"The Adventure of the Beryl Coronet", one of the 56 short Sherlock Holmes stories written by British author Sir Arthur Conan Doyle, is the eleventh of the twelve stories collected in The Adventures of Sherlock Holmes. The story was first published in Strand Magazine in May 1892.
A banker, Mr. Alexander Holder of Streatham makes a loan of £50,000 to a socially prominent client, who leaves the Beryl Coronet — one of the most valuable public possessions in existence — as security. Holder feels that he must not leave this rare and precious piece of jewellery in his personal safe at the bank, and so he takes it home with him to lock it up there. He is awoken in the night by a noise, enters his dressing room, and is horrified to see his son Arthur with the coronet in his hands, apparently trying to bend it. Holder's niece Mary comes at the sound of all the shouting and, seeing the damaged coronet, faints dead away. Three beryls are missing from it. In a panic, he travels to see Holmes, who agrees to take the case.
The case against Arthur seems rather damning,yet Holmes is not convinced of his guilt. Why has Arthur clammed up? Why is he refusing to give a statement of any kind? How could Arthur have broken the coronet (even Holmes, who has exceptionally strong hands, can't do it) and without making any noise? Could any other people in the household be involved, such as the staff, or Mary? Could some visitor, such as the maid's wooden-legged boyfriend, or Arthur's rakish friend Sir George Burnwell, have something to do with what happened to the coronet? The failure to resolve the case will result in Mr. Holder's dishonour, and a national scandal.
Holmes sets about not only cogitating the details that he learns from Holder, but also examining tracks and traces in the snow outside. Eventually, Holmes solves the mystery, and Holder is flabbergasted to find that his niece was in league with a notorious criminal, although apparently she is unaware of his character. The two of them escape justice; however, Holmes is convinced that they will receive their punishment in due time. Arthur's motive in allowing his father to think he was the thief was because he was in love with his cousin Mary and suspected she was in on the theft.