Complete tangent, but alumina, aka aluminum oxide, is usually considered the second hardest naturally occurring material. When it is found in nature, it is given the mineral name corundum and is clear. But if there are some impurities in it, you can get colours. Red corundum is called Ruby, and blue is called Sapphire. In the beauty industry, the same material (mixed with magnetite) is called emery, and lends its name to emery board, and is used in nail files. In the tech industry, it’s used to make the extremely scratch resistant coating on most modern phone screens (basically nothing but diamond will scratch it).
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