MIT Study shows making hydrogen with soda cans and seawater is scalable and sustainable

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4. Where do you see that?

I don't see Al2O3 mentioned in the article, other than the need to disrupt the preexisting Al2O3 coating on the aluminum before proceeding.

I see two byproducts mentioned:

Extensive research on AWRs has shown that the reaction products can include AlOOH (boehmite) or Al(OH)3 (bayerite or gibbsite).

Even if Al2O3 was a byproduct of this process, it's pretty unlikely it would be in the form of a gemstone. Your lawn chair and other metallic aluminum objects are covered in a thin layer of Al2O3, but no one would mistake them for sapphire.