(source: Wikipedia)
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.
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, Cut, Clarity
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.[8] 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]
Emeralds are green by definition
(the name is derived from the Ancient Greek word smaragdos, meaning
green).[9] Emeralds are the green variety of beryl,
a mineral which comes in many other colors that are sometimes also used as
gems, such as blue aquamarine,
yellow heliodor, pink morganite, red beryl or bixbite, not to be confused with
bixbyite, and colorless goshenite.[10]
Clarity
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.[11] 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.[12] The U.S.
Federal Trade
Commission requires the disclosure of this treatment when an oil
treated emerald is sold.[13] 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. 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.[citation needed]
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
localities
Spanish-made emerald and gold
pendant exhibited at Victoria and Albert
Museum.[14]
Emeralds in antiquity have been
mined in Egypt since 1500 BCE, and India,
and Austria since at least the 14th century CE.[15]
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.[16][17][18][19] Emerald production in Colombia has
increased drastically in the last decade, increasing by 78% from 2000 to 2010.[20] The three main emerald mining areas in
Colombia are Muzo, Coscuez, and Chivor.[21] Rare 'trapiche' emeralds are found in
Colombia, distinguished by a six-pointed radial
pattern made of ray-like spokes of dark carbon impurities.[citation needed]
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.[22] In the first half of 2011 the Kagem
mines produced 3.74 tons of emeralds.[23]
Emeralds are found all over the
world in countries such as Afghanistan, Australia, Austria, Brazil,[24] 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 1997 emeralds were discovered in the
Yukon.[25]
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.[verification needed]
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.[26] 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.[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 (attributable to
E.M. Flanigen),[27] 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”.[28] 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.[29]
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."[30]
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 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.[31]
India's most famous temple, the
Madurai Minakshiamman temple, has its chief deity as goddess Minakshi whose
idol is made of emerald, most likely carved out of a single emerald stone.
Notable
emeralds
Emerald
|
Origin
|
Colombia
|
|
Brazil
|
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