The Adventures of Sherlock Holmes - The Adventure of the Beryl Coronet
Described as a very peculiar case. Alex Holder of the bank Holder & Stevenson of Threadneedle street was visited by a nobleman. The noble offered up his beryl coronet (a jeweled crown of sorts) as collateral for a loan of £50,000. Holder gave up his own money for the loan.
He took the crown home, told his kids Arthur and Mary (adopted niece). He woke up in the middle of the night to see his son holding a twisted up crown with three of the jewels missing. The son was arrested.
The daughter had run away with the neighbor/friend, Sir George Burnwell.
She had handed the crown through a window to him which the brother had witnessed. Arthur chased Sir George and fought for the crown, thus twisting it. A piece broke off and Sir George got away. The son didn't want to say anything in order to protect Mary.
Here's a picture of a jeweled coronet I found online