The official court website has sometimes refered to Madeleine as wearing 'her own tiara'. OK, the PR department is know to make mistakes from time to time. But imo this still is a strong indication that the modern fringe is now really owned by Princess Madeleine.
If that is the case, it would definitely make sense that Sofia also receives a personal tiara. And if a new tiara is bought for her, it does not matter if it should leave the family in case of a divorce. I mean a divorce will always be expensive, but there would not be a lost heirloom. And what would CP do with his ex-wife's tiara? Give it to wife no. 2? Anyway, let's not speculate too much about divorce, they have to marry first
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Giving her a personal tiara does not mean that she will be barred from using the historic tiaras from time to time. She will just be on the same footing as Madeleine. And it doesn't even mean that she will necessarily wear it at her wedding. She could still wear a family tiara at the wedding start wearing her own later on. Although the weddings seem a perfect event to break in a new tiara.
CP Mette-Marit and CP Mary both received a new tiara from their in-laws and wore them at their weddings. Does this mean that we think they were not accepted by the family? I don't think so. And the SWR has made it very clear that they accept Sofia. They invited her to private events for years, included her in all sorts of official events since the engagement, etc. Still, people who dislike her will always look for signs that her future in-laws dislike her. Wearing a certain tiara won't change that.
I do not believe that a new tiara would come as gift from the groom. It seems rather the tradition that the new parents-in-law give it (at least this is what happened in Norway and Denmark, Belgium is a different story because Mathilde's tiara was a gift from the nobility). As for Carl Philip designing one: I hope not! Firstly, contemporary tiaras are very often rather awful. Secondly his designs so far aren't indicating that he could pull off a good tiara design. Just my opinion.
A good option is to buy an antique tiara. Usually one gets the most flattering designs for a better price than in a new tiara. But I don't think that they would buy at auction. That would mean that the price tag would be all over the papers and the internet! But it is also possibly to dicreetly buy antique pieces at a jeweller. Probably that's what was done with Mary and Mette-Marit's tiaras, because no one ever found out anything about them being auctioned.