"Set with diamonds, sapphires, rubies and emeralds, the bejewelled piece features the cypher of her father, King George VI," Zack Stone, a fine jewelry expert from U.K. retailer Steven Stone, shared with WWD.

Trooping the Colour in the U.K. is a celebration of the reigning monarch's birthday. It's held every year in June, regardless of when their actual birthday is, and it involves a lot of fanfare with a parade and a dramatic RAF flyover.

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King Charles and Queen Camilla appeared at the annual Trooping the Colour celebration as the monarch continues to undergo treatment for cancer.

Senior working royals including Prince William, Kate Middleton, Princess Anne, Prince Edward and Queen Camilla will reportedly take part in the king's birthday parade.