@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.
@freemo I could be wrong on the reasoning, but I believe the term sapphire exclusively referred to the blue variant (and possibly lapis lazuli) from an etymology perspective. It is a VERY old word, in fact.
Maybe it's not a marketing gimmick just natural language evolution, but in that case, why call them sapphire or ruby at all? Just call em *Corundum(or *Sapphire), where * is the RGBAOCCCC (rgba+optical orientation+the 4Cs) of the gem. Maybe it doesn't roll off the tongue? XD
WHO DEVISED THIS TERRIBLE NAMING SCHEME? I NEED ANSWERS FREEMO, WHY ARE PEOPLE LIKE THIS?!