Princess Kate is participating in the traditional St. Patrick's Day custom of presenting shamrocks to the Irish Guards.

She wore a shamrock broach on her coat that the Irish Guards loan to royal women.

The regiment gave Princess Mary, the Princess Royal, the Cartier brooch in 1961. Its provenance is unknown.

After Princess Mary's death, the brooch was purchased by the Irish Guards at an auction of her personal jewelry.

The Queen Mother wore the brooch before Princess Anne was seen wearing it at events related to the Irish Guard.

Princess Kate has also been seen wearing the brooch at the annual presentation of shamrocks.

Kate has an extensive personal jewelry collection, some inherited from Princess Diana and others borrowed from the Queen.

The shamrock brooch is unique as it is not owned by someone in the royal family.

The shamrock presentation is a century-old tradition started by Queen Alexandra in 1901.