It has been reported that it was a gift from the Duke to celebrate Prince George’s birth.
The small band is made from white gold and is decorated with diamonds, experts at Joshua James jewellery commented.
They said: “The third ring she wears is an eternity ring. Eternity rings are often used to mark special milestones in a relationship.
“In this case, it was a gift from her husband to mark the birth of their first child, Prince George.
“Made from 18ct white gold, it features 0.23ct of white diamonds.
“Kate’s eternity band comes from London-based designer Annoushka, and is valued at £1,200.”
Eternity rings are a common gift to celebrate milestones such as the birth of a child.
The royal will usually wear the three rings stacked on her wedding finger.
“The order in which these are worn actually has a great significance.
“Wearing your wedding ring first means that it is closest to your heart, followed by the engagement and then eternity ring.
“It looks like Kate Middleton follows this tradition.”
Never miss a royal update again by signing up to the Daily Express Royal Family newsletter
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 £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 Diana’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 £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.