Kate Middleton and Prince William, both 38, first met at the University of St Andrews in 2001. The couple dated for nearly 10 years before their engagement was announced.
The stunning jewel is also thought to be worth a staggering sum of money.
“Created by Garrard & Co. (the former Crown Jeweller), it is now valued at around £300,000,” the experts added.
“However, much of its value is sentimental. This engagement ring was passed down from Princess Diana, William’s mother.”
Just a few months after their engagement was announced, Kate and Prince William married at Westminster Abbey.
Kate walked down the aisle wearing a custom-made dress designed by Sarah Burton for Alexander McQueen.
Sticking with royal tradition, she borrowed a royal tiara from the Queen’s collection.
The mother-of-three chose the diamond Cartier Halo Tiara which she styled with a white veil.
When the couple exchanged their vows, Kate was also given a Welsh gold wedding band from her husband.
Before Prince William gave the ring to Kate, he was given it by his grandmother the Queen.
The experts stated: “The plain, slender band shows off the incredibly rare and beautiful gold that it was made from, free of decorative elements that embellish many modern wedding rings.
“The ring was gifted to Prince William by the Queen following the engagement.”
Kate will often wear both her engagement ring and wedding band when she is seen out.
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.