boil, but more serious in its effects
2. A gemstone, especially a garnet, cut with a convex back and a cabochon surface
3. Also called London brown, a dark grayish, red-brown color
adjective;
1. Having the color carbuncle

This word dates to the early 13th century with the definition "fiery jewel." It comes from Old Northern French carbuncle ("carbuncle-stone" or "carbuncle, boil") which derives from Latin carbunculus ("red gem" or "red, inflamed spot"). Carbunculus is literally "a little coal" from carbo ("coal"), which derives from Proto-Indo-European *ker- ("heat, fire, to burn"). Carbo is also the forebear of carbon. In English, this word originally only applied to red gems like rubies and garnets. Using carbuncle to refer to tumors and boils dates to the late 14th century.