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.

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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!