What is the name of the gem stolen in 'The Adventure of the Blue Carbuncle'?

The gem stolen in “The Adventure of the Blue Carbuncle” is the Blue Carbuncle.

The jewel is a valuable blue gemstone stolen from the hotel room of the Countess of Morcar. The police arrest John Horner, a plumber who had been working in the room, but the real trail leads through an accidental chain involving a Christmas goose. Commissionaire Peterson finds the gem inside the bird after recovering the goose and a hat from a street scuffle.

Holmes traces the goose through London’s poultry trade and identifies James Ryder, a hotel attendant, as the thief. Ryder had hidden the stone by feeding it to a goose belonging to his sister, Mrs Oakshott, but mistakenly chose the wrong bird. Holmes’s decision to let Ryder escape is one of the story’s most memorable moral turns.

The Blue Carbuncle is not the Hope Diamond or the Koh-i-Noor. In the story, “carbuncle” refers to a red gemstone in older usage, but Doyle’s title specifies a blue stone. The tale was first published in The Strand Magazine in January 1892.