The piece is estimated to have a value of around £300,000.
While Prince Harry was reportedly in possession of the jewel for many years, he gave it to Prince William before he proposed.
Prince Harry still made sure to incorporate his mother into the ring he gave to Meghan Markle, however.
Meghan has a trilogy engagement ring which was custom designed by the Duke of Sussex.
It consisted of a diamond from Botswana and a jewel from Princess Diana’s collection.
In 2019, it was redesigned to be fitted with a new diamond-studded gold band and the ring is thought to be worth between £120,000 and £250,000.
Both rings have a special meaning as they allow the Duchesses to be close to their late mother-in-law.
Experts at jewellers Joshua James commented on Kate’s jewel.
They told Express.co.uk: “Kate’s engagement ring is incredibly famous, known for sparking a huge increase in sales of sapphire engagement rings back in 2010.
“It features a 12-carat oval sapphire set in 18k white gold, surrounded by 14 solitaire diamonds.
“However, much of its value is sentimental as this engagement ring was passed down from Princess Diana, William’s mother.”
As is tradition in the Royal Family, both Kate and Meghan were given Welsh gold wedding bands.
The slim gold bands are made from a rare material and created for members of the Royal Family.
They have been worn by royals including Queen Elizabeth II and Princess Diana.
Both Kate and Meghan are regularly spotted wearing their matching wedding bands.
The experts also shared analysis on Welsh gold wedding bands.
“Kate’s simple gold wedding ring is crafted from a special nugget of Welsh gold – a tradition that dates back to 1923 – by Wartski, a jeweller based in London,” they stated.
“The value of a Welsh gold ring depends on the weight and purity of the gold used, but will likely be upwards of £2,000.
“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.”