Duchess Kate is always extremely stylish and fashionable in public. When it comes to her jewelry, though, there's one rule must follow: diamonds can't be worn before 6 p.m. 

The etiquette expert Myka Meier tells the British Express: "Other jewels are worn pre-6pm." She explains: "At night, you'll see the diamonds come out, and that's in order to not come across as flashy in your appearance."

Royal style rule: No diamonds before 6 p.m.

During the day, Duchess Kate wears more inconspicuous pieces of jewelry that are in a cheaper price range.

However, there are exceptions, as Gala reports. Of course, eye-catching jewelry can also be worn during the day at wedding celebrations. Engagement rings can also be seen at any time of the day.

Duchess Kate wore Lady Diana's jewelry

The etiquette expert and former royal butler Grant Harrold tells the BBC of another peculiarity: Tiaras are typically reserved for married women.

"For married ladies it was a sign of status and would show you were taken and not looking for a husband," he reports.

"The more significant event, the more likely you are to see them wearing diamonds," Harrold adds.

Duchess Kate showed up in 2017, for example, elegant with Diana's favourite tiara at a ball given by Queen Elizabeth II to honour her.

So: diamonds are off limits before 6 p.m. and no tiaras for single royals!