May Birthstone - Emerald

Physical and Chemical Properties

Chemical composition

Emerald is a green to blue green variety of the mineral Beryl, which has a slightly complex chemical formula: Al2Be3(Si6O18)a silicate of Beryllium and Aluminum., 7.5 to 8 hard, 2.65 to 2.75 in specific gravity.

Emeralds, as all other beryls, can range from being opaque to transparent. Beryl has fairly low optical properties. The refractive index ranges from 1.57 to 1.60 with many gems falling in the 1.572 - 1.580 range. Beryl has a low birefringence and are only weakly doubly refractive. It would take a piece of beryl about 15 inches thick to show a double image. The dispersion of beryl is also quite low (0.014) and emerald, as other beryl gems, has to essentially make it on color and pleasing appearance without the brilliant flashes of light that we associate with diamonds or gems with a very high refractive index or dispersion. Emeralds are usually clouded by many inclusions.

Colors

Emerald owes its color to a small percentage of Chromium Oxide in the crystal lattice---up to 3%. Strangely, this is the same oxide in the same percentage that imparts the red color to ruby when it is in the corundum lattice. Emeralds vary in color from light to dark green and most have inclusions, which are made of substances that become trapped in the formation of the gem. Completely transparent emeralds can be so rare that they are valued higher than diamonds.

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