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 is 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.