A few years ago crown princess Mathilde was seen wearing a diamond rose brooch...
At the 60th birthday of the prince of Wales, in London (2008):
And at the King's Feast in Brussels (2008):
And at the wedding of prince William of Wales and Catherine Middleton, in London (2011):
However this diamond brooch does not belong to Mathilde. It's a loan from Belgian jeweller Wolfers.
The brooch is described as a "trembling brooch", i.e. with some diamonds mounted on little springs. They move slightly with the movements of the person, which multiplicates the glittering effect.