From Halopedia, the Halo wiki

Were you looking for Iron Skull?
There is more information available on this subject at Iron (element) on the English Wikipedia.
A segment of iron ore.

Iron is a chemical element with the symbol Fe (Latin: ferrum) and atomic number 26. Iron is a group 8 and period 4 element on the periodic table. Iron is a lustrous, silvery soft metal. It is one of the few ferromagnetic elements.

Iron is used by the UNSC in a wide array of functions, including use with tungsten to create rounds for the Magnetic Accelerator Cannons, used by the UNSC Navy warships as high-penetration weapons. The metal was also used in the composition of the M645 FTP-HE round used in the ARC-920 railgun.