She teamed it with glittery heels, believed to be Jimmy Choo's £525 Romy style and a jewel topped satin clutch bag.
It looks the same style of gown she wore to Prince Charles's birthday a year ago, but in white.
Speaking ahead of the gala, Amanda Pullinger, CEO of 100 Women in Finance, said: "Our Philanthropic Themes are Women's & Family Health, Education and Investing in the Next Generation".
Chris Jackson/pool photo via Getty Images The gorgeous gown was designed by Gucci.
This is thought to be the first time that the Duchess of Cambridge has worn the earrings at a public event, although it certainly isn't the first time she has paid tribute to Princess Diana by wearing pieces from her jewellery collection.
The gala dinner, which was held at the Victoria and Albert Museum, benefited the Mentally Healthy Schools program that's part of the royals' Heads Together foundation.More news: Winter Weather Advisory: Messy travel as snow-ice-rain resurges
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Launched by Kate a year ago, the "Mentally Health Schools" program is one of the legacies of the royals' Heads Together campaign, which helps teachers and school leaders better support children's mental well-being. Picture: PA Kate Middleton was greeted before entering the conference.
Earlier today Kate attended a round table discussion at the Mental Health in Education conference at Mercers' Hall.
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Child development Gill Connell said at that time that active listening is one of the most important ways for parents to send their message to their children.
Of course William was looking as handsome as ever. Subscribe to HuffPost's Watching the Royals newsletter for all things Windsor (and beyond).