My very elementary answer, until more sophisticated people pass by:
Similarly to silicon, aluminium is light and easily oxidized, as compared to iron and nickel.
Earth composes of much metal, a bit of oxygen, little carbon, nitrogen, hydrogen...
So you find oxidized aluminium and silicon at Earth's surface, and metallic iron and nickel in depth.
That is, nearly every stone or pebble in Earth's crust is silica or an alumino-silicate.
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This segregation happened because Earth is big enough that gravitation and radioactivity have melted it. Expect the same for the biggest asteroids, down to some 500km size - all objects with a round shape. Smaller objects, with an irregular shape, are not differentiated into core, crust and the like.