Leonor wore the ruby earrings Queen Sofía has never lent to anyone: not even her daughters Cristina and Elena had that honour

At the Princess of Asturias Awards in the Campoamor Theatre in Oviedo, Princess Leonor didn't come alone: with her were her parents, her sister Infanta Sofía and her grandmother, Queen Sofía. But the story of the evening was the earrings.

They are drop earrings with cabochon-cut ruby medallions surrounded by diamonds. Queen Sofía keeps them in her personal jewellery collection. They are not part of the Niarchos set, the jewels shipping magnate Stavros Niarchos gave her for her wedding to Juan Carlos in 1962, although she usually pairs them with it. A photograph from 1976 shows the queen wearing exactly these.

The detail that makes the story: according to journalist José Moreno, who covers the Spanish court, no other member of the royal family has ever worn them. Not even Infantas Cristina and Elena, the queen's daughters. When your grandmother takes something out of the box she never gave even her own daughters, that's a message about who's next in line, delivered without a single word.

The dress is by designer Aníbal Laguna, in shades of plum and aubergine, the colour fashion editors have declared the most wanted this autumn. A boat neckline, short sleeves, a fitted bodice and an A-line midi skirt. The silhouette is pure fifties, and the comparison with Grace Kelly makes itself. The colour also set her apart from the darker tones her mother and sister usually choose.

The rest was understated: black pointed Carolina Herrera slingbacks with shiny embellishments, a bag in the colour of the dress and her usual half-up hairstyle, which kept the earrings on show. Her make-up was almost blush-free. Make-up artist Paola Moyano says that works precisely because her blue eyes are the focus anyway.

The week had other looks too. At Thursday's concert Leonor wore a black, structured suit with an eighties spirit, with earrings belonging to her mother, Queen Letizia, an ear cuff by Moliane and a large silver bracelet. For Friday's audiences she chose a coral three-piece by Spanish brand Mirto: a wrap jacket, a waistcoat and wide high-waisted trousers, with nude heels and modest jewellery.

A modern monarchy is built on photographs, and every piece of jewellery in them is a sentence. The heiress to the Spanish crown is learning that language - and her grandmother, it seems, is giving her lessons from her own jewellery box.