Here's why:

* Solid State: Unlike the outer core, which is liquid, the inner core is solid due to immense pressure.

* Composition: It's primarily made of iron and nickel, with trace amounts of other elements.

* Extreme Conditions: The inner core experiences incredibly high temperatures (around 5,200° Celsius) and immense pressure, roughly 3.5 million times that at sea level.

While the crust and mantle are also rocky, they are much less solid. The crust is relatively thin and brittle, while the mantle is a semi-solid, viscous layer.