Here are some definitions of the word carbuncle.
A carbuncle is an archaic name for a deep-red cabochon cut gemstone usually the red garnet, and specifically, almandine. It was called by the Ancient Greeks anthrax and is found in the East Indies. When a carbuncle is held up to the sun, the gem loses its deep red tinge, and becomes of the colour of bright burning coal. The name carbuncle belongs for the most part to ruby sapphire, though carbuncle has been also given to red spinel and garnet. The word carbuncle has been used in this context four times in the bible. (Exodus 28:17 and 39:10; Ezekiel 28:13; Isaiah 54:12).
In heraldry a carbuncle is a charge or bearing representing the precious stone. A carbuncle has eight sceptres or staves radiating from a common centre; four of which make a common cross, and the other four a saltire.
Cockney Rhyming Slang:
Carbuncle is Cockney rhyming slang for uncle. As in “My old carbuncle sat me on his knee”.
Carbuncle is the name of a vicious and deadly computer virus. The carbuncle virus is spotted, deleted and controlled by all modern anti-virus software…… yeah!
A carbuncle is a cluster of boils on the surface of the skin, or an abscess larger than a boil, usually caused by bacterial infection of staphylococcus aureus, or staph. An infection of more than one carbuncle is known as carbunculosis. Carbuncles are typically groups of infected hair follicles. A single infected hair follicle would be called a boil or furuncle. Carbuncles are typically localised, deep skin infections. Other names for Carbuncles include: Skin infection, staphylococcal, staph, staph skin infection, carbunculosis, carbuncles.
Carbuncle in mythology is a mythical beast reportedly sighted in the Americas by the early Spanish conquistadors. The carbuncle was very rarely seen, but was reported to have a shining jewel on the creatures forehead.
As you can see the carbuncle definitions range from positive (a precious stone), through mythology and religion, to negative (skin disease). Quite metaphorical for life, really.