Aluminum

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“The mineral corundum which includes the precious stones ruby, sapphire, and topaz, contains alumina, or aluminum oxide, Al2O3. It is found in nature crystallized in rhombohedral forms, having a specific gravity of 3.9, and is scarcely inferior in hardness to diamond.” —Hallock 1905

Source

V. I. Hallock I.C.S. Refrence Library (Scranton: International Textbook Company, 1905) 11:60

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