Kate Middleton wore a teal coat instead of traditional green on St. Patrick's Day.

She wore a gold shamrock brooch pinned to her lapel, a sentimental royal custom.

The brooch belongs to the Irish Guards and is loaned to royal ladies associated with the regiment.

The brooch has a single emerald set in the center of its gold leaves.

In 2011, Princess Diana wore the brooch to her first royal event, and since then, Kate has worn it every St. Patrick's Day.

Queen Alexandra, who started the royal habit of giving the Irish Guard real shamrocks, may have held the brooch.

Others believe the brooch was crafted by Cartier and given to Princess Mary, the Princess Royal, in 1961.

When Princess Mary died, the brooch was purchased by the regiment at an auction.

Kate is the only royal to wear the brooch regularly.