The Duchess of Cambridge was wearing a dark Jenny Packham dress, but it was the Cartier diamond known as the Nizam of Hyderabad, given to Queen Elizabeth II in 1947 for her wedding to the Duke of Edinburgh, that had tongues wagging.
Other guests at the gala dinner -- the duchess's first official engagement of the year -- included Elizabeth Hurley, rocker Bryan Adams and artist Grayson Perry.
The duchess said she was "delighted" to be able to support the gallery.
"The gallery's achievements are exceptional," she told guests.
"They hold the most extensive collection of portraits in the world, and their unique and brilliant exhibitions never fail to inspire us all."