When King Charles III and Queen Camilla are officially crowned at Westminster Abbey on Saturday, the Duchess of Sussex won’t be the only thing missing. The controversy-stirring Kohinoor diamond — the 105-carat sparkler at the center of the violet crown Camilla was expected to wear — won’t make an appearance. The royals have good reason to want to keep the gem out of Saturday’s coronation festivities. The crown jewel of the crown jewels is widely considered an ill-gotten spoil of Britain’s colonial conquests, and calls for the British to return the stone to India have grown increasingly loud since the...