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Emerald

From Wikipedia, the free encyclopedia
(January 10th, 2012)
Emerald

Emerald crystal from Muzo, Colombia
General
Category Beryl variety
Chemical formula Be3Al2(SiO3)6
Crystal symmetry (6/m 2/m 2/m) – Dihexagonal Dipyramidal
Unit cell a = 9.21 Å, c = 9.19 Å; Z = 2
Identification
Molar mass 537.50
Color Green shades
Crystal habit Massive to well Crystalline
Crystal system Hexagonal (6/m 2/m 2/m)Space group: P6/mсc
Cleavage Imperfect on the [0001]
Fracture Conchoidal
Mohs scalehardness 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
Ultravioletfluorescence None (some fracture filling materials used to improve emerald's clarity do fluoresce, but the stone itself does not)
References [1]

Emerald is 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 10 point Mohs scale of mineral hardness.[2] Most emeralds are highly included, so their toughness (resistance to breakage) is classified as generally poor.

Contents

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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"); its original source being either the Hebrew word אזמרגד izmargad meaning "emerald" or "green"[3] or the Sanskrit word मरकत marakata meaning "emerald." The name could also be related to the Semitic word baraq (בָּרָק ;البُراق; "lightning" or "shine") (cf. Hebrew: ברקת bareqeth and Arabic: برق barq "lightning"). It is the same source for the names Persian (زمرّد zomorrod), Turkish (zümrüt), Sanskrit (मरकत ; marakata), Kannada (ಪಚ್ಚೆ ; Pacche), Telugu (Paccha), Georgian (ზურმუხტი; zurmukhti), Russian (изумруд; izumrud)[4] and Armenian zmruxt.

Properties determining value

Cut emeralds

Emeralds, like all colored gemstones, are graded using four basic parameters – the four Cs of Connoisseurship: Color, Cut, Clarity and Crystal. The last C, crystal is simply used as a synonym that begins with C for transparency or what gemologists call diaphaneity. Before the 20th century, jewelers used the term water as in "a gem of the finest water"[5] to express the combination of two qualities, color and crystal. Normally, in the grading of colored gemstones, color is by far the most important criterion. However, in the grading of emerald, crystal 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.[6]

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 emeraldspurchased 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.'[7]

Color

Scientifically speaking, 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, blue, and red — the hues found adjacent to green on the spectral color wheel — 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. In addition, the hue of an emerald must be bright (vivid). Gray is the normal saturation modifier or mask found in emerald; a grayish-green hue is a dull green.

Clarity

Emerald tends to have numerous inclusions and surface breaking fissures. Unlike diamond, where the loupe standard, i.e. 10X 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", to enhance the apparent clarity. 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.[6] This relative crystal non-uniformity makes emeralds more likely than other gemstones to be cut into cabochons, rather than faceted shapes.

Treatments

Most emeralds are oiled as part of the post lapidary process, in order to improve their clarity. 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. The U.S. Federal Trade Commission requires the disclosure of this treatment when a treated emerald is sold.[8] The use of oil is traditional and largely accepted by the gem trade. Other treatments, for example the use of green-tinted oil, are not acceptable in the trade. The laboratory community has recently standardized the language for grading the clarity of emeralds. Gems are graded on a four step scale; none, minor, moderate and highly enhanced. Note that these categories reflect levels of enhancement notclarity. 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 to be similar in quality may actually be quite far apart in treatment level, 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 an enhancement level graded moderate should cost 40–50% less than an identical stone graded none.

Emerald localities

Spanish emerald and gold pendant exhibited at Victoria and Albert Museum.

Emeralds in antiquity[when?] were mined by the Egyptians and in India and Austria.[9]

A rare type of emerald known as a trapiche emerald is occasionally found in the mines of Colombia. A trapiche emerald exhibits a "star" pattern; it has raylike spokes of dark carbon impurities that give the emerald a six-pointed radial pattern.[citation needed] Emeralds come from three main emerald mining areas in Colombia: Muzo, Coscuez, and Chivor.[10] Emeralds are also found in other countries, such as Afghanistan, Australia, Austria,Brazil, 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, Zambia and Zimbabwe.[1] In the US, emeralds have been found in Connecticut, Montana, Nevada, North Carolina and South Carolina.[1] In 1998 emeralds were discovered in theYukon.[citation needed]

Synthetic emerald

Emerald showing its hexagonal structure

Emerald is a rare and valuable gemstone and, as such, it has provided the incentive for developing synthetic emeralds. 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. Because Chatham's emeralds do not have any water and contain traces of vanadate, molybdenum and vanadium, a lithium vanadate flux process is probably involved. The other large producer of flux emeralds was Pierre Gilson Sr., which has been on the market since 1964. Gilson's emeralds are usually grown on natural colorless beryl seeds which become 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.[11] Gilson sold his production laboratory to a Japanese firm in the 1980s, but production has ceased since; so did 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.[citation needed] Later, from 1965 to 1970, the Linde Division of Union Carbide produced completely synthetic emeralds by hydrothermal synthesis. According to their patents,[12][13] 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. They have succeeded to synthesize emeralds that have similar chemical composition as emeralds in alkaline deposits in Colombia, hence they are called “Colombian Created Emeralds” or “Tairus Created Emeralds”.[14] 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.[15]

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."[16]

Emerald in different cultures, and emerald lore

The Gachala Emerald is one of the largest gem emeralds in the world, at 858 carats (172 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, Cancer and sometimes Gemini. One of the quainter anecdotes on emeralds was 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 after.[17] "The Attenbury Emeralds" is a detective story in which Lord Peter Wimsey must solve several emerald-related mysteries. In The Wizard of Oz, the Wizard lives in a city made entirely of emerald, known as the Emerald City.

Notable emeralds

Emerald Origin
Chalk Emerald Colombia
Duke of Devonshire Emerald
Gachala Emerald
Mogul Mughal Emerald
Bahia Emerald Brazil

Gallery

See also

References

  1. ^ a b c "Emerald at Mindat". Mindat.org. 2010-07-19. Retrieved 2010-07-30.
  2. ^ a b Hurlbut, Cornelius S. Jr, & Kammerling, Robert C., 1991, Gemology, p. 203, John Wiley & Sons, New York ISBN 0471526673
  3. ^ Fernie, William Thomas, MD (1907). Precious Stones for Curative Wear; and other remedial uses; likewise the nobler metals. Bristol: John Wright & Co..
  4. ^ Harper, Douglas. "emerald". Online Etymology Dictionary.
  5. ^ 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, &c. by V. Ball. 2nd ed., edited by William Crooke; Vol. II, pp. 44, 58
  6. ^ a b Wise, R. W. (2001) Secrets of the Gem Trade: the connoisseur's guide to precious gemstones. Brunswick House PressISBN 0972822380; p. 108
  7. ^ Read, Peter (2008) Gemmology 3rd rev. ed. NAG Press; p. 218
  8. ^ "Guides for the Jewelry, Precious Metals, and Pewter Industries". Ftc.gov. 1996-05-30. Retrieved 2010-07-30.
  9. ^ Giuliani, G.; Chaussidon, M; Schubnel, HJ; Piat, DH; Rollion-Bard, C; France-Lanord, C; Giard, D; De Narvaez, D et al. (2000). "Oxygen Isotopes and Emerald Trade Routes Since Antiquity". Science 287 (5453): 631–3.doi:10.1126/science.287.5453.631. PMID 10649992.
  10. ^ Emerald Mining Areas in Colombia, with location map of these three districts.
  11. ^ Nassau, K., 1980, Gems Made By Man, Gemological Institute of America, ISBN 0873110161
  12. ^ U.S. Patent 3,567,642
  13. ^ U.S. Patent 3,567,643
  14. ^ Karl Schmetzer, Dietmar Schwartz, Heinz-Jurgen Bernhardt, Tobias Hager “A new type of Tairus hydrothermally-grown synthetic emerald, colored by vanadium and copper” Journal of Gemmology of Gemmological Association of Great Britain, September, Vol. 30 (1–2) 2006–2007, pp. 59–74
  15. ^ Hurlbut, Cornelius S. Jr, & Kammerling, Robert C., 1991, Gemology, p. 81, John Wiley & Sons, New York ISBN 0471526673
  16. ^ "Guides for the Jewelry, Precious Metals, and Pewter Industries". Ftc.gov. 1996-05-30. Retrieved 2010-07-30.
  17. ^ Kunz, George Frederick (1915). Magic of Jewels and Charms. Philadelphia: Lippincott Company. p. 305.ISBN 0766143228.

Further reading

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