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Aluminum exposed to air very quickly grows a thin passive layer of aluminum oxide (unlike steel rust, this layer is stable). Anodizing forces it to grow a thicker layer.

Aluminum oxide is hard stuff: 9 on the Mohs hardness scale (talc = 1, diamond = 10). https://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness

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Also note that pure crystalline aluminum oxide is called corundum, or when doped with other elements, the precious gemstones ruby and sapphire, rocks of exceptional hardness and chemical resistance.

> chemical resistance

I remember from a freshman chem lab that trying to dissolve aluminum into acid doesn't work because acid promotes oxide growth, so we dissolved it in NaOH, which was supposed to attack the oxide, then acidified the solution. Are the aluminum oxide minerals similarly susceptible, or do they also resist alkali?

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