Many models of Gas giants have a supercompressed "metallic" hydrogen core in which the huge magnetic field is generated. perhaps this would be a shell overlying the rocky and icy core - also supercompressed.

On Earth, the density of the core is about 13.9 g/cc from memory, but iron at surface conditions is only about 5.5 (don't hold me to the exact numbers, but it's something like that. The pressure of all the stuff on top packs everything closer together).

On a larger scale, even Stars can have "solid" cores, but temperature fights comaction. Cool old White Dwarfs have very compressed internal material and in neutron stars, it's "degenerate matter" with the electrons pushed into the protons and the remaining neutrons packed together like passengers on a Tokyo subway.

And there are are black holes, of course - not just matter but space-time itself becomes "degenerate"!

In most of these cases, if you somehow collected the proverbial matchbox-full of the material from the inner layers and opened the box on Earth, it would be quite spectacular as it decompacted...