I was not aware of the announcement of this forecoming State visit. Great news, we can fairly expect some nice level of royal glitter!
Regarding Infanta Leonor's possible first tiara appearance, I hope she will be able to attend. Leonor is the only Crown princess of her generation who has still not worn a tiara publicly (Ingrid-Alexandra of Norway, Elisabeth of Belgium and Catharina-Amalia of the Netherlands have). But it will probably depend on her training schedule in the Army: of course, it could be adapted to enable her to attend the State dinner, but I don't know if the Spanish Royal Family would be keen, in terms of public relations, to allow Leonor to "elope" her military duties to attend a representative function to which her presence is not, strictly speaking, necessary. Time will tell...
If she is able to attend, I doubt that we will see her in a new tiara (though I would be of course very glad if it happens). But considering Spain's economic hardships and the lukewarm popularity of monarchy in Spain, I suspect that King Felipe VI will be very careful to avoid any element suggesting that the Royal family has enough cash in their pockets to buy a new tiara, or that they got one from any foreign donor...
Another thought about the Spanish royal jewels: my impression is that Queen Letizia has the disposal of the "Joyas de pasar" + her Ansorena wedding-gift fleur-de-lys tiara + a few minor jewels (bracelets, earrings...) given or lent by Queen Sofia, but not much more. All the other tiaras belong personally either to Queen Sofia (the Mellerio floral tiara, the Prussian tiara, the Niarchos ruby necklace/tiara...) or to King Juan-Carlos (Mellerio Shell tiara...).
Due to King Juan-Carlos' financial misconducts, King Felipe VI had announced that he renounced in advance to any inheritance from his father, but I don't know if this would include also the royal jewellery...
As for Queen Sofia, she seems to have correct relations with the current royal couple, but she might also want to preserve the interests of her two daughters and of her grandchildren, so we can't be sure if all her jewels will go to Felipe VI/Letizia/Leonor - though Infantas Elena and Christina and their children will have scarcely any need of royal jewellery in their life...
Anyway, I hope at least all the historical jewels (whether from the Spanish or from the Greek side of the family) will end up with King Felipe VI and Infanta Leonor, while Elena and Christina could get their mother's modern (and in my view much less elegant) jewels...
To conclude, if Leonor is present at the State dinner for King Charles III and Queen Camilla, I hope to see her wearing the Mellerio Shell tiara, which is such a lovely and delicate tiara, perfect for a young woman. Or even better, a surprise jewel: either Queen Sofia of Greece's circle tiara (often worn in the 1950s/1960s by Princess Irene of Greece, then worn only once in the late 1980s by Infanta Elena) or Queen Frederika's pearl and diamond necklace/tiara, both supposedly belonging to Princess Irene of Greece...
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It seems Charles and Camilla are making a formal visit to Spain in the first part of 2024. It resulted in speculation about Leonor's first tiara.
Will she wear one of the family pieces or did she get a new one for herself?
It will be interesting to see what she gets to wear.