What are Emeralds?
Emeralds are gem-quality specimens of the beryl mineral family with a rich, distinctly green color. They are found in igneous, metamorphic, and sedimentary rocks in a small number of locations worldwide.
For over 5000 years, emeralds have been one of the most desirable and valuable colored gemstones. Ancient civilizations in Africa, Asia, and South America independently discovered emeralds and made them their gemstone of highest esteem.
Today emerald, together with ruby and sapphire, form the "big three" of colored stones. The "big three" generate more economic activity than all other colored stones combined. In 2011 the value of emeralds imported into the United States exceeded the value of rubies and sapphires combined. The value of imported emeralds also exceeded the value of all colored stones outside of the "big three" combined.
Emerald's Green Color
Beryl, the mineral of which emerald is a variety, has a chemical composition of Be3Al2(SiO3)6. When pure, beryl is colorless and known as "goshenite." Trace amounts of chromium or vanadium in the mineral cause it to develop a green color. Trace amounts of iron will tint emerald a bluish green or a yellowish green color depending upon its oxidation state.
Emerald is defined by its green color. To be an emerald, a specimen must have a distinctly green color that falls in the range from bluish green to green to slightly yellowish green. To be an emerald, the specimen must also have a rich color. Stones with weak saturation or light tone should be called "green beryl." If the beryl's color is greenish blue then it is an "aquamarine." If it is greenish yellow it is "heliodor."
This color definition is a source of confusion. Which hue, tone, and saturation combinations are the dividing lines between "green beryl" and "emerald"? Professionals in the gem and jewelry trade disagree on where the lines should be drawn. Some believe that the name "emerald" should be used when chromium is the cause of the green color, and that stones colored by vanadium should be called "green beryl." Calling a stone an "emerald" instead of a "green beryl" has a significant impact upon its price and marketability. This "color confusion" exists within the United States. In some other countries, any beryl with a green color - no matter how light - is called an "emerald."
Physical Properties of Emerald
|Color||A distinctly green color that ranges between bluish green and slightly yellowish green. Stones with a light tone or a low saturation should be called "green beryl" instead of emerald.|
|Clarity||Almost every natural emerald has eye-visible characteristics that can be inclusions, surface-reaching fractures, or healed fractures.|
|Diaphaneity||Transparent to translucent|
|Cleavage||One direction of imperfect cleavage|
|Durability||Emerald is very hard, but almost all specimens have inclusions and surface-reaching fractures that compromise their durability.|
|Mohs Hardness||7.5 to 8|
|Specific Gravity||2.7 to 2.8|
Emerald's green color is caused by trace amounts of chromium or vanadium.
|Crystal System||Hexagonal. Often as prismatic crystals.|
Use of the Name "Yellow Emerald"
By definition, emeralds are gem-quality specimens of the beryl mineral family with a rich, distinctly green color. It is inappropriate to use the name "emerald" when marketing a beryl of any other color.
The Federal Trade Commission publishes a set of Guides for the Jewelry, Precious Metals and Pewter Industries. In the next revision of these guides, they propose language stating that "it is unfair or deceptive to mark or describe a product with an incorrect varietal name." The name "yellow emerald" will be held up as an example of a name that could be misleading "based upon consumer perception evidence." 
Clarity, Treatments, and Durability
Emerald has a Mohs hardness of 7.5 to 8, which is normally a very good hardness for jewelry use. Even with that hardness, emerald has a durability issue. Most emerald contains numerous inclusions and surface-reaching fractures. These weaken the stone, cause it to be brittle, and make it subject to breakage.
This is an expected characteristic of emerald. It is rare to find an emerald that does not have inclusions and surface-reaching fractures that can be seen with the unaided eye. Under low magnification, most emeralds are said to have a "garden" of these features.
To improve appearance, most cut emeralds are treated with oils that enter the fractures and make them less obvious. Some are fracture-filled with resins or other materials. Although these treatments improve appearance, they generally do not improve the durability of the gem.
With that information, emerald should be considered a fragile stone that is best worn on special occasions rather than daily. Emerald is better suited for earrings and pendants that are usually subjected to less impact and abrasion than rings and bracelets. Settings that protect the stone are much safer than those that present the stone to impact and abrasion. Cleaning emeralds should be done carefully. Steam and ultrasonic cleaning can remove oils. A light washing in warm water with a mild soap is safer for cleaning and should be done only when needed.
Geologic and Geographic Occurrence
Beryl is a rare mineral with a chemical composition of Be3Al2(SiO3)6. It is rare because beryllium is an element that occurs in very small amounts in the Earth’s crust. It is unusual for enough beryllium to be present in one location to form minerals. In addition, the conditions in which beryllium is present in significant amounts are different from the conditions where chromium and vanadium, the sources of emerald’s green color, occur. This is why emerald is rare and only found in a small number of locations.
Today, most emerald production originates in four source countries: Colombia, Zambia, Brazil, and Zimbabwe. These countries reliably produce commercial amounts of emeralds. Minor amounts of production or irregular production comes from Madagascar, Nigeria, Afghanistan, Pakistan, Canada, Russia, and a few other countries.
Even though the conditions for the formation of emerald are very unlikely, the gem has been found in a diversity of rock types. In Colombia, the country that has supplied most of the world’s emeralds, black organic shale and carbonaceous limestone, both sedimentary rocks, are the ores for many emerald deposits. The shale is thought to be the source of chromium, and the beryllium is thought to have been delivered by ascending fluids.
Many of the world's emerald deposits have formed in areas of contact metamorphism. A granitic magma can serve as a source of beryllium, and nearby carbonaceous schist or gneiss can serve as a source of chromium or vanadium. The emeralds usually form in the schist or gneiss or in the margins of a nearby pegmatite. Mafic and ultramafic rocks can also serve as sources for chromium or vanadium.
Emeralds are rarely mined from alluvial deposits. Emerald is usually a fractured stone that does not have the alluvial durability to persist great distances from its source. Emerald also has a specific gravity of 2.7 to 2.8, which is not significantly different from quartz, feldspar, and other common materials found in stream sediments. It therefore does not concentrate with high-density grains which are segregated in the stream and more easily recovered by mining.
Emerald Mining in the United States
Very few emeralds are mined in the United States. North Carolina has been a sporadic producer of emeralds in small quantities from a few tiny mines since the late 1800s. The Crabtree Emerald Mine was once operated by Tiffany and Company and a series of property owners between 1894 and the 1990s. Many fine clear emeralds were produced, and tons of emerald-bearing pegmatite were sold as "emerald matrix" for slabbing and cabochon cutting. The cabochons displayed emerald and tourmaline prisms in a white matrix of quartz and feldspar. A specimen of the Crabtree Pegmatite is shown on this page.
Today, North American Emerald Mines operates a small mine near Hiddenite, North Carolina. Between 1995 and 2010, over 20,000 carats of emeralds had been produced, including a six-inch-long, 1,869-carat crystal that is now in the Houston Museum of Natural Science and valued at $3.5 million. A crushed stone quarry on the same property is operated with an eye open for signs of the hydrothermal veins and pockets that sometimes contain emerald. It is one of the only gemstone mines in the world that sells the country rock. 
The first synthetic emeralds were produced in the mid-1800s, but it was not until the 1930s that Carroll Chatham began producing emerald in commercial quantities. To date, several companies including Chatham Created Gems, Gilson, Kyocera Corporation, Lennix, Seiko Corporation, Biron Corporation, Lechleitner, and Regency, have produced synthetic emeralds by flux and hydrothermal processes. 
Synthetic emeralds, also known as lab-created emeralds, have the same chemical composition and crystal structure as natural emeralds. They are sold beside natural emeralds in most mall jewelry stores in the United States. When compared to natural emeralds, the synthetics have superior clarity and a more uniform appearance than natural stones of equivalent cost. Many consumers purchase them for their attractive appearance at a lower cost.
 Gems: Their Sources, Descriptions and Identification: Michael O’Donoghue; Elsevier; sixth edition; 873 pages; 2006.|
 Gemstones of the World: Walter Schumann; Sterling Publishing; fifth edition; 320 pages; 2013.
 Emeralds: Fred Ward; Fred Ward Gem Books; third edition; 64 pages; 2010.
 Emeralds: A Passionate Guide: The Emeralds, the People, their Secrets: Ronald Ringsrud; Lith Publishing; 382 pages; 2009.
 Rock and Gem: Ronald L. Bonewitz; The Smithsonian Institution, Dorling Kindersley Publishing; 360 pages; 2008.
 2011 Minerals Yearbook: Gemstones: Donald W. Olson; The United States Geological Survey; 21 pages; 2013.
 Big Crabtree Emerald Mine: Spruce Pine District, Mitchell County, North Carolina, USA; Mindat.org; accessed July 2014.
 Guides for the Jewelry, Precious Metals, and Pewter Industries, proposed revisions; Federal Trade Commission; 16 CFR Part 23, 2015. Link to excerpt.
Imitation Emeralds and Alternative Stones
"Imitations" are materials that have a similar appearance to natural gems and are used in their place. They are often manufactured specifically to serve as substitutes. Green glass, synthetic green spinel, green cubic zirconia, and green yttrium aluminum garnet are common imitations used in place of emerald.
"Alternative stones" are other natural stones with a green color that are purchased by people who simply want a green gem. They might prefer to own an emerald, but they select the alternative stone instead because of its lower price or other characteristics. Chrome diopside and chrome tourmaline are deep green gems that some people purchase when they want a green gem. Tsavorite garnet is another gem with a wonderful green color. Dyed quartz can be a beautiful stone at a very low cost. Several examples of alternative stones and synthetic emerald are shown in the photo above. The best rule for buying gemstones is: "Buy what you like!"
Contributor: Hobart King
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