Kate Middleton mixed a bit of old with the new as she carried out another successful engagement, proving she has more than settled into her position as the Princess of Wales. While Kate deviated from traditional St Patrick’s Day fashion, she kept one detail - a brooch - which references back to many royals before her, including Princess Anne and the Queen Mother. And it just so happens to be sentimental to Kate for another reason - it was the first ever accessory she wore while carrying out her first official royal engagement. 

• Princess Catherine got into the St Patrick’s Day spirit, honoring the Irish Guards regiment with shamrocks, as per royal tradition

• For the appearance, Kate wore the Irish Guards brooch – the first brooch she ever wore as a working royal, but this simple gold brooch has an interesting origin story

• In other royal news, why there could be a second balcony appearance at King Charles’ coronation in ‘illuminating’ moment

While Kate Middleton sidestepped one tradition this St Patrick’s Day – opting for a vibrant teal outfit compared to the usual traditional green – the Princess of Wales honored an age old royal custom.

And it involves a brooch which is truly unlike any other which has been adorned by royal figures in the past.

Despite being photographed on the likes of the Queen Mother and Anne, the Princess Royal, the golden shamrock brooch isn’t thought to belong to the royal family at all. This is because the brooch is thought to belong to the Irish Guards, with whom Kate greeted at her St Patrick’s appearance.

The royal family jewelry website, the Court Jeweller, explains that the Irish Guards brooch "is loaned out to royal ladies who are associated with the regiment."

The somewhat surprising idea that a royal must request to wear the brooch is only part of the mystery around this simple gold brooch.

Officially known as the Irish Guards Brooch, the textured gold accessory features a single emerald set in the center of its leaves.

Another reason the brooch is so special is that it was the first one ever worn by Kate during her first official engagement as a royal, as per jewelry expert Maxwell Stone, who also estimates this brooch to be worth around $3K and featured 18K carat gold.

Ever since that historic first for her, Kate has sported the shamrock brooch on St Patrick’s Day celebrations since 2011, and she’s the only royal to have worn it since then.

The fascination around this brooch doesn’t end there – it also has something of a mysterious origin.

Some historians cite Queen Alexandra as the first owner of the brooch. This is partly because Alexandra began another custom still observed today – the handing out of actual shamrocks to the Irish Guard. Kate was seen carrying this tradition out again this year.

However, another origin story for the gold brooch includes Prince William himself.

In the book, Up the Micks! An Illustrated History of the Irish Guards, a book written by James Wilson, which features a foreword by the now Prince of Wales, the brooch is said to have been crafted by Cartier and then presented to Princess Mary, the Princess Royal, by the regiment in 1961.

After Princess Mary died, the brooch was purchased by the regiment at an auction of her personal jewelry. It was worn by the Queen Mother, by request.

While it might be puzzling that a jewel so associated with the royals doesn’t actually belong to them, it isn’t the only shamrock themed piece in their collection.

The Irish Guards brooch is a much simpler style than other brooches representing Ireland and/or Northern Ireland – including one which was part of the late Queen Elizabeth’s collection and went on display for the Platinum Jubilee.

Featuring a small bouquet of diamond shamrocks, secured by an emerald ribbon, this tribute to Northern Ireland was gifted to the Queen in 2012 and has been worn by Her Majesty many times in a short period, including at Royal Ascot in 2015 and during her state visit to Germany the following week.

A jewelry expert explained to woman&home, “The Queen’s Shamrock of Ireland brooch features three diamond shamrocks, with each leaf containing one round diamond, set in white gold. The shamrocks are secured by an emerald ribbon and the level of detail of the squared baguettes along the stem is exquisite.”

The expert estimated it to be worth approximately $23,000.