The royal family are pretty lucky when it comes to jewellery. Women like the Duchess of Cambridge and the Duchess of Sussex are often allowed to raid the Queen's personal jewellery box for priceless pieces. As well as historic designs, they are also gifted bespoke items by their husbands. But the origin of Kate Middleton's citrine ring is proving to be quite the mystery.

The square yellow ring was first spotted by royal fans at the wedding of Prince Harry and Meghan Markle. Middleton also accessorised with the stone at Wimbledon on Sunday, matching it to her sunny Dolce & Gabbana dress.

Many believe that the ring was a push present from Prince William, reports Harper's Bazaar. These sentimental gifts are a tradition for the royal family, given to women who have recently given birth. Since Middleton welcomed little Prince Louis into the world in April, this theory makes sense — especially when you consider that citrine is a stone with links to joy, warmth, and general positivity.

However, Harper's Bazaar has also noted that Middleton may have had the ring for much much longer. The magazine pointed out that the royal had been seen wearing a very similar ring while out celebrating her 26th birthday in 2008 and at a royal wedding later that year. A tricky one, right?

The International Gem Society says that it's not the size of the citrine that affects its value but the intensity of its colour. The more intense the stone, the more expensive it is.

But Ringspo's jewellery expert Alastair Smith tells me that this particular ring isn't actually that valuable. "Citrine isn't particularly rare," he states. "It's a softer semi-precious stone, which means that it's not as suitable for everyday wear as Middleton's sapphire engagement ring. Although the centre stone looks to be around five to seven carats in weight, the price of the ring would likely be between £500 and £1,000, depending on the quality of the stone."

While the citrine ring may be rather thrifty, the 36-year-old royal has showed off a range of other (much more expensive) jewellery over the years. According to the Daily Mail, the contents of the royal's jewellery box total an estimated £600,000. However, this figure doesn't include some gifts from other members of the royal family, making her collection more likely to be worth £1 million.

Here's a few of Middleton's priciest jewels: