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The most valuable gemstone in the world (based on weight) is emerald, easily recognized by its lustrous green shine caused by trace amounts of chromium and, less often, iron. The major sources for emerald are Colombia and Africa.
Historically first used by the Egyptians, the first high-profile admirer of the stone was Queen Cleopatra, who had her own mines during her reign. At least one of her mines has been discovered by archeologists, but they were beat to it by looters.
The most valuable emeralds were discovered in Colombia. The Aztecs and the Incas used emeralds in their worship rituals, and in the 1500's, when Spanish Conquistadors arrived, the gems were quickly taken away, only to be lost at sea in shipwrecks.
Emerald has been believed to have medicinal purposes, and said to have properties able to soothe strained and tired eyes, break fevers, prevent epileptic attacks, and even cure digestive problems if used in a "gem elixir." Additionally, it’s had a role in improving concentration.
Unlike most gems, emeralds have found their way into mythos and fantasy, best noted in L. Frank Baum's book (and its consequent film) The Wizard of Oz. In the story, Dorothy visits the Emerald City and needs to wear protective, green-tinted glasses to shield her eyes from the shine of the resplendent greenery.
Clean your emerald with a soft, dry cloth. Avoid sudden temperature changes, household chemicals and ultrasonic jewelry cleaners. Emerald is often treated with oil to fill fissures and fractures that are characteristic of emerald. Hence, you should never clean an emerald in an ultrasonic cleaner because this oil could be removed or damaged, making the fissures more visible. In addition, do not clean emerald in hot soapy water since it too can remove this oil.