"The Adventure of the Beryl Coronet" is a Sherlock Holmes story by Sir Arthur Conan Doyle. In this tale, Holmes is approached by Alexander Holder, a banker, who is distressed over the theft of a valuable coronet (a small crown). Holder's niece, Mary, has been accused of the theft, as she was the last person to handle the coronet. However, Holmes suspects foul play and investigates the case. He uncovers a plot involving a family member who had a gambling addiction and coerced Mary into helping him steal the coronet. With Holmes's keen deduction and quick thinking, the true culprit is revealed, and the coronet is recovered.