The Earth's inner core is not made of rock in the traditional sense. It's primarily composed of solid iron with a small percentage of nickel.

Here's why:

* Extreme Pressure: The immense pressure at the Earth's core, around 3.5 million times the pressure at sea level, forces the iron and nickel into a solid state, despite the incredibly high temperatures.

* No Typical Rocks: The conditions in the inner core are so extreme that the minerals we find in rocks at the Earth's surface wouldn't be stable there.

So, while we can't say there's "rock" in the inner core, it's a solid metal sphere made mostly of iron and some nickel.