Emerald, the green variety of Beryl, is the most famous and valuable green gemstone. Its beautiful green color, combined with durability and rarity, make it one of the most expensive gemstones. Deep green is the most desired color in Emeralds. In general the paler the color of an Emerald, the lesser its value. Very pale colored stones are not called Emeralds but rather "Green Beryl". They are sometimes heat treated, which causes their color to turn blue and transform into Aquamarine. Besides for Emerald, the mineral Beryl also has other important gem varieties, including blue Aquamarine, pink Morganite, and yellow Heliodor/Golden Beryl Pure Beryl is white; the green color of Emerald is usually caused by chromium impurities, and occasionally by vanadium impurities. Emerald is by far the most valuable gemstone variety of Beryl, being one of the few precious gemstones. Emeralds are notorious for their flaws. Flawless stones are very uncommon, and are noted for their great value. Some actually prefer Emeralds with minute flaws over flawless Emeralds, as this proves authenticity. Flaws are often hidden by treating the Emeralds with oil or synthetic lubricants, and this is a common practice in the industry. Though Colombian Emeralds have traditionally been the highest quality Emeralds with the finest green color, a new source of Emerald from the African country of Zambia has been producing deep green Emeralds with fewer flaws. Many Emerald fakes and doublets are known. Two pale colored stones may be glued together with a deep green paste, creating a stone resembling Emerald. Faceted green glass also resembles Emerald, and it may be coated with a hard substance to mask its low hardness. Synthetic Emeralds are also sold to unwary buyers without them knowing the stone is synthetic. Experts can distinguish all these fakes, and it is especially important to only purchase Emeralds from reliable dealers. Experts can also determine if an Emerald was treated with oil or a lubricant to mask internal flaws. A rare and unusual form of Emerald, known as "Trapiche Emerald", is characterized by star-shaped rays that emanate from the center of a stone in a hexagonal pattern. These rays appear much like asterism, but, unlike asterism, are not caused by light reflection from tiny parallel inclusions, but by black carbon impurities that form in a star-shaped pattern. These Trapiche Emeralds are only found in the Boyaca Emerald mining district of Colombia, and are cut into cabochons. Though Emerald has good hardness, it is a brittle stone. It may develop internal cracks if banged hard or if subject to extreme temperature change. Emeralds that were treated to mask internal flaws should never be cleaned with an ultrasonic jewelry cleaner, nor should they be washed with soap. These practices may remove the oiling treatment and expose the hidden internal flaws.
Transparent emeralds are faceted into gem cuts for jewelry, and make one of the most popular jewelry gemstones. Emerald is very sensitive to pressure and banging, and the well-known emerald cut was developed specifically for this gem to reduce the amount of pressure during cutting. Translucent Emeralds are cut and polished into cabochons and beads, as are Trapiche Emeralds. Emerald is the birthstone of May.
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