The term allochromatic is used in reference to gem materials that derive their color from elements that are not an essential part of their chemical composition. In other words: the gemstone's color is due to impurities. A good example of an allochromatic gemstone is sapphire which, when pure, is colorless. Blue sapphires get their color from titanium (Ti4+) and iron (Fe2+) ion impurities. Neither of these two elements are essential to form sapphire which is an aluminum oxide (Al2O3). When the coloring elements are an essential part of a gem material's chemical build up we speak of idiochromatic gemstones.