It's rocky in the sense that it has elements heavier than hydrogen and helium, but not in the sense that it's solid. Temperature within the cores reaches 40k Celsius, so everything is molten.
At the pressures and temperatures we’re not talking about either gases or liquids but a supercritical fluid and even more exotic phases, including liquid metallic hydrogen which is thought to be an excellent solvent.
I suppose it means the boundary is a massive and eternal dust storm. That's the only way I imagine solids and gases to "mix".