Among the jewels was the Princess' Ruby Parure Tiara - a piece she regularly wears to regal events and which has been in the Danish royal family since 1869.The tiara was left to Prince Frederik by Queen Ingrid after she died in 2000, who then gifted the jewels to Mary.Crown Princess Mary dazzled as she arrived at the event in one of her most glamorous golden gowns and wore a maroon sash draped elegantly across her shoulders.
Crown Princess Mary was seen pointing out the sights to her royal counterpart as they cruised along, enjoying a sunny afternoon at the start of a three day state visit.
Both were still wearing the glamorous pastel outfits they had arrived in to meet at the airport.
Queen Mathilde smiled as she stood to attention alongside her husband during the official welcome at Copenhagen Airport.
Kisses, hugs and curtsies were exchanged by the Belgian and Danish royals, and Mathilde and Mary seemed particularly pleased to see one another.
In the following weeks, the bodies of eight-year-old friends Julie and Melissa who had been kidnapped in Liege in June 1995 were found in a basement at one of Dutroux's houses, having died of starvation.
Following the grim discovery, Belgian authorities subsequently found the bodies of An Marchal, 17, and Efeje Lambrecks, 19, buried in a garden at another of Dutroux's properties.
But Prince Frederik had wisely ditched his navy checked suit jacket in favour of a warmer grey overcoat as they took to the river.
In the morning, Queen Mathilde was a vision in powder blue as she and King Philippe touched down in Denmark for a three day state visit.
During his trial, Dardenne explained that her tormentor had adopted an alternative name and insisted that he was protecting her from mysterious "wicked" people, all the while subjecting her to sustained sexual abuse.