The detail in Princess Eugenie's engagement ring that makes it so special

While Kate Middleton's engagement ring is one of the most iconic pieces of royal jewellery, having been passed down from her late mother-in-law, Diana, Princess of Wales, it's not the only special piece in the family.

Princess Eugenie's engagement ring contains a rare detail that makes it one of the more intriguing rings on any royal finger and it features a sweet nod to another royal lady.

At first glance, Eugenie's pink centre stone may look like a very pale garnet or ruby, but her ring is actually made up of a large pink sapphire surrounded by a halo of diamonds.

The stone is a padparadscha sapphire, also known as a 'lotus blossom sapphire', an incredibly rare stone that takes its stunning hue from the mix of red and yellow in the gem.

Experts have valued the stone at more than $12,000 AUD and that's not including the cost of all the diamonds that make up the halo.

The design is similar to the Duchess of Cambridge's ring, although Eugenie's is more rounded whereas Kate's has an oval deep-blue sapphire.

Princess Eugenie's ring also bears a striking resemblance to her mother's engagement ring.

Eugenie's husband, Jack Brooksbank, actually designed the ring for his wife with his mother-in-law's jewellery in mind, taking inspiration from the piece Prince Andrew used to propose to Sarah Ferguson in 1986.

Fergie's ring was a Garrard & Co design and featured 10 stunning diamonds surrounding a large ruby.

It was noted by many royal fans at the time to be a beautiful contrast to Diana's blue sapphire, which was also designed by Garrard.

The pair were close friends and their rings showed off the way their different personalities complimented one another.

Now we're just waiting to see what kind of ring Beatrice will one day sport on her left hand; will she too opt for a stone like mum's, or will she go for something a little different?