At the time, Kate was in attendance with Prince William, 37, at the annual Diplomatic Reception at Buckingham Palace.
The soiree was attended by members of the Diplomatic Corps based in London — who support the work of the royal family, including the hosting of state visits and the Queen's regular audiences with ambassadors and high commissioners.
There she showed off a sparkly new diamond ring on her right hand, which was speculated to be a Christmas present from William.
What Kate Wore noted this is particularly unusual for Kate as she tends to only wear three rings stacked on her left ring finger: her engagement ring, wedding ring and eternity ring.
Meanwhile, the Queen wore what was believed to be an updated version of an emerald necklace - the Greville Emerald Necklace.
Prince Charles and the Camilla, the Duchess of Cornwall, were also in attendance - unlike Prince Harry and Meghan Markle, who were noticeably absent from the reception.