I doubt very much it is a costume piece. More likely made of gilded silver, like a lot of the jewellery worn with the "bunad" (the traditional attire with regional variations). This type of jewellery is called "søljer". It looks like the three "pendants" in the crown are just gilded silver buttons pressed to get a concave shape. The difference in appearance is probably down to a trick of the light depending on how it hits and reflects in the concave shapes.

I have tried to look for photos of similar headpieces, but it doesn't seem to very normal. There are actually crowns though that can be used by brides if they marry in "bunad", but I haven't seen any diadem in my google search like the one worn by the Princess. Still, she is a Princess and if anyone should be able to wear a diadem (or tiara) like that it would be our Princess Astrid!







I found this one made with a coin with the portrait of the Princess' father, King Olav V, rather charming: