Philippe and Mathilde announce their royal engagement, September 1999 (HERWIG VERGULT/AFP via Getty Images)

Twenty years ago, the royal watchers of the world were preparing for the wedding of a future monarch. Prince Philippe, Duke of Brabant (and heir to the Belgian throne) proposed to Mathilde d’Udekem d’Acoz in September 1999, and today, we’re starting our coverage of their milestone anniversary with a look at their engagement announcement — and her engagement ring!

Philippe and Mathilde greet the press, September 1999 (HERWIG VERGULT/AFP via Getty Images)

Thirty-nine-year-old Prince Philippe surprised the people of Belgium on Friday, September 10, 1999, by announcing his engagement to a young Belgian aristocrat, Mathilde d’Udekem d’Acoz. The couple had dated in secret for three years. Mathilde, a 26-year-old speech therapist, was introduced to the nation in a press conference a few days later. Commentators spoke favorably of Mathilde’s ability to speak some Flemish as well as French, hoping that she could become a unifying figure for a divided country. Journalist Toby Helm wrote that the bride-to-be was “destined to be the first Belgian-born queen [of Belgium], a prospect that has come as a pleasant surprise to a country held together by little other than its football team, royal family and love of chips and chocolate.”

Philippe and Mathilde pose with their families during the official engagement media event, September 1999 (HERWIG VERGULT/AFP via Getty Images)

The September 13th photo call at Laeken, which the Associated Press called “chaotic,” was attended by numerous members of both families. Royals in attendance included King Albert II, Queen Paola, Queen Fabiola, Princess Astrid, Prince Lorenz, Prince Amedeo, Princess Maria Laura, Prince Joachim, Princess Luisa Maria, and Prince Laurent. Mathilde’s parents, Patrick and Anna Maria, were there with her three surviving siblings: Elisabeth, Helene, and Charles-Henri. During one interview, Philippe shared that he and Mathilde were both “enormously touched” by the good wishes they had received from the public. The royal engagement came on the heels of several major scandals in Belgium, and Philippe also expressed the hope that their “marriage will help to restore the image of Belgium a little.”

Mathilde and Philippe wave to crowds below greet the crowd from the balcony of the provincial palace in Arlon, October 1999 (GUY MOSSAY/AFP via Getty Images)

A few weeks after the engagement announcement, Philippe and Mathilde began a series of visits to various parts of the country, allowing the people to meet the future queen consort. Press reports hailed Mathilde as a “photogenic” aristocrat who “reinvigorated interest in the royal family and transformed the prince from an awkward recluse with little public support into an acceptable heir to the throne.” The Associated Press noted that the local visits were going swimmingly, with the couple “kissing babies, waving from balconies and smiling to huge, adoring crowds across Belgium.”

Delphine Boel in Brussels, March 2008 (Mark Renders/Getty Images)

But just as the royal family was riding high on public support following the engagement announcement, a major scandal rocked the monarchy. A new biography of Queen Paola was published in October 1999, and it included a very public statement of an old rumor: that King Albert had fathered an illegitimate child. The book named 31-year-old artist Delphine Boel as Philippe’s half-sister. Press reports noted that book’s publication “put a damper on the festivities” ahead of the royal wedding and further divided Flemish and French speakers in the country, with some viewing the book as part of a plot to “undermine the monarchy and the unity of Belgium.”

ROBERT VANDEN BRUGGE/AFP via Getty Images

But the book’s publication didn’t halt the plans for the upcoming wedding or put a damper on public (and media) enthusiasm about Mathilde. The couple celebrated their engagement with a ball in the indoor gardens on the grounds of the Royal Palace in Brussels on November 13, 1999. They were photographed smiling as they danced together for the first time. (King Albert and Queen Paola were also pictured dancing happily together, apparently unbothered by public revelations about the earlier difficulties in their marriage.) A photo of Mathilde from the ball made the cover of Paris Match, with a headline declaring her “Queen of Hearts.” Mathilde’s ruby engagement ring was also front-and-center in images from the party.

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Philippe had commissioned the ring from a Belgian jewelry firm, Wolfers. It features a dark red Burmese ruby set in gold and surrounded by a halo of diamonds. Additional diamonds decorate the side of the band, giving the ring a vintage feel.

Pascal Le Segretain/Getty Images

Mathilde has continued to wear the engagement ring on her left hand throughout her marriage. Above, she wears it in Paris in October 2012.

Pascal Le Segretain/Getty Images

And here, you’ll spot the ring on her hand during the inauguration of King Willem-Alexander of the Netherlands in Amsterdam in April 2013. You’ll note that Mathilde doesn’t stack her engagement ring and wedding band; instead, like many Belgians, she wears her wedding band on her right hand.

Mark Renders/Getty Images

Today, you’ll often see the engagement ring replaced by a different jewel. For their tenth wedding anniversary in 2009, Philippe commissioned a second gorgeous ring for Mathilde to wear.

Olivier Matthys/Getty Images

Wolfers also made this spectacular diamond and sapphire ring for Mathilde. She often swaps out her engagement ring for the sapphire ring at public events. Perhaps we’ll see a third ring added to her collection this year to mark their twentieth anniversary?