The sapphire and diamond ring may be the most prominent piece of Diana’s jewellery Kate owns and she is rarely seen without it.
The ring has a central 12 carat oval sapphire that is set in white gold.
The Duchess has also borrowed jewels including her Collingwood pearl earrings and a sapphire set that matches her ring.
Both royals have also borrowed the Lover’s Knot Tiara from the Queen’s collection, which was one of Princess Diana’s favourite jewels while in the Royal Family.
Just last week, Kate was seen at Buckingham Palace wearing a pair of sapphire earrings that were thought to be Princess Diana’s.
Not only are the jewels stunning to look at, she could have a more personal reason for sporting Diana’s collection.
Wearing the items could highlight the similarities between the royals, Le Vian CEO Eddie LeVian claimed.
“There is nothing quite like jewellery with a sense of history,” he told Express.co.uk.
“When it comes to the royals, a jewel with a story endures forever.
The Duchess could use her jewellery to connect with Diana, Eddie suggested.
He added: “There is a poignancy when we see Kate wearing jewels that once belonged to her husband’s late mother Princess Diana, a woman she never met in person.
“Jewellery, worn next to the skin, is in my view the most personal of all accessories.
“These beautiful royal jewels somehow connect these women’s journeys, and the milestones in their lives.”
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How did Kate Middleton become a royal?
Kate Middleton, or Catherine, Duchess of Cambridge, was born on 9 January 1982 at the Royal Berkshire Hospital, Reading, England.
She was born to Michael and Carole Middleton, a former flight dispatcher and air hostess. The couple founded a party supplies company, Party Pieces, in 1987 – now estimated to be worth Â£30 million.
Kate has a younger sister Pippa Matthews, 34, and a younger brother James Middleton, 31.
Kate and her family moved to Amman, Jordan, in 1984, where the future Duchess was educated at an English-speaking nursery school.
After moving back to Berkshire the family sent Kate to private school, St Andrews School. She went on to briefly study at Downe House before moving onto Marlborough College where she boarded.
For university the young Kate attended the University of St Andrews in Fife, Scotland. Here she was awarded an undergraduate MA (2:1 Hons) in history of art.
Relationship with Prince William
Kate met Prince William when they were both studying at St Andrews, and both living in St Salvator’s Hall. Famously, William reportedly took a fancy to Kate after she wore a see-through dress during a fashion show.
In 2003 the couple began dating and lived together during their second year of university.
The couple split in April 2007, however, in October they were back together.
Prince William proposed to Kate Middleton with his late mother Princess Dianaâ€™s engagement ring in October 2010. The couple were holidaying in Kenya at the time.
On 29 April 2011 they married at Westminster Abbey with a whopping 26 million watching the event live.
Kate wore an Alexander McQueen dress worth a reported Â£250,000 on the big day.
Kate assumed the title “Her Royal Highness The Duchess of Cambridge” and also became the Duchess of Cambridge on the day.