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A ruby is a crystal of alumina, aluminum oxide \((Al_2O_3)\) containing a trace of chromium \((III)\) ions replacing some of the aluminum ions. In ruby, each \(Al^{3+}\) ion and \(Cr^{3+}\) ion is surrounded by six oxide ions in an octahedral arrangement. The origin of the color of emeralds is similar to that of the color of rubies. However, the bulk of an emerald crystal is composed of beryl, beryllium aluminum silicate \((Be_3 Al_2(SiO_3)_6)\) instead of the alumina which forms rubies. The color is produced by chromium \((III)\) ions, which replace some of the aluminum ions in the crystal. In emeralds, the \(Cr^{3+}\) is surrounded by six silicate ions, rather than the six oxide ions in ruby. Therefore, the color (green) of emeralds is different from that of ruby