The iconic jewel has a central sapphire which is surrounded by diamonds.
Kate was given the ring when Prince William proposed in 2010 but it originally belonged to Princess Diana.
The Princess of Wales was gifted the jewel when she became engaged to Prince Charles and the ring was reportedly bought for £28,000.
Due to its prominence in the Royal Family, it is now worth an estimated £300,000.
READ MORE: Kate Middleton £2,000 earrings ‘refreshing twist’
The Duchess of Cambridge is rarely seen without the ring which is sure to be a sweet reminder of her mother-in-law.
Kate also has a collection of other royal jewels that once belonged to Princess Diana.
Diamond and Sapphire Earrings
The subtle drop earrings match the engagement ring and have been refashioned by Kate, according to Le Vian CEO Eddie LeVian.
He told Express.co.uk: “These sapphire and diamond earrings, first worn by Diana, have been refashioned by Kate to her personal taste.
“Originally, they featured both a round stud and an oval one that dangled beneath it.
“Kate, however, has had the stud removed, opting for a simple, single drop earring instead.”
Princess Diana often wore the sapphire ring and earrings with a matching necklace and the stone was believed to be a favourite of hers.
“Sapphires were a favourite of Diana’s – just think of her famous engagement ring, and these earnings matched it perfectly,” Eddie continued.
“Kate’s modernised version of these earrings retains the beauty of the original, but are more understated and versatile.”
Collingwood Drop Pearl Earrings
The timeless pearl earrings were worn by Princess Diana over the years and have since been seen on the Duchess of Cambridge.
The jewellery pieces were personal to the Princess of Wales as she was first spotted wearing them before her wedding.
Eddie continued: “Laden with regal history, these sleek, sophisticated and versatile earrings were first worn by Princess Diana prior to her wedding.
“Timeless drop pearls in a teardrop shape, they are clasped in diamonds.
“Kate was first spotted wearing these at the Spanish State Banquet in 2017 when King Felipe and Queen Letizia of Spain visited the United Kingdom.
“She has gone on to wear them many times since.”