The special meaning behind Kate Middleton's bold yellow state dinner brooch.

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Kate Middleton and her husband Prince William attended a fancy state dinner at Buckingham Palace overnight in honour of visiting royals King Willem-Alexander and Queen Maxima of the Netherlands.

But while all eyes were on the Duchess’ stunning tiara and her Cinderalla-esque dress, there was another special piece of jewellery that she was spotted wearing.

For the first time ever, the Duchess was photographed wearing a royal family order.

The yellow brooch, which featured a small portrait of the Queen, is the same honour which was presented to Princess Diana in 1981.

According to Good Housekeeping, the royal family order is essentially a prestigious honour symbolised with a badge.

The yellow brooch is presented by the Queen to female royal family members as a thank you for their service and work during royal engagements.

While Princess Diana received the honour in the same year she married Prince Charles, Kate has received hers seven years following her royal wedding to Prince William.

Kate Middleton royal order
It's the first time the Duchess has been photographed wearing the Royal Family Order. Image: Getty.

Camilla, Duchess of Cornwall, received the honour in 2007, just two years after marrying Prince Charles.

But there's one key detail that differentiates Kate's Order to the other brooches presented to royal family members.

In the past the brooches were made with ivory, but the Duchess' was made with glass instead.

This is believed to be because of Prince William's years of conservation efforts, particularly his tireless campaigning for a ban on ivory.

At the state dinner, Kate also wore her signature tiara, the Lover's Knot.

According to a Order of Splendor, the tiara was created for Queen Mary and inherited by Queen Elizabeth. She wore it early in her reign before loaning it to Princess Diana.

princess diana
Princess Di wore the same tiara and earrings combo during a visit to the United States. Image: Getty.

The Duchess also wore a necklace which hasn't been seen since Queen Elizabeth The Queen Mother wore it.

She also wore pearl earrings previously belonging to Diana.