emerald

Emerald
Emerald with host rock
General
Category	Beryl variety
Chemical formula	Beryllium aluminium silicate with chromium, Be3Al2(SiO3)6::Cr
Identification
Color	Green
Crystal habit	Hexagonal Crystals
Crystal system	Hexagonal
Cleavage	Poor Basal Cleavage (Seldom Visible)
Fracture	Conchoidal
Mohs Scale hardness	7.5 - 8.0
Luster	Vitreous
Streak	White
Specific gravity	2.70 - 2.78
Refractive index	1.576 - 1.582
Pleochroism	Distinct, Blue-Green/Yellow-Green

Emeralds are a variety of the mineral beryl (Be₃Al₂(SiO₃)₆,) colored green by trace amounts of chromium and sometimes vanadium.^[1] Beryl has a hardness of 7.5 - 8 on the 10 point Mohs scale of mineral hardness.^[1] Most emeralds are highly included, so their brittleness (resistance to breakage) is classified as generally poor. The word "emerald" comes from Latin smaragdus, via Greek smaragdos, its original source being a Semitic word izmargad or the Sanskrit word, marakata, meaning "emerald" or "green".^[2]

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. Prior to the 20th Century jewelers used the term water as in "a gem of the finest water"^[3] 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.^[4]

Color

Scientifically speaking, color is divided into three components: hue, saturation and tone. Yellow and blue, the hues found adjacent to green on the spectral color wheel, are the normal secondary hues found in emerald. Emeralds occur in hues ranging from yellowish green to bluish green. The primary hue must, of course, be green. Only gems that are medium to dark in tone are considered emerald. Light toned gems are known by the species name, green beryl. In addition, the hue must be bright (vivid). Gray is the normal saturation modifier or mask found in emerald. A grayish green hue is a dull green hue.

Clarity

From the collection at the National Museum of Natural History

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.^[4] 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.^[5] 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 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 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

Emeralds in antiquity were mined by the Egyptians and in Austria, as well as Swat in northern Pakistan.^[6][7]

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. It is named for the trapiche, a grinding wheel used to process sugarcane in the region. Colombian emeralds are generally the most prized due to their transparency and fire. Some of the most rare emeralds come from three main emerald mining areas in Colombia: Muzo, Coscuez, and Chivor. Fine emeralds are also found in other countries, such as Zambia, Brazil, Zimbabwe, Madagascar, Pakistan, India, Afghanistan and Russia. In the US, emeralds can be found in Hiddenite, North Carolina. In 1998, emeralds were discovered in the Yukon.

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.^[8] Gilson sold his production laboratory to a Japanese firm in the 1980s, but production has ceased since, so did Chatham's, after the San Francisco earthquake in 1989.^{[citation needed]}

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 (US3,567,642 and US3,567,643), 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.”^{[citation needed]}

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.^[9]

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."^[10]

Wispy veil-like inclusions are common in flux-grown synthetic emeralds.

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 more quaint anecdotes on emeralds was by the 16th-century historian Brantome, 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 them borne of woman there hath not arisen a greater Man. XI, 11) which referred to John the Baptist. Brantome 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 who died soon after.^[11]

In some cultures, the emerald is the traditional gift for the 55th wedding anniversary. It is also used as a 20th and 35th wedding anniversary stone.

The Authorized King James Version of the Bible, in Exodus 28:18 and 39:11, lists "emerald" as one of the precious stones in the breastplate of the high priest of the Jews; but modern consensus is that this is probably a mistranslation. (See Hoshen.)

Ireland is often referred to, especially in America, as the "Emerald Isle".

See also

• List of minerals
• Diamond
• Ruby
• Sapphire

Gallery

More from the National Museum of Natural History

Notes

References

• Cooper, J.C. (Ed.) (1992). Brewer's Myth and Legend. New York: Cassell Publishers Ltd. ISBN 0-304-34084-7.
• Sinkankas, John (1994). Emerald & Other Beryls. Geoscience Press. ISBN 0-8019-7114-4
• Hurlbut, Cornelius S.; Klein, Cornelis (1985). Manual of Mineralogy (20th ed.). New York: John Wiley and Sons. ISBN 0-471-80580-7
• Weinstein, Michael (1958). The World of Jewel Stones. Sheriden House.
• Nassau, Kurt (1980). Gems made by man. Gemological Institute of America. ISBN 0-87311-016-1
• Ali, Saleem H. (2006). The Emerald City: Emerald mining in Brazil (+Gemstone mining in other countries) http://www.uvm.edu/envnr/gemecology/brazil.html
• Wise, Richard W., Secrets of the Gem Trade, The Connoisseur's Guide To Precious Gemstones (2001), Brunswick House Press. ISBN 0-9728223-8-0. Website: [1]
• Ball, V., & Crooke, W., Travels In India by Jean-Baptiste Tavernier, Oriental Reprint Corporation, New Delhi, India.

External links

Wikimedia Commons has media related to: Emeralds

• Historical References of Emeralds (full texts on) Famous Emeralds, and many of full text references used here.
• ICA's Emerald Page International Colored Stone Emerald Page
• Mindat Emerald Page Mindat on Emerald with locality information
• Webmineral Beryl Page Webmineral with crystallographic and mineralogical information