car·bun·cle 
(kärbŭng′kəl)
(kärbŭng′kəl)n.
1. A painful localized bacterial infection of the skin and subcutaneous tissue that usually has several openings through which pus is discharged.
2.
a. A red precious stone, especially a deep-red garnet cut as a cabochon.
b. A mythical gemstone said to emit light even in total darkness.
[Middle English, from Old French, from Latin carbunculus, small glowing ember, carbuncle, diminutive of carbō, carbōn-, coal; see ker-3 in the Appendix of Indo-European roots.]
carbun′cled adj.
car·buncu·lar (-kyə-lər) adj.