The Princess of Wales was the picture of royalty on Tuesday evening to attend the King's Diplomatic Reception at Buckingham Palace.
LOOK: Princess Kate just stepped out in a royal wedding heirloom – and nobody noticed
The wife of Prince William wore a truly stunning red sequin dress for the occasion designed by trusted royal designer Jenny Packham, known as the 'Elodie Gown'. Elevating her beaded scarlet-hued ensemble, the royal also wore the late Queen Elizabeth II's earrings, Princess Margret's Lotus Flower tiara and a poignant brooch emblazoned with a portrait of the late monarch.
WATCH: Royal jewels - a look at the British royals' most stunning tiaras
Princess Kate's curious brooch is actually the Royal Family Order. The rare jewellery piece depicts a young Queen Elizabeth II in evening dress wearing the ribbon and star of the Order of the Garter.
The delicate pendant, painted on ivory (glass since 2017), is bordered by diamonds and mounted by a Tudor Crown in diamonds and red enamel.
TRENDING: 10 times Princess Kate dazzled in show-stopping tiaras
The Princess of Wales wore the Royal Family Order
On the back of the brooch is the royal cypher and St Edward's Crown in gold and enamel, fastened onto a chartreuse silk ribbon secured in a bow. But why does Princess Kate have the Royal Family Order?
The extremely rare jewellery piece is only ever worn to formal events, and is a touching emblem of the former Majesty's trust in the Princess of Wales.
READ: Inside Prince William & Princess Kate's luxury five-star hotel in Boston
SEE: 5 stunning emerald chokers to recreate Princess Kate's look
The brooches are a sign that the wearer has been made a part of the Royal Family Order, an honour gifted to female members of the Firm by the monarch.
According to the MailOnline, just 15 women were granted the honour by the late Queen during her reign. The honour is believed to have been introduced by George IV in the 1800s, as women in the royal court typically did not have the same commemorative medals as their male counterparts.
The Princess is one of 15 royal women who have the honour
There are seven living recipients: the Princess of Wales, the Queen Consort, the Countess of Wessex, the Princess Royal, the Duchess of Kent, the Duchess of Gloucester and the Honourable Lady Ogilvy.
Kate joined her husband, the Prince of Wales, along with King Charles and Queen Consort Camilla at the glittering event, which saw the monarch and other members of the royal family welcome over 500 members of the Diplomatic Corps in the State Rooms at Buckingham Palace.
Like this story? Sign up to The Royal Life newsletter to get your weekly dose of royal lifestyle inspiration, from the must-see fashion moments to sneak peeks into royal homes and wellness news.