Describe the story of "The Adventure of the Blue Carbuncle" as a report.

In this 1892 short story by Sir Arthur Conan Doyle, a combination of coincidences and mix-ups allow Sherlock Holmes to solve the mystery of a stolen carbuncle, a precious gem.
The Christmas season, however—not a jewel—opens the story in what Holmes describes to Watson as one of those "whimsical little incidents." Peterson, a "very honest" police officer, approaches Holmes with a hat and a Christmas goose he has retrieved after the man who was carrying the goose was attacked. Both the assailant and the owner fled from the policeman, so Peterson comes to Holmes wondering if the owner of the items can be tracked down. As Holmes thinks this is impossible, Peterson takes the goose home to cook, and Holmes keeps the hat.
Unrelated to Holmes, a priceless blue carbuncle is stolen from the Countess of Morcar's hotel room. Unlike the usual carbuncle, a bright red gem, this stone, which was found in China, is blue.
However, the two incidents come together when the blue carbuncle is found inside the goose. Now, it becomes imperative to track the owner of the goose, and Holmes is on the trail.
More than just a mystery, ethics and justice come into play. John Horner, an innocent man who happens to have a criminal record, is framed for the crime. The real thieves are the countess's maid and James Ryder, the mastermind behind the plot and a fearful, cringing little man. Holmes, who has the freedom to make moral determinations that a police officer might not, decides to let Ryder flee to the European continent, determining that serving jail time would only harden him into a real criminal. Plus, once he is gone, Horner will go free because there is now no case against him.
Thematically, therefore, the story questions the value of imprisonment as a form of justice, and Holmes enacts some of the mercy we associate with the Christmas season. As Holmes says to Watson:

This fellow will not go wrong again; he is too terribly frightened. Send him to gaol now, and you make him a gaol-bird for life. Besides, it is the season of forgiveness. Chance has put in our way a most singular and whimsical problem, and its solution is its own reward.