A London-based jeweler is offering an item that turns existing rings into convertible pieces—jewelry that can be worn different ways. The eight designs in the Jessica McCormack Diamond Party Jacket collection (jessicamccormack.com) wrap around solitaire-stone rings and change their looks. “A Diamond Party Jacket can breathe new life into an old jewel and give an added dimension to a new one,” says Jessica McCormack. Prices start at $7,000. McCormack can also create custom jackets.