Arthur Conan Doyle (1859-1930) wrote his Sherlock Holmes stories strictly as a source of income; it always bothered him that most readers of his generation were far less interested in his "serious" work. Having read some of Doyle's "serious" work, I have to confess, I prefer the Sherlock Holmes stories as well.
While some of the "Adventures of Sherlock Holmes" are a tad gruesome, this one is more lighthearted and no one dies, if you don't count a Christmas goose or two. If you've seen the BBC Jeremy Brett adaptation, it is very close. We include it here because the Christmas season is a part of the background of the story, and it does get occasionally incorporated in Christmas story collections.
Incidentally, a "carbuncle" is a kind of garnet, usually in the chemical composition almandine [Fe3Al2(SiO4)3], which occurs in shades of red, so a blue carbuncle, if such a thing were known in Doyle's day, would be very rare indeed. Ironically, if I understand my sources right, grayish-blue garnets made of a different chemical composition pyrope [Mg3Al2Si3O12] were discovered in Madagascar within the last two decades.