The Nuttall Encyclopedia
The crown of the ancient Lombard kings, a golden circlet studded with jewels, and so called as enclosing a ring of iron said to have been one of the nails of the cross, beaten out; Napoleon had it brought from Monza, and crowned himself with it as king of Italy. It is now in Vienna.
Wood, James, ed. Entry for 'Iron Crown'. The Nuttall Encyclopedia. https://www.studylight.org/encyclopedias/eng/nut/i/iron-crown.html. Frederick Warne & Co Ltd. London. 1900.