I suspect.

The Belgian Royal House more so than any other reigning royal house has dissipated, what was in the past, a very grand collection.

Queen Mathilde does not have a huge collection and she isn't adverse to wearing jewels - I seem to recall she even borrowed a brooch from a French jeweller to wear to a Scandinavian wedding a few years - If she had a stash of jewels at home just gathering dust, I suspect she would find a way of using them rather than borrowing jewels.