Difference between revisions of "Emerald"

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Emerald is the Chromium bearing variety of [[Beryl]] with a "grass green" hue and a slight yellow or blue secondary hue. The purer the green, the finer the quality of the stone.<br />
 
Emerald is the Chromium bearing variety of [[Beryl]] with a "grass green" hue and a slight yellow or blue secondary hue. The purer the green, the finer the quality of the stone.<br />
In 1960 H. Epsig described the color of Emerald as: " .. warmen Grün einer Wiese im Frühling .." (warm green of a meadow in spring).
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In 1960 H. Espig described the color of Emerald as: " .. warmen Grün einer Wiese im Frühling .." (warm green of a meadow in spring).
  
 
There are two other varieties of Beryl that are also green, [[Vanadium Beryl]] and [[Green Beryl]]. Vanadium Beryl is also known as "Vanadium-Emerald" and can exhibit colors like the Chromium bearing cousin but is usually much less included. "Green Beryl" however is a pale green Beryl that shows more resemblance with [[Aquamarine]] and is colored by Iron. One will hardly ever confuse Green Beryl with Emerald unless you are at the selling end of the market.
 
There are two other varieties of Beryl that are also green, [[Vanadium Beryl]] and [[Green Beryl]]. Vanadium Beryl is also known as "Vanadium-Emerald" and can exhibit colors like the Chromium bearing cousin but is usually much less included. "Green Beryl" however is a pale green Beryl that shows more resemblance with [[Aquamarine]] and is colored by Iron. One will hardly ever confuse Green Beryl with Emerald unless you are at the selling end of the market.
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==Synthetics==
 
==Synthetics==
  
Emerald can be synthezised by different methods. The most in the market today are of hydrothermal and flux-melt origin.
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Emerald can be synthezised by different methods. The most in the market today are of hydrothermal and flux-melt origin.<br /.
 
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The coloring agents of synthetic Emerald vary by manufactorer.<br />
 +
{| {{table}}
 +
|! Manufactorer
 +
|! Method
 +
|! Coloring agent
 +
|-
 +
| Chatham
 +
| Flux
 +
| Cr
 +
|-
 +
| Igmerald
 +
| Flux
 +
| Cr, Ni
 +
|-
 +
|Gilson
 +
| Hydrothermal
 +
| Cr, Ni
 +
|}
 
==Imitations==
 
==Imitations==
  

Revision as of 09:08, 28 October 2006

Emerald
Chemical composition Be3Al2(SiO3)6 Beryllium aluminum silicate
Crystal system Hexagonal
Habit Prismatic
Cleavage Imperfect, basal
Fracture Conchoidal to uneven
Hardness 7.5 (brittle)
Optic nature Uniaxial -
Refractive index 1.566 - 1.600
Birefringence 0.004 - 0.010
Dispersion Low, 0.014
Specific gravity 2.67 - 2.78
Lustre Vitreous
Pleochroism Weak to distinct

Color

Emerald is the Chromium bearing variety of Beryl with a "grass green" hue and a slight yellow or blue secondary hue. The purer the green, the finer the quality of the stone.
In 1960 H. Espig described the color of Emerald as: " .. warmen Grün einer Wiese im Frühling .." (warm green of a meadow in spring).

There are two other varieties of Beryl that are also green, Vanadium Beryl and Green Beryl. Vanadium Beryl is also known as "Vanadium-Emerald" and can exhibit colors like the Chromium bearing cousin but is usually much less included. "Green Beryl" however is a pale green Beryl that shows more resemblance with Aquamarine and is colored by Iron. One will hardly ever confuse Green Beryl with Emerald unless you are at the selling end of the market.

Tsavorite Garnet is a gemstone that comes close to Emerald in color, but has a much "harder" color than the Chromium colored Emeralds.

Diagnostics

The trained eye should have little trouble recognizing an Emerald due to its silky rich green color, the main challenge however is distinguishing natural Emerald from the various types of synthetic Emerald.

Magnification

Emerald is classed as a "type III" in the GIA clarity grading system. This means that stones are commonly included and these inclusions are mostly visible to the trained eye. Only few Emeralds are "eye clean".
In general eye visible inclusions in Emerald are forgiven if they show good color and transparancy overall.

With a 10x loupe or a microscope at low magnification typical inclusions are seen which often give vital clues about its origin, being it locality or natural/synthetic.

Optical and Physical Properties

The optical and physical properties of Emerald vary with location. Synthetic Emeralds usually have lower values.
nω = 1.575 - 1.600, nε = 1.566 - 1.588 for natural Emerald and nε = 1.560, nω = 1.565 for synthetic Emerald.
Birefringence of natural Emerald ranges from 0.004 to 0.010, while synthetic Emerald will hardly ever go over 0.004.

The specific gravity of synthetic Emerald is slightly to distinctive lower than of the natural ones. Natural Emerals will sink more rapidly in a heavy liquid with SG = 2.65 (bromoform diluted with toluol).


Source Refractive Index Birefringence Specific gravity
Australia 1.570-1.579 0.005 - 0.007 2.67 - 2.70
Brazil 1.566 - 1.575 0.005 2.68 - 2.70
Colombia 1.568 - 1.586 0.006 2.69 - 2.71
India 1.585 - 1.593 0.007 2.73 - 2.74
Pakistan 1.588 - 1.600 0.007 2.75 - 2.78
South-Africa 1.586 - 1.593 0.007 2.75
Tanzania 1.578 - 1.585 0.007 2.74
Zambia 1.580 - 1.590 0.004 - 0.010 2.71 -2.76
Zimbabwe 1.586 - 1.593 0.007 2.73 - 2.77
Hydrothermal synthetic 1.560 - 1.563 0.003 2.67 - 2.69
Flux synthetic 1.560 - 1.565 0.003 - 0.004 2.65 - 2.66

Spectroscope

Natural Chromium bearing Emeralds will usually show a distinct spectrum with a doublet in the red, two lines in the orange red region and a weak band in the yellow part.
Synthetic Emerald shows the same absorption pattern with the addition of a line in the blue (at 477nm), this line may however also be seen in fine quality natural Emerald.

Chelsea Colour Filter

Although the Chelsea Colour Filter was designed to discriminate between Emerald and its simulants in the 1930's it has lost that purpose with the arrival of synthetic Emeralds. It's primary use was to detect chromium in Columbian and Siberian Emeralds.
As synthetic Emeralds are also colored by Chromium this tool is no longer very reliable, this is especially true now we know that Vandaium and Iron may also be coloring agents in Emerald aswell. The Chromium will cause a fluorescence in natural daylight, but the Iron content that is present in Emeralds from certain locations dulls or totally surpresses that. Hence there will be a different (not red) reaction to the Chelsea Filter.
In the past it was suggested that if an Emerald would light up bright red under the CF, the stone would either be of Columbian origin or be synthetic. In other words, all synthetics will show up bright red due to rich Chromium content. This is no longer viable as Iron may be introduced into the synthezising process to mimic a more natural fluorescence reaction.

The use of the "Emerald Filter" set may be useful as an addition to the Chelsea Colour Filter.

Treatments

Enhancement of Emerald is regarded a routine to mask inclusions. About 90% of all Emeralds on the market have undergone some kind of treatment.
Most common enhancement is oiling, waxing or impregnation with materials that have a RI close to that or Emerald. Opticon is a brandname for a substance used to impregnate Emerald.

Synthetics

Emerald can be synthezised by different methods. The most in the market today are of hydrothermal and flux-melt origin.<br /. The coloring agents of synthetic Emerald vary by manufactorer.

! Manufactorer ! Method ! Coloring agent
Chatham Flux Cr
Igmerald Flux Cr, Ni
Gilson Hydrothermal Cr, Ni

Imitations

Most simulants of Emerald, as Tsavorite and Glass, should be easily detected with the trained unaided eye. There are unfortunatly also doublets and triplet imitations on the market that may be similar in appearance to Emerald.

Sources

  • Gems Their Sources, Descriptions and Identification 4th Edition (1990) - Robert Webster/ B.W. Anderson
  • Secrets of the Gem Trade (2003) - Richard W. Wise
  • A Sudents' Guide to Spectroscopy (2003) - Colin H. Winter
  • Gem Identification Made Easy 3rd edition (2006) - A.C. Bonanno/ Antoinette Matlins
  • The colour of Igmerald .. - Dr. Karl Schmetzer/Dr. Lore Kiefert - The Journal of Gemmology, July 1998

External Links