Of course I know what the word means now. I'll forego posting an image of one until the end, in case you haven't read or seen this tale. But it's odd that I still don't associate the word with the object. If someone were to ask me what a carbuncle was before this refresher course, I would probably have guessed it was something like a boil or a goiter--in other words, some protuberence on the human body. So let's see exactly how I got this impression.
...Well it's very interesting. I suppose we've come far enough now that I can reveal that the carbuncle of the story is a jewel, "a forty-grain weight of crystalized carbon". In other words, a blue diamond. Or maybe not, as some have pointed out that Holmes never refers to the jewel as such. But my original guess about the boils would also be right. That's because the word has taken off in two very different directions. Once again, it all goes back to etymology.
"Carbuncle" comes from Latin and means, "little coal". (That "-cle" on the end turns out to be diminutive.) It was first used to describe gems of a fiery color, such as rubies and garnets. (I guess it's assumed that the little coal is glowing.) Only later did it come to describe an inflamed sore or boil, which like a coal, though not so much like a jewel, is glowing red. You would think that a jewel and a boil would be about as far apart, as concepts go, as you could get, but apparently the associative mind does or did make that leap at some point. Sir Arthur Conan Doyle's mind takes it even further, making the red jewel glow blue...
"The Adventure of the Blue Carbuncle" is actually a nice little holiday story and as we are rapidly advancing on that season, you might like to either read or listen to here. And when you've done that, you might enjoy some interesting questions about the story that I happened upon here.
(Well, it was either this or a picture of a flaming pustule...)