Kate Middleton opted for a vibrant teal outfit instead of traditional green for St. Patrick's Day.

Princess Diana honored a royal custom by wearing a golden shamrock brooch.

The brooch is not thought to be in the royal family's possession, but is loaned to royal ladies associated with the Irish Guards regiment.

The brooch is officially known as the Irish Guards Brooch and features a single emerald set in the center of its leaves.

Only Kate Middleton has worn the shamrock brooch on St. Patrick's Day since 2011.

Some say Queen Alexandra was the first owner of the brooch, while others say it was given to Princess Mary by the Irish Guards regiment in 1961.

In 2012, the late Queen Elizabeth received a shamrock brooch with a diamond bouquet on an emerald ribbon.

The Queen's Shamrock of Ireland brooch has three white gold diamond shamrocks with intricate stem decoration.

The Irish Guards Brooch is simpler than other Irish/Northern Irish brooches.