Beryllium aluminum silicate crystals

Rubies are deep red and emeralds are brilliant green, yet the color of both gemstones is caused by the same ion, Cr. The difference lies in the crystal that hosts the ion. Rubies are crystals of aluminum oxide (AI2O3) in which about 1% of the Al ions are replaced by Cr " " ions. Emeralds, by contrast, are crystals of beryllium aluminum silicate [Be3Al2(Si03)6] in which a similar percentage of the Al " ions are replaced by Cr. The imbedded Cr " ion is red in the aluminum oxide crystal but green in the beryllium aluminum silicate crystal. Why ... 

A colorless mineral known as corundum (composed of aluminum oxide) is colorless. A red variety of corundum known as ruby, a precious stone, owes its color to impurities of chromium within the crystal structure of corundum. Blue and violet varieties of corundum are classified as sapphires, the blue being the result of iron and titanium impurities, and the violet of vanadium impurities within the corundum crystal structure. Another colorless mineral is beryl (composed of beryllium aluminum silicate) but blue aquamarine, green emerald, and pink morganite, are precious varieties of beryl including different impurities aquamarine includes iron, emerald chromium and vanadium, and morganite manganese. 

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