If I am not mistaken the base is the same as the one of the emerald tiara, which has been used over the years by various princesses And in more recent times also in more and unexpected variations (upstanding pearls) too!! Furtherdown this page, in another thread, I posted a photo showing Queen Juliana posing for an official portrait (with her husband) where HM wore the emerald tiara 'upside' down, something Princess Irene also has done. Princess Margriet has used it frequently over the years, like this photo shows from the Silver Wedding festivities/May 1962:
And here is the emerald tiara on the head of another jewel-loving lady: