In ‘The Adventure of the Blue Carbuncle,’ Sherlock Holmes gets possession of the famous jewel by accident, since it is found in the crop of a goose that Peterson, the commissionaire, rescues from a brawl. He successfully connects the finding of the carbuncle in the goose with its theft from Countess of Morcar at the Hotel Cosmopolitan.
Sherlock Holmes’ actions and deductions in “The Blue Carbuncle” illustrate that he is knowledgeable about human psychology, even those aspects which are foreign to his own personality. For example, several of the deductions he explains to Watson depend on his understanding of human nature. Upon examination of the felt hat, he states that the owner has foresight, because he special-ordered an elastic hat-securer, but his foresight has weakened, since the elastic is broken and he has not replaced it. Holmes understands that not everyone considers the possibility of wind blowing away a hat, but because this man did (and took steps to prevent it) he had more foresight than most people. Similarly, Holmes points out the ink applied to the faded spots on the felt as evidence that the man has not entirely lost his self-respect. Although he may not be able, financially, to replace the hat, he at least finds a way to improve its appearance.
Another part of human psychology that Holmes understands is the appeal of jewels, especially those that are unique — “…it is a nucleus and focus of crime. Every good stone is.” Even though the blue carbuncle is less than 20 years old, it already has several crimes (e.g. murders, robberies) connected with it, and Holmes states that every facet of older and larger jewels may represent a heinous crime, even murder. He knows that brilliantly colored and well cut precious stones can lead human beings to do almost anything in order to possess them.
Finally, Sherlock Holmes’ method of getting information from the Covent Garden goose salesman, Mr. Breckinridge, shows his familiarity with gamblers and with what makes them tick. Since Holmes notices a “Pink ‘un” (a British newspaper focusing on sport, especially horse racing) in Breckinridge’s pocket, he knows that, if he makes his questions about the geese into a bet, Breckinridge will not think twice before answering — even though when asked directly he refused to answer. The tactic is successful, and Holmes solves the mystery, simultaneously revealing his considerable understanding of the psychology of human beings.