Tuesday, June 26, 2012

What makes Rubies Red

An uncut ruby crystal on a limestone matrix
Photo by Rob Lavinsky

Why rubies are red is a question that many people ask themselves when they see this beautiful gemstone that is a member of the larger family of gemstones that are termed corundum that are naturally occurring crystals of aluminum oxide. Pure corundum itself is colorless, but the interjection of those specific metallic ions as an impurity is what gives all these gems their color. It is a specific type of impurity chromium ions that are found replacing some of the aluminum ions in the ruby crystal creating the red color. Other members of the corundum family are also colored by the same mechanism only with different ions, or combinations of ions replacing the aluminum.

In the case of rubies the color you see, red, is because all the rest of the colors of the rainbow are absorbed within the crystal structure allowing you to see only red. The entire range of colors that is seen in the family of corundum gemstones is also based on the same mechanism of absorbation caused by different metallic ions that have different levels of energy. The ruby actually has two large absorption bands one of which transmits blue, and the other transmits red.  These bands are 480 nm in blue and 610 nm in red because the 610 nm is more highly energetic and the light coming from the 480 nm scale. In effect the red light coming from the gem blanks out the blue light.

A cut ruby with inclusions
Photo by Humanfeather

The amount of chrome that replaces the aluminum ions in the crystal lattice determines the density of the red that is going to appear; the more chromium there is the deeper the red color will be.

Geologically it takes a special set of circumstances to produce corundum, and especially the variety that we call Ruby. The best rubies in the world are produced in the Mogok Valley of Myanmar. Other rubies are also found in Sri Lanka, Madagascar, Africa and in the Cowee River Valley outside Franklin, North Carolina.

The finest color for Ruby is what they call pigeon blood red that is a deep red with almost bluish flashes when moved around in the daylight. Rubies also fluoresce under ultraviolet light source, and in some specimens it is even possible to see they also fluoresce under the influence of sunlight.

A red version of this star sapphire would be a star ruby
Photo by Ligar

A large ruby gem weighing more than 10 carats is the most expensive gemstone on earth being higher-priced than the finest diamonds. Large masses of imperfect ruby crystals are used extensively for carving. Many of the rubies that are on the market have been heat-treated to remove the needles of titanium dioxide that causes them to become cloudy; insert cases however the same needles of rutile produce a star effect called asterism that causes a star to appear on the face of a cabochon cut stone. Some of the stones can be almost as valuable as a regular faceted gem.

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