The "blue carbuncle" is a fictional gemstone featured in the Sherlock Holmes story titled "The Adventure of the Blue Carbuncle," written by Arthur Conan Doyle. In the story, the blue carbuncle is a valuable jewel that is stolen and later found in the crop of a goose. Its significance lies in the mystery surrounding its theft and the investigation led by Sherlock Holmes. The blue carbuncle symbolizes themes of crime, morality, and the unexpected twists of fate. The narrative explores how a seemingly trivial event, like the discovery of a goose, can lead to the unraveling of a complex crime. The story highlights Holmes's deductive reasoning and the importance of observation in solving mysteries.