I was thinking about Mathilde but it counts for every royal in general. I can understand that the spouses of HH Prince Pieter-Christiaan or Prince Maurits van Oranje-Nassau, Van Vollenhoven have no need for grand jewels. The same counts for the spouse of HRH The Duke of Parma.
I can imagine that todays excellent quality alternatives for diamonds and other gemstones (which still have a price tag anyway) would be a great solution for these ladies. When the Duchess of Parma is invited for a grand gala, she will probably borrow jewels from the Orange-Nassau collection while she can probably look even much grander with a new purchase made from "eco diamonds" or something.
I have seen with my own eyes, at Christmas Dinner, how fantastic those eco diamonds glittered. In fact all these royal ladies can glitz and glam as if they were Russian Grand Duchesses.