What are the inner and outer cores made of?
The inner core is solid with density up to 13 gm/cc, at the center. The outer core comprises solid-like low-viscosity liquid that reflect secondary seismic waves.
Perhaps, owing to the basic difference in structure between low-viscosity liquid above and higher-density solid below under higher temperature and higher pressure, the inner core seems to spin a little faster than outer core, over the orbit period,
Seismic wave propagation during earthquake events, give all these clues to the patterns of layers, from surface to the center. Any abrupt change in the propagation discloses discontinuity.
Both the nearly annular outer of width about 1400 miles and nearly spherical inner core of radius about 800 miles might contain iron, nickel, platinum, gold et al.