Thanks Erik, so the story about the gemstones inherted by Prince Hendrik of the Netherlands from his mother Qeen Anna Paulovna was correct but not the part that this bow brooch (or: sévigné) was made for his spouse Princess Amalia.
It was made for Prince Hendrik's niece, the young Queen Wilhelmina, instead. Always nice to learn how history can be a little different. Also interesting that apparently Prince Hendrik had 'loose' gemstones somewhere in a cassette. (I understand no other jewel was broken up to make this bow brooch).