Allochromatic
from the Antique Jewelry University
The term allochromatic is used in referral to gem materials that derive their color from elements that are not an essential part of their chemical composition. Their color is due to impurities. A good example of an allochromatic gemstone is sapphire that, 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.