They actually have quite a few jewels that can be worn as tiaras:
The diamond and pearl tiara of Princess Charlotte
The diamond fringe necklace/tiara of Princess Charlotte
The small diamond tiara of Princess Grace
the Bains des Mer diamond and ruby tiara
The modern diamond necklace/tiara of Princess Grace
The sapphire necklace/tiara of Princess Caroline
The diamond flowers of Princess Caroline
The sapphire necklace/tiara of Princess Charlene
The diamond aigrette of Princess Charlene
That makes for more pieces than the Belgian Queen has for a nation that has only very few tiara-events.

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I've always wondered why the princely family doesn't own more tiaras.

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She had cancelled her visit to the netherlands during her pregnacny in 2014. There was a tiara event at Paleis het Loo

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There is also princess Gracia's small diamond tiara. That is simple in its lines and light to wear. It would suit Charlene but she seems to dislike wearing tiaras. Did she wear a tiara to another event than her wedding ball? She did wear them for photoshoots but for actual events?

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That would be great, it makes a gorgeous fringe tiara in that configuration.

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On second thought. Perhaps mounting the fringe necklace onto a tiara frame could be an option for a tiara for Charlene. Classic but modern, light and (thus) wearable. Donate the spray and ocean tiaras to a charity.

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Stunning piece of jewellery on a truly stunning woman IMHO. Caroline is the only member of the Grimaldi family who has a regal demeanour and who can actually pull off wearing grand jewels. The Reed corsage does not need to be converted into a tiara because what's the point? The last time Caroline wore a tiara must be a decade ago. Charlene doesn't wear tiaras.They do not really suit her and she doesn't mingle with royals anyway. Pity the grand jewels like the Monaco pearl tiara and the fringe necklace/tiara are never worn.

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Princess Caroline of Hanover is wearing her Chaumet Reed Stomacher with Princess Charlotte’s Diamond Earrings and Princess Grace’s Van Cleef & Arpels Diamond Bracelet at this year’s ‘Sunset Ball de la Rose’ in Monte Carlo .