Crown Jewels of Ireland
This is one of the more hilarious cases of stolen jewelry. The Crown Jewels of Ireland consisted of three pieces: a star of the Order of St. Patrick and a diamond brooch. They were crafted with 394 stones, which originally belonged to Queen Charlotte.
However, due to poor security (the valuables were left in a library of all places), some mastermind thief managed to break the case and leave with the items. Nobody knows who took it for certain, though the finger of blame points toward Francis Shackleton, the brother of a famous explorer. Again, check your attics and basements, especially if your last name is Shackleton.