@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
@johnabs While blue sapphire and other colored sapphire does indeed have things in it other than aluminum oxide giving it its color dont forget clear sapphires. Those are just crystalline aluminum oxide.
Did not know that the layer on aluminum was amorphous though, fair point.
@freemo I didn't forget, I was just saying that applying the term "sapphire" to a piece of corundum without any colored impurities is likely just a marketing gimmick, since most people don't know what corundum is, but they recognize the terms ruby/sapphire. The problem is rubies MUST be red by definition, whereas sapphires used to refer to blue corundum, the term has been relaxed to include any color including clear (which kind of defeats the point of the term corundum/sapphire completely).
@johnabs Clear corundum occurs naturally and is often called clear saphire. So I wouldnt really call that a gimmick. Ruby, yes, they must be red, they are just red sapphires as you know. But no sapphire was never restricted to just blue as far as I know.the other color of sapphires are still called sapphires, at least since the 80s when i started collecting them.
@johnabs Oh well yes if your talking ancient times then sapphire originally referred to only the blue ones. In fact they had no idea clear sapphires were sapphires at all.
Sapphire and Ruby are terms generally used more in marketing, though i still wouldn't call a gimmick, its just how the word evolved. Geologists almost always call it corundum in my expiernce, particularly when talking in the general sense (inclusive of all colors).
@johnabs Also dont get started on the absurdity of human naming things... I've been ready to use a constructed language for decades :)