I agree. It's very similar to https://en.wikipedia.org/wiki/Corundum https://en.wikipedia.org/wiki/Corundum that is transparent unless it has some minor contami

I agree. It's very similar to https://en.wikipedia.org/wiki/Corundum https://en.wikipedia.org/wiki/Corundum that is transparent unless it has some minor contamination that gives it a color and is call "ruby", "sapphire", ... Nobody call ruby a metal.