Ruby is a gemstone variety of the aluminum oxide mineral corundum (Al2O3). The red color from which its name is derived (the Latin word ruber translates to ‘red’ in English) is related to the presence of trace amounts of chromium in the mineral structure. Ruby is a precious stone commonly used in jewelry, together with sapphire, emerald and diamond.
Information source: Mindat.org
Image: Ruby crystals found in Vietnam. Source: Parent Géry, Wikimedia Commons.