Emerald
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 |
Contents
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.
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
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
Sources
- Gems Their Sources, Descriptions and Identification 4th Edition (1990) - Robert Webster/ B.W. Anderson
- Secrets of the Gem Trade (2003) - Richard W. Wise