Queen Elizabeth II
Queen Elizabeth II is seen at the Chichester Theatre while visiting West Sussex on Nov. 30, 2017, in Chichester, United Kingdom. Getty Images/Stuart C. Wilson

Queen Elizabeth II has a wide array of jewelry, but there’s one particular piece that she always wears on her birthday.

Every year on Trooping the Colour, Her Majesty is photographed with one brooch that she seems to love the most, and there’s a fascinating story behind it.

Grant Mobley, a renowned diamond expert and gemologist, recently told Express that one of the monarch’s favorite brooches is her Brigade of Guards brooch. The stunning piece has a military history and is a nod to the armed forces.

“This brooch is the perfect accessory for the day as it combines the badges of the five regiments – the Grenadier, Coldstream, Scots, Irish, and Welsh Guards – which take turns every year trooping their colors for Her Majesty,” he said.

The Queen’s favorite brooch was actually created for the Queen Mother several years ago. But the monarch has spotted using it at the Trooping the Colour for the past 30 years. And since the piece has been with the royal family for years, it is already considered a priceless accessory.

Mobley also shared some of the intricate details of the piece. He said that the five symbols of the brooch are enclosed in an oval frame bearing the words, “Quinque Juncta In Uno,” which means “five joined as one.”

In the coming years, the Queen’s brooch may be passed on to another member of the royal family and it’s not Camilla Parker Bowles. Mobley said that when Prince William is crowned King, his wife, Kate Middleton, may be photographed with the Brigade of Guards brooch.

Meanwhile, Mobley said that there’s another reason why the Queen may seem to be fond of the brooch. The expert said that it’s because it’s made with diamonds, which are Her Majesty’s favorite.

“The entire brooch is very fitting considering the Queen’s love of diamonds – it is made out of rare natural diamonds and topped by a crown,” he said.