The Iron Crown of Lombardy was made in the Middle Ages. Legend maintains its central band was beaten from a nail of the True Cross. It was used in the coronation of the first Holy Roman Emperor Charlemagne on Christmas day 800 and was worn again in 1805 by Napoleon when he was crowned King of Italy.
I'm not really very knowledgeable about the distribution of the nails but I'm sure there's a strong case for skepticism about their authenticity. A nail wouldn't have to be especially remarkable to be the embodiment of an apocryphal association.
Perhaps, it would most likely make it a little too heavy and cumbersome for whichever person would be next to be crowned with it! Something a little simpler might serve just as well.
I’ve never worn a crown,¹ but they look uncomfortable.
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1. The cardboard ones from Burger King excluded. Although in that case I can testify that yes, they are uncomfortable.
I've never worn a crown either, apart from the paper ones you get out of Christmas crackers. There does seem to be a noticeable absence of padding in the proper ones!
