I’ve been researching Italy’s transition from the Fall of Rome to becoming a part of the Holy Roman Empire and I came across the hypothesis that the Iron Crown of the Lombards was already used in the coronation of Charlemagne as King of Italy. The first definite use we know of was Conrad III in 1128, but it might have been used earlier. The last instance was Ferdinand of Austria in 1838.
At first it seemed like this might be a relatively long time period, but after some thinking I realised there are others that have been used over the same, if not longer, periods. What’s your candidate for longest used crown?
Depends on how you want to view it but I would nominate the royal insignias of Hungary.
Primarily the Crown of St. Stephen, named after King Istvan I., who converted to Christianity in 1000 AD and received a crown and hereditary title of "apostolic king" from the pope. Unfortunately, this one was lost and thus is not the actual crown existing today.
The one that is commonly referred to as St. Stephen's Crown has a somewhat hazy origin story with many mythical elements but I believe the consensus is that it was started (having added various stuff at later times) under the reign of Bela III. (1172-1196). Since Bela was educated in Byzantium that would also explain the distinct Byzantine base design.
Anyway, even if we take the crown's dates to be its first and last use in a crowning ceremony and assume that the tradition started after Bela, we have a probable starting date of 1196 with the coronation of Imre I. and ending with the crowning of Charles I. of Austria-Hungary (as Károly IV.) in 1916. Thus the Crown of St. Stephen would have been in use 10 years more than the Iron Crown.
Same thing for the Hungarian sceptre, which is just weird by European standards, although there we find some elements that are older and have just been assembled and made into a sceptre then; the crystal orb engraved with three lions on the head is thought to date to 10^th century Egypt, to give the oldest example.
Though if you want to go the longest we could take a look at the Hungarian coronation mantle. According to its own embroidery it was ordered by Istvan I. in 1031 and donated to the Church of the Virgin Mary in Székesfehérvár until it was turned into a coronation piece in the late 12^th century and again used until 1916. Shorter time of use as an insignia but much older in total.
Sources:
Endre Tóth, Károly Szelényi: Die heilige Krone von Ungarn. Könige und Krönungen
