Thank you for these pictures, and for the possible link between Princess Astrid's pendant and Queen Sonja's earrings.
I am pretty sure that the brooch worn by Princess Astrid is this one, pictured here among Queen Maud's wedding gifts. According to the caption, it was a gift from her husband, Prince Charles of Denmark (later King Haakon VII of Norway):
(pictures first posted by grandudchessella - on this board?)