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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.

The sapphire in this ring would be colorless if it didn't contain iron and titanium ion impurities. Gemstones which derive their color from impurities are allochromatic.
Lang Antiques
Lang Antiques