Arcanium is a metal used to manipulate magical currents. Josho mentions that "...they use it to harness manashields around military bases. Even manticores can't belch their way through the junk." It seems to have similar properties to the chaos metal that was once used to contain Rainbow Dash in the Blue Shelf laboratory.
Arcanium was used to build the cage surrounding the book in the Noble Jury's engine room, in order to keep its energy from hindering Rainbow Dash. It was also used in the Silvadelian "death water" bomb, and in the radar-cloaking devices on Searonese manaships. Rainbow also mentions it being used to secure spellbooks back in Equestria.