I'm really uncertain how best to answer this question. Its not that you did not make your question clear, it is that more or less there is no "one simple answer". http://en.wikipedia.org/wiki/Gas_giant Deals with the Jovian Planets in our solar system and we soon discover that although the 4 Jovian Planets (Gas Giants) are similar, they are vastly different when it comes to the gases in the atmosphere which may have some impact on what is at their core. "Uranus and Neptune may be considered a separate subclass of giant planets, 'ice giants', as they are mostly composed of ice, rock, as well as gases of water, ammonia and methane, (snip for length)... ...Gas giants may have a rocky or metallic core—in fact, such a core is thought to be required for a gas giant to form—but the majority of its mass is in the form of the gaseous hydrogen and helium, with traces of water, methane, ammonia, and other hydrogen compounds..." Saturn and Jupiter both have higher levels of Hydrogen and Helium thus their distinctive "hue" or color - those being mostly in the reds to yellows and Neptune and Uranus being blue to green. Composition of the atmosphere could have a huge impact on the composition of the core. Compression of say Hydrogen under higher pressures make liquid Hydrogen, compress that further and you end up with a solid - or "metallic hydrogen". We can be fairly confident that metallic hydrogen exists at the core of Jupiter since it is abundant in hydrogen. How Large are those cores? Unknown. We can calculate very roughly (which is a smart way to say "guess") due to the various sizes and compositions I'll just post a link to the page on each planet, you can scroll down to composition/Internal Structure for each. Jupiter: http://en.wikipedia.org/wiki/Jupiter Saturn: http://en.wikipedia.org/wiki/Saturn Uranus: http://en.wikipedia.org/wiki/Uranus Neptune: http://en.wikipedia.org/wiki/Neptune There are other theories out there. Due to some carbonization of some in falling matter, and the high pressures and greater heating at the cores, some have theorized that there is a diamond at the core of the Jovian planets. A diamond half the size of our moon. Ultimately we could be wrong no matter what. We have made reasonable assumptions about the conditions and processes that form planets. We assume that the Earth has a dense molten heavy metal core that spins. For all we know it could be a core of some other non-metal which under pressure and heat has taken on the characteristics of a spinning sphere of metal. (aka, creating a magnetic field). When it comes the Outer Gas Giants we could be more in the dark. Perhaps a "solid core" is never reached. Perhaps due t the pressures and heat and the materials the core if made of there is only denser and denser liquidity of the core.