Jupiter consists mostly of liquid hydrogen, a large portion of which (shown in darker gray) is metallic hydrogen. Deep inside, an icy core must overly a rocky and metallic core, but the size and mass of these cores is too small to significantly affect observations of Jupiter's properties, so we can only make educated guesses as to their size.
     The structure of Saturn is similar to that of Jupiter, but it is much less compressed under its weight, so it has far less metallic hydrogen.
     Uranus and Neptune have relatively small amounts of hydrogen -- just a few Earth masses -- above their icy and metallic cores, so we can be more confident about the structure of those cores. For Neptune, which has more mass than Uranus, but is smaller, the core must be most of the mass, while for Uranus, the hydrogen "mantle" must be more significant, and perhaps even exceed the mass of the core.