Mineral that would be colorless if chemically pure, but which commonly exhibits a range of colors due to the presence of small quantities of one or more coloring elements. Chief among these elements are those having atomic numbers 22 to 29; namely, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, and copper. Corundum, beryl, spinel, and quartz are examples of allochromatic gemstones.