Stone or rock, is a naturally occurring solid aggregate of one or more minerals or mineraloids. They have been used by mankind throughout history. From the Stone Age, stones have been used for tools. The minerals and metals found in stones have been essential to human civilization.
Ruby / Corundum
Ruby / Corundum is a crystalline form of aluminum oxide (Al2O3) typically containing traces of iron, titanium, vanadium and chromium. It is one of the naturally transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, ruby if red and padparadscha if pink-orange. All other colors are called sapphire, e.g., "green sapphire" for a green specimen. Because of corundum's hardness (pure corundum is defined to have 9.0 Mohs), it can scratch almost every other mineral.