Source : Webster's Revised Unabridged Dictionary (1913)

Carbuncle \Car"bun*cle\, n. [L. carbunculus a little coal, a
   bright kind of precious stone, a kind of tumor, dim. of carbo
   coal: cf. F. carboncle. See {Carbon}.]
   1. (Min.) A beautiful gem of a deep red color (with a mixture
      of scarlet) called by the Greeks anthrax; found in the
      East Indies. When held up to the sun, it loses its deep
      tinge, and becomes of the color of burning coal. The name
      belongs for the most part to ruby sapphire, though it has
      been also given to red spinel and garnet.

   2. (Med.) A very painful acute local inflammation of the
      subcutaneous tissue, esp. of the trunk or back of the
      neck, characterized by brawny hardness of the affected
      parts, sloughing of the skin and deeper tissues, and
      marked constitutional depression. It differs from a boil
      in size, tendency to spread, and the absence of a central
      core, and is frequently fatal. It is also called
      {anthrax}.

   3. (Her.) A charge or bearing supposed to represent the
      precious stone. It has eight scepters or staves radiating
      from a common center. Called also {escarbuncle}.

Source : WordNet®

carbuncle
     n 1: deep-red cabochon-cut garnet cut without facets
     2: an infection larger than a boil and with several openings
        for discharge of pus