Iron Crown of Lombardy
is so called from a narrow band of iron within it, said to be beaten out of one of the nails used at the Crucifixion. This band is about three-eighths of an inch broad, and one-tenth of an inch in thickness. According to tradition, the nail was first given to Constantine by his mother, who discovered the cross. The outer circlet of the crown is of beaten gold, and set with precious stones. The crown is preserved with great care at Monza, near Milan; and Napoleon, like his predecessor Charlemagne, was crowned with it. After the war between Austria and Italy, the Iron Crown was delivered by the former power to Victor Immanuel.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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