For questions about the Earth's core. The center of the Earth is primarily iron-nickel.
For questions about the Earth's core.
Following the chonditic model of the Earth and other evidence (eg. mass distribution, active magnetic field), the core is primarily composed of iron and nickel. The inner core is ~1220 km diameter and solid due to high pressure. This is surrounded by the liquid outer core which is ~2260 km thick. The solid:liquid boundary produces a strong seismic reflector and S waves do not pass through the outer core.
The outer core is actively convecting and is believed to act as a 'self exciting dynamo' which produces the Earth's active magnetic field.