The Princess of Wales is reportedly not allowed to wear this before 6pm

Being a royal entitles you to a lot of privileges - private chefs, a designer wardrobe and having a palace for a home - the list goes on. But a royal title also comes with a hefty rulebook - something Kate Middleton, now Princess of Wales, had to learn the hard way when she entered the royal family. 

From enduring terrifying kidnapping training (yes, really) to learning the correct way to descend a staircase, Kate is thought to have taken Princess lessons to learn the ropes.

Most of the rules for royal women seem to surround fashion, from the insistence on see-through nail varnish and wearing tights, to a ban on tracksuits and a dislike of denim. Even Kate's trademark wedges are reportedly frowned upon in the royal rulebook.

It was a different fashion rule that resurfaced this week however, and it involves diamonds.

There are reportedly strict rules on wearing diamonds within the royal fold, with family members said to be banned from wearing them before a certain hour in the day.

Yes, really. According to royal etiquette expert Myka Meier via News.com.au, royal women are not allowed to wear diamonds during daytime.

"Other jewels are worn pre-6 pm," the royal etiquette expert explained to the news station. "Before 6pm, you’ll see metallics, gemstones, pearls, sapphires. At night, you’ll see the diamonds come out, and that’s in order to not come across as flashy in your appearance."

After over a decade as a royal, Princess Kate is certainly well-versed in the royal rulebook, credited with not putting a foot wrong and actually keeping the royal family afloat these past few years.

"[The royal family] realise the power of Kate and the potential of Kate and William as this young, glamorous, dynamic duo who actually do have the ability to reshape and project the monarchy into the future," Royal expert Katie Nicholl explained earlier this year on True Royalty's Kate: The Making of a Modern Queen.

Well, that’s that.