Emerald
Emerald is a variety of the
mineral beryl, colored green by trace amounts of
chromium and sometimes
iron. It is highly prized as a
gemstone and by weight is the most valuable gemstone in the world, although it is often made less so by inclusions, which all emeralds have to some degree. Beryl has a
hardness of 7.5 on the 10 point Mohs scale of hardness. However, this Mohs rating can decrease, depending on the number and severity of inclusions in a particular stone.
Most emeralds are oiled as part of the post lapidary process.
Encyclopedia
| Emerald |
|---|
Emerald with inclusions |
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| General |
|---|
| Category | Mineral |
| Chemical formula | Beryllium aluminium silicate with chromium, Be3Al26Cr |
| Identification |
|---|
| Color | Green |
| Crystal habit | Hexagonal Crystals |
| Crystal system | Hexagonal |
| Cleavage | Poor Basal Cleavage |
| Fracture | Conchoidal |
| Mohs Scale hardness | 7.5 - 8.0 |
| Lustre | Vitreous |
| Refractive index | 1.576 - 1.582 |
| Pleochroism | Distinct, Blue-Green/Yellow-Green |
| Streak | White |
| Specific gravity | 2.70 - 2.78 |
|
Emerald is a variety of the
mineral beryl, colored green by trace amounts of
chromium and sometimes
iron. It is highly prized as a
gemstone and by weight is the most valuable gemstone in the world, although it is often made less so by inclusions, which all emeralds have to some degree. Beryl has a
hardness of 7.5 on the 10 point Mohs scale of hardness. However, this Mohs rating can decrease, depending on the number and severity of inclusions in a particular stone.
Most emeralds are oiled as part of the post lapidary process. The amount of oil entering an emerald microfissure is roughly equivalent to the size of a period in print.
Emeralds come in many shades of green and bluish green. There is a wide spectrum of clarity, along with various numbers of inclusions. Most emeralds are highly included, so it is quite rare to find an emerald with only minor inclusions.
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.
Synthetic emerald
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 C. Chatam. Because Chatam'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 is 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 and a typical seven-month growth run produces emeralds crystals of 7 mm of thickness .
Hydrothermal synthetic emeralds have been attributed to IG-Farben, Nacken, Chatam and others but the first satisfactory commercial product was that of Johann Lechleitner of Inbruck, Austria, which appeared on the market on the 1960's. These stones were initially sold under the names "Emerita" and "Symeralds" and they were grew 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 on acid conditions. Later, from 1965 to 1970, the Linde Division of Union Carbide produced completely synthetic emeralds by hydrothermal synthesis. According to their patents acidic conditions are essential to prevent the chromium from precipitating. Also, it is important that the silicon containing nutrient be kept away from the other ingredients in order to prevent nucleation and confine growth to the seed crystals. Growth occurs by a diffusion-reaction process, assisted by convection. Typical growth conditions include pressures of 700-1400 bars at temperatures of 500oC to 600oC with a temperature gradient of 10 to 25 C. Growth rates as fast as 1/3 mm per day can be attained.
Flux-grown synthetic emeralds
fluoresce a dull red with
long wave ultraviolet light, due to an indicator added during the process of synthesizing the emerald, whereas natural specimens do not.
Synthetic emeralds are often referred to as "created", as their chemical and gemological composition is exactly same as its natural counterparts. The
Federal Trade Commission has very strict regulations as to what can and what can not be called "synthetic" stone. The FTC says: "...[created stone must have] essentially the same optical, physical, and chemical properties as the stone named."
Furthermore, all natural emeralds, with the exception of the red
Bixbite beryls from Utah which are anhydrous, have water inclusions, as emerald is of hydrothermal origin. Flux synthetic emeralds have no water, an integral part of any natural beryl . Hydrothermally-grown emeralds, however, contain water molecules.
Wispy veil-like inclusions are common in flux-grown synthetic emeralds.
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Cultural and historical/mythical usage
Emerald is regarded as the traditional
birthstone for May, as well as the traditional gemstone for the astrological signs of Cancer and Taurus. According to legend, the emerald has the power to protect the
chastity of the wearer and to ward off evil spirits. Furthermore, it was believed to cure
epilepsy and dysentery, and to aid weak eyesight. The Emerald is also a symbol of Pride.
According to
Rebbenu Bachya, the
Hebrew word "Nofech" in Exodus 28:18 means "Emerald", and was the stone on the Ephod representing the
tribe of Judah. According to other commentaries, "Nofech" means "
garnet", and another stone, the "Bareqet", representing the tribe of Levi, is thought to be the emerald.
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.
Famous emeralds
References
- Cooper, J.C. . Brewer's Myth and Legend. New York: Cassell Publishers Ltd. ISBN 0-304-34084-7.
- Sinkankas, John . Emerald & Other Beryls. Geoscience Press. ISBN 0-8019-7114-4
- Hurlbut, Cornelius S.; Klein, Cornelis . Manual of Mineralogy . New York: John Wiley and Sons. ISBN 0-471-80580-7
- Weinstein, Michael . The World of Jewel Stones. Sheriden House.
