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Interesting fact of the day. All aluminum has a skin of sapphire. When you cut a piece of aluminum, and expose the pure aluminum to the air it very quickly reacts with the air on the surface forming aluminum oxide, which is identical to lab and natural sapphire, except much more pure.
@freemo It appears this 4nm layer is amorphous (or at least polycrystalline), not corundum/sapphire, unless produced by plasma oxidation with the express purpose of producing hardened aluminum oxide for increased hardness. Additionally, while clear, pure, alpha-crystalline aluminum oxide is often called sapphire (for what I think must be a marketing gimmick) it is actually just pure corundum, as sapphire is corundum with the metal ion inclusions that aren't red (which is ruby).
But the aluminum passivity layer is pretty sick from a materials/chemical perspective for sure :3
