It's been known for decades that the Earth's core (which is mostly iron and nickel, as opposed to the mantle which is made of silicates) has a liquid outer layer and a solid inner layer. This is apparent because seismic shear waves can propagate through the inner layer but not the outer layer, which is diagnostic of the medium being liquid and not solid. (The shear or S-waves get to the inner core because when a seismic pressure wave, which can travel through liquid, hits the inner-core-outer-core boundary, some of the energy is converted into shear waves. Jofer please correct me if this is inaccurate.)

This article states that the inner core has two different concentric zones that have very different crystalline properties. In particular, the inner sanctum is anisotropic with respect to seismic wave propagation, and the outer sheath of the inner core is much more isotropic. This suggest that they have different geologic histories--the innermost core's crystallographic anisotropy may have resulted from an event early in Earth's history that deformed the crystals, much as mountain building events deform the crystals in Earth's crust and upper mantle. Then, the younger outer sheath of the inner core cooled and crystallized around the older inner bit later on, after the deforming event transpired.