Emerald

For other uses, see Emerald (disambiguation).
Emerald

Emerald crystal from Muzo, Colombia
General
Category Beryl variety
Formula
(repeating unit)		 Be3Al2(SiO3)6
Crystal system Hexagonal (6/m 2/m 2/m) Space group: P6/mсc
Unit cell a = 9.21 Å, c = 9.19 Å; Z = 2
Identification
Formula mass 537.50
Color Green shades to colorless
Crystal habit Massive to well Crystalline
Crystal symmetry (6/m 2/m 2/m) – Dihexagonal Dipyramidal
Cleavage Imperfect on the [0001]
Fracture Conchoidal
Mohs scale hardness 7.5–8
Luster Vitreous
Streak White
Diaphaneity Transparent to opaque
Specific gravity Average 2.76
Optical properties Uniaxial (-)
Refractive index nω = 1.564–1.595,
nε = 1.568–1.602
Birefringence δ = 0.0040–0.0070
Ultraviolet fluorescence None (some fracture filling materials used to improve emerald's clarity do fluoresce, but the stone itself does not)
References [1]

Emerald is a gemstone and a variety of the mineral beryl (Be3Al2(SiO3)6) colored green by trace amounts of chromium and sometimes vanadium.[2] Beryl has a hardness of 7.5–8 on the Mohs scale.[2] Most emeralds are highly included, so their toughness (resistance to breakage) is classified as generally poor. It is a cyclosilicate.

Etymology

The word "emerald" is derived (via Old French: esmeraude and Middle English: emeraude), from Vulgar Latin: esmaralda/esmaraldus, a variant of Latin smaragdus, which originated in Greek: σμάραγδος (smaragdos; "green gem").[3]

Properties determining value

Cut emeralds

Emeralds, like all colored gemstones, are graded using four basic parameters–the four Cs of Connoisseurship: Color, Clarity, Cut and Carat weight. Before the 20th century, jewelers used the term water, as in "a gem of the finest water",[4] to express the combination of two qualities: color and clarity. Normally, in the grading of colored gemstones, color is by far the most important criterion. However, in the grading of emeralds, clarity is considered a close second. Both are necessary conditions. A fine emerald must possess not only a pure verdant green hue as described below, but also a high degree of transparency to be considered a top gem.[5]

In the 1960s, the American jewelry industry changed the definition of "emerald" to include the green vanadium-bearing beryl as emerald. As a result, vanadium emeralds purchased as emeralds in the United States are not recognized as such in the UK and Europe. In America, the distinction between traditional emeralds and the new vanadium kind is often reflected in the use of terms such as "Colombian Emerald".[6]

Color

In gemology,[7] color is divided into three components: hue, saturation and tone. Emeralds occur in hues ranging from yellow-green to blue-green, with the primary hue necessarily being green. Yellow and blue are the normal secondary hues found in emeralds. Only gems that are medium to dark in tone are considered emerald; light-toned gems are known instead by the species name green beryl. The finest emerald are approximately 75% tone on a scale where 0% tone would be colorless and 100% would be opaque black. In addition, a fine stone should be well saturated; the hue of an emerald should be bright (vivid). Gray is the normal saturation modifier or mask found in emerald; a grayish-green hue is a dull green hue.[5]

Clarity

Brazilian emerald (grass-green variety of the mineral beryl) in a quartz-pegmatite matrix with typical hexagonal, prismatic crystals.[8]

Emerald tends to have numerous inclusions and surface breaking fissures. Unlike diamond, where the loupe standard, i.e. 10× magnification, is used to grade clarity, emerald is graded by eye. Thus, if an emerald has no visible inclusions to the eye (assuming normal visual acuity) it is considered flawless. Stones that lack surface breaking fissures are extremely rare and therefore almost all emeralds are treated ("oiled", see below) to enhance the apparent clarity. The inclusions and fissures within an emerald are sometime described as "Jardin" (French for 'garden'), because of their mossy appearance.[9] These imperfections within the stone are unique to each emerald and can be used to identify a particular stone. Eye-clean stones of a vivid primary green hue (as described above) with no more than 15% of any secondary hue or combination (either blue or yellow) of a medium-dark tone command the highest prices.[5] This relative crystal non-uniformity makes emeralds more likely than other gemstones to be cut into cabochons, rather than faceted shapes. Faceted Emeralds are most commonly given the Oval cut, or the signature Emerald cut, a rectangular cut with facets around the top edge.

Treatments

Most emeralds are oiled as part of the post-lapidary process, in order to fill in surface-reaching cracks, improving their clarity and stability. Cedar oil, having a similar refractive index, is often used in this generally accepted practice. Other liquids, including synthetic oils and polymers with refractive indexes close to that of emerald such as Opticon, are also used. These treatments are typically applied in a vacuum chamber under mild heat, to open the pores of the stone and allow the fracture-filling agent to be absorbed more effectively.[10] The U.S. Federal Trade Commission requires the disclosure of this treatment when an oil treated emerald is sold.[11] The use of oil is traditional and largely accepted by the gem trade, although oil treated emeralds are worth much less than un-treated emeralds of similar quality. Other treatments, for example the use of green-tinted oil, are not acceptable in the trade. Gems are graded on a four-step scale; none, minor, moderate and highly enhanced. These categories reflect levels of enhancement, not clarity. A gem graded none on the enhancement scale may still exhibit visible inclusions. Laboratories tend to apply these criteria differently. Some gem labs consider the mere presence of oil or polymers to constitute enhancement. Others may ignore traces of oil if the presence of the material does not materially improve the look of the gemstone.

Given that the vast majority of all emeralds are treated as described above, and the fact that two stones that appear visually similar may actually be quite far apart in treatment level and therefore in value, a consumer considering a purchase of an expensive emerald is well advised to insist upon a treatment report from a reputable gemological laboratory. All other factors being equal, a high quality emerald with moderate enhancement should cost much less than an identical stone graded none.[5]

Emerald mines

Spanish-made emerald and gold pendant exhibited at Victoria and Albert Museum.[12]

Emeralds in antiquity have been mined in Egypt since 1500 BCE, and India, and Austria since at least the 14th century CE.[13]

Colombia is by far the world's largest producer of emeralds, constituting 50–95% of the world production, with the number depending on the year, source and grade.[14][15][16][17] Emerald production in Colombia has increased drastically in the last decade, increasing by 78% from 2000 to 2010.[18] The three main emerald mining areas in Colombia are Muzo, Coscuez, and Chivor.[19] Rare 'trapiche' emeralds are found in Colombia, distinguished by a six-pointed radial pattern made of ray-like spokes of dark carbon impurities.

Zambia is the world's second biggest producer, with its Kafubu River area deposits (Kagem Mines) about 45 km (28 mi) southwest of Kitwe responsible for 20% of the world's production of gem quality stones in 2004.[20] In the first half of 2011 the Kagem mines produced 3.74 tons of emeralds.

Emeralds are found all over the world in countries such as Afghanistan, Australia, Austria, Brazil,[21] Bulgaria, Cambodia, Canada, China, Egypt, Ethiopia, France, Germany, India, Italy, Kazakhstan, Madagascar, Mozambique, Namibia, Nigeria, Norway, Pakistan, Russia, Somalia, South Africa, Spain, Switzerland, Tanzania, United States(US), Zambia, and Zimbabwe.[1] In the US, emeralds have been found in Connecticut, Montana, Nevada, North Carolina, and South Carolina.[1] In Canada, in 1997 emeralds were discovered in the Yukon.[22]

Origin determinations

Since the onset of concerns regarding diamond origins, research has been conducted to determine if the mining location could be determined for an emerald already in circulation. Traditional research used qualitative guidelines such as an emerald’s color, style and quality of cutting, type of fracture filling, and/or the anthropological origins of the artifacts bearing the mineral to determine the emerald's mine location. More recent studies using energy dispersive X-ray spectroscopy methods have uncovered trace chemical element differences between emeralds; even emeralds mined within close proximity to one another. American gemologist David Cronin and his colleagues have extensively examined the chemical signatures of emeralds resulting from fluid dynamics and subtle precipitation mechanisms, and their research demonstrated the chemical homogeneity of emeralds from the same mining location and the statistical differences that exist between emeralds from different mining locations, including those between the three locations: Muzo, Coscuez, and Chivor, in Colombia, South America.[23]

Synthetic emerald

Emerald showing its hexagonal structure

Both hydrothermal and flux-growth synthetics have been produced, and a method has been developed for producing an emerald overgrowth on colorless beryl. The first commercially successful emerald synthesis process was that of Carroll Chatham, likely involving a lithium vanadate flux process, as Chatham's emeralds do not have any water and contain traces of vanadate, molybdenum and vanadium. The other large producer of flux emeralds was Pierre Gilson Sr., whose products have been on the market since 1964. Gilson's emeralds are usually grown on natural colorless beryl seeds, which are coated on both sides. Growth occurs at the rate of 1 mm per month, a typical seven-month growth run producing emerald crystals of 7 mm of thickness.[24] Gilson sold his production laboratory to a Japanese firm in the 1980s, but production has since ceased; so has Chatham's, after the 1989 San Francisco earthquake.

Hydrothermal synthetic emeralds have been attributed to IG Farben, Nacken, Tairus, and others, but the first satisfactory commercial product was that of Johann Lechleitner of Innsbruck, Austria, which appeared on the market in the 1960s. These stones were initially sold under the names "Emerita" and "Symeralds", and they were grown as a thin layer of emerald on top of natural colorless beryl stones. Although not much is known about the original process, it is assumed that Leichleitner emeralds were grown in acid conditions. Later, from 1965 to 1970, the Linde Division of Union Carbide produced completely synthetic emeralds by hydrothermal synthesis. According to their patents (attributable to E.M. Flanigen),[25] acidic conditions are essential to prevent the chromium (which is used as the colorant) from precipitating. Also, it is important that the silicon-containing nutrient be kept away from the other ingredients to prevent nucleation and confine growth to the seed crystals. Growth occurs by a diffusion-reaction process, assisted by convection. The largest producer of hydrothermal emeralds today is Tairus in Russia, which has succeeded in synthesizing emeralds with chemical composition similar to emeralds in alkaline deposits in Colombia, and whose products are thus known as “Colombian Created Emeralds” or “Tairus Created Emeralds”.[26] Luminescence in ultraviolet light is considered a supplementary test when making a natural vs. synthetic determination, as many, but not all, natural emeralds are inert to ultraviolet light. Many synthetics are also UV inert.[27]

Synthetic emeralds are often referred to as "created", as their chemical and gemological composition is the same as their natural counterparts. The U.S. Federal Trade Commission (FTC) has very strict regulations as to what can and what cannot be called "synthetic" stone. The FTC says: "§ 23.23(c) It is unfair or deceptive to use the word "laboratory-grown," "laboratory-created," "[manufacturer name]-created," or "synthetic" with the name of any natural stone to describe any industry product unless such industry product has essentially the same optical, physical, and chemical properties as the stone named."[11]

Emerald in different cultures, and emerald lore

The Gachala Emerald is one of the largest gem emeralds in the world, at 858 carats (171.6 g). This stone was found in 1967 at La Vega de San Juan mine in Gachalá, Colombia. It is housed at the National Museum of Natural History of the Smithsonian Institution in Washington, D.C.

Emerald is regarded as the traditional birthstone for May as well as the traditional gemstone for the astrological signs of Taurus, Gemini, and sometimes Cancer.

One of the quainter anecdotes about emeralds was told by the 16th-century historian Brantôme, who referred to the many impressive emeralds the Spanish under Cortez had brought back to Europe from Latin America. On one of Cortez's most notable emeralds he had the text engraved, Inter Natos Mulierum non sur-rexit mayor ("Among those born of woman there hath not arisen a greater," Matthew 11:11) which referred to John the Baptist. Brantôme considered engraving such a beautiful and simple product of nature sacrilegious and considered this act the cause for Cortez's loss of an extremely precious pearl (to which he dedicated a work, A beautiful and incomparable pearl), and even for the death of King Charles IX of France, who died soon afterward.[28]

The chief deity of one of India's most famous temple, the Meenakshi Amman Temple in Madurai, is the goddess Meenakshi, whose idol is traditionally thought to be made of emerald.[29]

Notable emeralds

Emerald Origin
Chalk Emerald Colombia
Duke of Devonshire Emerald
Gachala Emerald
Mogul Mughal Emerald
Patricia Emerald[30] Year 1920, 632 carats uncut, 12 sides-dihexagonal Colombia
Bahia Emerald Brazil
Emerald of Saint Louis, at the National Museum of Natural History (France), Paris Austria, probably Habachtal

Gallery

See also

References

  1. 1 2 3 "Emerald at Mindat". Mindat.org. 2010-07-19. Retrieved 2010-07-30.
  2. 1 2 Hurlbut, Cornelius S. Jr. and Kammerling, Robert C. (1991) Gemology, John Wiley & Sons, New York, p. 203, ISBN 0-471-52667-3.
  3. ↑ Harper, Douglas. "emerald". Online Etymology Dictionary.
  4. ↑ Tavernier, J. B. (1925) Travels in India by Jean Baptiste Tavernier, Baron of Aubonne; translated from the original French edition of 1676 with a biographical sketch of the author, notes, appendices, etc. by V. Ball. 2nd ed., edited by William Crooke; Vol. II, pp. 44, 58
  5. 1 2 3 4 Wise, R. W. (2001) Secrets of the Gem Trade: the connoisseur's guide to precious gemstones. Brunswick House Press, p. 108, ISBN 0-9728223-8-0.
  6. ↑ Read, Peter (2008) Gemmology, 3rd rev. ed., NAG Press, p. 218, ISBN 0719803616.
  7. ↑ Grading Fancy-Color Diamonds. Gemological Institute of America
  8. ↑ Bonewitz, R. (2005). Rock and gem. New York: DK Pub. pp. 292-293. ISBN 0756633427.
  9. ↑ Emerald Quality Factors. Gemological Institute of America.
  10. ↑ Liccini, Mark. Understanding Emerald Enhancements and Treatments. International Gem Society
  11. 1 2 "Guides for the Jewelry, Precious Metals, and Pewter Industries". U.S. Federal Trade Commission. 1996-05-30. Retrieved 2010-07-30.
  12. ↑ "Pendant | V&A Search the Collections". Victoria and Albert Museum. Given by Dame Joan Evans. Retrieved 30 Jan 2014. Museum item number M.138-1975
  13. ↑ Giuliani G, Chaussidon M, Schubnel HJ, Piat DH, Rollion-Bard C, France-Lanord C, Giard D, de Narvaez D, Rondeau B (2000). "Oxygen Isotopes and Emerald Trade Routes Since Antiquity" (PDF). Science 287 (5453): 631–3. Bibcode:2000Sci...287..631G. doi:10.1126/science.287.5453.631. PMID 10649992.
  14. ↑ Badawy, Manuela (June 13, 2012). "Emeralds seek the 'De Beers' treatment". Reuters. Retrieved 14 June 2012.
  15. ↑ Dydyński, Krzysztof (2003). Colombia. Lonely Planet. p. 21. ISBN 0-86442-674-7.
  16. ↑ Branquet, Y. Laumenier, B. Cheilletz, A. & Giuliani, G. (1999). "Emeralds in the Eastern Cordillera of Colombia. Two tectonic settings for one mineralization". Geology 27 (7): 597–600. Bibcode:1999Geo....27..597B. doi:10.1130/0091-7613(1999)027<0597:EITECO>2.3.CO;2.
  17. ↑ Carrillo, V. (2001). Compilación y análisis de la información geológica referente a la explotación esmeraldífera en Colombia. Informe de contrato 124. INGEOMINAS
  18. ↑ Wacaster, Susan (March 2012). "2010 Minerals Yearbook: Colombia [ADVANCE RELEASE]" (PDF). United States Geological Survey. Retrieved 7 August 2012.
  19. ↑ Emerald Mining Areas in Colombia, with location map of these three districts.
  20. ↑ Behling, Steve and Wilson, Wendell E. (January 1, 2010) "The Kagem emerald mine: Kafubu Area, Zambia", The Mineralogical Record  – via HighBeam (subscription required)
  21. ↑ "Maior esmeralda do mundo, encontrada no Brasil, será leiloada no Canadá". UOL (2012-01-18)
  22. ↑ Emeralds in the Yukon Territory. Yukon Geological Survey.
  23. ↑ Cronin, David; Rendle, Andy (2012). "Determining the geographical origins of natural emeralds through nondestructive chemical fingerprinting". Journal of Gemmology 33: 1–13.
  24. ↑ Nassau, K. (1980) Gems Made By Man, Gemological Institute of America, ISBN 0-87311-016-1.
  25. ↑ Geological Magazine "Hydrothermal process for growing crystals having the structure of beryl in an alkaline halide medium" U.S. Patent 3,567,642 Issue date: March 2, 1971
  26. ↑ Schmetzer, Karl; Schwartz, Dietmar; Bernhardt, Heinz-Jurgen and Tobias Hager (2006–2007). "A new type of Tairus hydrothermally-grown synthetic emerald, colored by vanadium and copper" (PDF). Journal of Gemmology of Gemmological Association of Great Britain 30 (1–2): 59–74.
  27. ↑ Hurlbut, Cornelius S. Jr. and Kammerling, Robert C. (1991) Gemology, John Wiley & Sons, New York, p. 81, ISBN 0-471-52667-3.
  28. ↑ Kunz, George Frederick (1915). Magic of Jewels and Charms. Philadelphia: Lippincott Company. p. 305. ISBN 0-7661-4322-8.
  29. ↑ "Meenakshi Temple - Meenakshi Temple of Madurai, Meenakshi Amman Temple Madurai India". www.madurai.org.uk. Retrieved 2016-01-18.
  30. ↑ "Patricia Emerald". AMNH. Retrieved 2015-08-22.

Further reading

External links

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