Nickel is a chemical element, with the symbol Ni and atomic number 28. Nickel is a white/gray soft metal and is one of the few ferromagnetic elements.
Nickel and Iron are notable for being the final elements produced by stellar nucleosynthesis, and are therefore the heaviest elements which do not require a red giant or supernova for formation. Nickel and iron are therefore the most abundant metals in metallic meteorites and in the dense-metal cores of planets such as Earth. Nickel and iron alloys are also the most common source of ferromagnetic materials in everyday use.
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. And they are also used in the M6 pistol series.