The Collaborative International Dictionary of English v.0.48:

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.]
   [1913 Webster]
   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.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "carbuncle":
   abscess, aposteme, bed sore, blain, bleb, blister, boil, bubo,
   bulla, bump, bunion, canker, canker sore, chancre, chancroid,
   chilblain, cold sore, corn, cyst, dilatation, dilation, distension,
   edema, eschar, felon, fester, festering, fever blister, fistula,
   furuncle, furunculus, gathering, gumboil, hemorrhoids,
   intumescence, kibe, lesion, lump, papula, papule, paronychia,
   parulis, petechia, piles, pimple, pock, polyp, pustule, rising,
   scab, sebaceous cyst, soft chancre, sore, stigma, sty, suppuration,
   swell, swelling, swollenness, tubercle, tumefaction, tumescence,
   tumidity, tumor, turgescence, turgescency, turgidity, ulcer,
   ulceration, wale, welt, wen, wheal, whelk, whitlow, wound



Easton's 1897 Bible Dictionary:

Carbuncle
   (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.