I understand that it was allegedly made from the Nails of the Crucifixion that St. Helena found, but from the wikipedia:
Blom (2002) stated that: "Constantine also understood the value of these objects in diplomacy"; several were sent off to various dignitaries, one of whom was Princess Theodelinda. She used her nail as part of her crown, the famous Iron Crown of Lombardy.
Theodelinda reigned around 600 AD, Constantine reigned c. 306-330. Is it a different Theodelinda? Or a different Constantine?
Neither. The crown is traditionally held to include a nail of the True Cross, and was probably part of Constantine's paraphernalia (possibly a component of his helmet), which was brought to Milan by Theodosius, the last emperor to rule over a united empire. However, there is a competing theory that holds that it was created by Lombard jewelers some time during the reign of Queen Theodolinda. The presence of a nail of the True Cross is contentious as well, since there's no mention of it prior to the 17th century. Chemical testing on the crown revealed that it's mainly composed of silver, not iron, however there is still debate as to where on the crown, precisely, the nail is contained.
The composition of the crown has been most recently analyzed by Valeriana Maspero (La corona ferrea. La storia del più antico e celebre simbolo del potere in Europa Monza, Vittone Editore) in 2003.