There's been a discussion in the thread Pre-Napoleonic Tiaras about the time the Bragança Tiara was first created, with Boris providing a lot of information.
I am fortunate to participate in a Brazilian Genealogy Newsgroup in which there are several respected historians, genealogists and authors. I questioned the participants about the tiara and Paulo Rezzutti (author of the book "Titilia and Demonão" - that's how D. Pedro I and Domitila de Castro Mello, Marqueza de Santos called each other when they were lovers) brought some lights about its origin.
When D. João VI, still living in Brazil at that time, arranged his son’s (D. Pedro I) marriage to Archduchess Maria Leopoldina (daughter of Francis II, Holy Roman Emperor) in 1817, he sent the Marquis of Marialva in a most exquisite and luxurious embassy to the Austrian Court. D. João VI sent along to almost everybody in the Austrian court expensive jewels and stones from Brazil as tokens of the occasion , including some spectacular diamonds to D. Leopoldina.
After D. Leopoldina died, D. Pedro I married D. Amélia (Amélie Auguste Eugénie Napoléone de Beauharnais) in 1829 and gave her a tiara. There is a document signed by D. Pedro I in 1830 in the archives of the Imperial Museum of Petrópolis in which he recognizes he is in debt with his children by D. Leopoldina because he used the diamonds that had belonged to their mother to make the tiara he gave to D. Amélia.
The very best diamonds Brazil has ever produced were from the XVIII century and I guess some of them were probably sent to a diamantaire before being given to D. Leopoldina. That is why the research conducted in 1976 by the Swidish for an expositon of the royal jewels on the occasion of King Carl Gustav and Queen Silvia’s wedding reported that the Bragança tiara was from the XVIII century.
Anyway, the Bragança tiara and other diamond jewels left by D. Amélia do her sister Queen Josephine of Sweden seem to have been made from D. Leopoldina’s diamonds.
Mr. Rezzutti also remembers that at some point he was informed that there is a letter from D. Amélia to her mother, Duchess Augusta, in which she says she wore the diamond tiara given by D. Pedro I at the first “Beija-Mão” (a ceremony of Portuguese and Brazilian usage when the nobles kissed the hand of the King and Queen to demonstrate their obidience) in Brazil. That might be the reason why the Bragança tiara was refered previously by Queen Josephine as Coronation Tiara.
I don’t consider this issue closed, but I tend to believe that the Bragança tiara was either mounted for D. Leopoldina (as a first version, then alterered to D. Amélia) or to D. Amélia, which means it is not pre-napoleonic.