I think we might have to look at previous Dutch-Norwegian State Visits for an indication what we might see.
- 1990 (Amsterdam) was a battle of the pearls - Queen Maud's tiara for Sonja and the Ornate Pearl tiara for Beatrx. Both ladies wore important necklaces - the large Ornaje pearls for the Dutch Queen and diamond collects + pearl string for her Norwegian counterpart.
See picture
- 2010 (Oslo) was a more or less a repeat for Queen Beatrix (who wore a different brooch), but Queen Sonja was the Ice Queen in glittering diamonds - Queen Josefina's diamond tiara, the Turkish earrings, collet necklaces and Queen Sofia's oval brooch. MM and Princess Astrid opted for their big guns - amethysts and turquoises.
See picture
Now, I believe that Queen Sonja enjoys wearing her jewels and alternating between her options. Given that she wore the emeralds at the 80th birthday celebrations in 2018, my guess would be for diamonds. They are colour-neutral and can easily be coordinated with the Dutch sash.
Princess Astrid will probably wear the Vasa tiara either with her collet necklace or the turquoises.
Mette-Marit might go for the complete amethyst parure or repeat her jewels from the 2006 Bulgarian state visit - daisy bandeau and Vifte tiara as necklace. Personally, I hope she's loaned the small setting of Queen Maud's pearl tiara with one of the collet necklaces for extra sparkle.
Maxima is the difficult one to predict. The Stuart tiara in the Lux. setting with an all-diamond necklace would be fitting, but she might also wear one of the ruby tiaras with a matching necklace (a nod to her hosts). I'm excluding the sapphires (worn in 2018), the Ornate Pearl tiara (worn in Norway too many times), the emerald one is too small. If she goes for pearls, I hope it will be the Antique pearl tiara.