Crown Princess Mary of Denmark's Stunning Brooch Had a Full-Circle Moment on Son Prince Christian's Birthday

The Connaught Sapphire Brooch made a previous appearance on another important day for Prince Christian 17 years ago

Crown Princess Mary of Denmark picked the perfect piece of royal history to complement her outfit for Prince Christian's 18th birthday celebrations.

The milestone birthday of Prince Christian, who is the second in line to the throne, was marked on Sunday with an appearance of the Danish royal family on the balcony of Amalienborg Palace in Copenhagen. Crown Princess Mary and Crown Prince Frederik joined their eldest son, Queen Margrethe and other members of the family for the outing kicking off the future king's birthday festivities.

Crown Princess Mary, 51, adorned her blue ensemble with a stunning accessory: the Connaught Sapphire Brooch.

The piece has a long history in the Danish royal family, starting with Prince Christian's great-great-grandmother, Princess Margaret of Connaught, according to The Court Jeweller. The brooch was passed down to Queen Ingrid of Denmark from her mother, then to the next generation with Queen Margrethe.

Related: Prince Christian of Denmark's Cousins Attend His Birthday Gala After Losing Royal Titles — Why One Was Missing

In addition to having a long family history, the brooch also has sentimental value to Crown Princess Mary. Queen Margrethe gifted the brooch to her daughter-in-law to celebrate Prince Christian's baptism, and Mary wore the piece at her son's christening in Jan. 2006 — making it the perfect choice for this weekend's festivities.

Since then, Princess Mary has sported the piece on several important occasions, including the 2011 christening of her twins, Prince Vincent and Princess Josephine, as well as Queen Margrethe's Golden Jubilee celebrations last year.

Later on Sunday, royals from around Europe gathered at Christiansborg Palace for a dinner gala. Crown Princess Mary changed into a navy gown for the bash, completing her look with another sweet accessory: the same tiara that she wore on her wedding day in 2004. The diamond sparkler was a gift from Queen Margrethe and Prince Henrik.

The family's evening looks took centerstage in new portraits released on Tuesday from the birthday celebrations. Crown Prince Frederik, dressed in a military uniform, and Crown Princess Mary posed with all four of their children with a scenic set of stairs as their background.

While Prince Christian sported his new Order of the Elephant regalia (the Danish Royal House revealed that Queen Margrethe recently awarded her grandson Denmark’s highest honor of chivalry), his sister Princess Isabella went without a tiara. At 16, her first appearance in a sparkling headpiece will likely come in a few years.

Princess Estelle of Sweden, 11, also did not wear a tiara for the event, which she attended alongside her parents, Crown Princess Victoria and Prince Daniel. However, other members of the next generation of royals wore stunning tiaras from their families' collections as they gathered for the big event.

As seen in a group photo marking the royal reunion, five future European monarchs posed together, many donning historic headtoppers. Prince Christian was all smiles beside Princess Estelle of Sweden, Princess Ingrid Alexandra of Norway, 19, Princess Catharina-Amalia of the Netherlands, 19, and Princess Elisabeth of Belgium, 21, in the snap that the Danish Royal House shared to Instagram.

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Related: Future Queens of Europe! Meet All the Female Heirs Set to Take the Throne

While guests at the birthday gala sported tiaras, ballgowns, military uniforms and sashes showing their honors, one attendee left without an important accessory. On Monday, the Danish royal palace had some fun on Instagram by posting a photo of a single sparkling shoe that was found after the palace event wrapped.

"Is it Cinderella who forgot her shoe last night?" courtiers captioned the snap. "When the guests at Her Majesty the Queen's gala dinner yesterday had gone home, this lonely stiletto shoe was left at Christiansborg Castle. The owner is welcome to contact us to get it back."

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