At first, I thought that the Maria Teresa came to the birthday dinner wearing Grand Duchess Joséphine-Charlotte's Diamond Scroll Tiara, but then helpful reader Shannon sent in this better angle:
... which shows that this is the Luxembourg Diamond Floral Tiara. I don't know much about the provenance on this one ... do we have any tiara historians who know a bit more about the Luxembourg collection and can fill us in?
(Quickly, though, can we give three cheers to Grand Duke Henri? He's not only holding his wife's fur wrap for her in the first picture above, he's also attending Margrethe's birthday party on his own birthday! He turns 55 today. Happy birthday, Your Royal Highness!)