Emerald
Emerald is a
gemstone, and a variety of the
mineral beryl (Be
3Al
2(SiO
3)
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.
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
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,
[citation needed] color is divided into three components:
hue,
saturation and
tone.
[7]
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 Greek word "smaragdus", meaning green).
[8] 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
red beryl or bixbite, not to be confused with bixbyite, and colorless
goshenite.
[9]
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 'the garden', because of their mossy
appearance.
[10]
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. 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. 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 severely less than an identical stone
graded none.
[5]
Emerald localities
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.
[citation needed]
Zambia
is the world's second biggest producer, with its Kafubu River area
deposits (Kagem Mines) about 45 km 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.
[21]
Emeralds are found all over the world in countries such as
Afghanistan,
Australia, Austria,
Brazil,
[22] 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.
[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.
[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.
[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.
[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),
[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."
[28]
Emerald in different cultures, and emerald lore
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.
[29]
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
Gallery
-
-
The
Chalk Emerald ring, containing a top-quality 37-carat emerald, also in the U.S. National Museum of Natural History.
-
The
Hooker Emerald Brooch, containing a 75-carat square-cut emerald, also in the U.S. National Museum of Natural History.
-
Emerald crystal (about 1 cm) in
calcite matrix,
Muzo, Colombia.
-
-
Typical low quality emerald.
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