Do you know about the core of the planet Jupiter?

Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. Jupiter is primarily composed of hydrogen with a quarter of its mass being helium; it may also have a rocky core of heavier elements. Because of its rapid rotation, Jupiter's shape is that of an oblate spheroid. The most recent probe to visit Jupiter was the Pluto-bound New Horizons spacecraft in late February 2007.

I just wikipedia'd it, and it says that the core is thought to consist of dense materials with a surrounding layer of liquid metallic hydrogen. But what are these "dense materials" in the core.