Emerald

For other uses, see (disambiguation)}}}.

Emerald
Emerald with inclusions
General
Category	Mineral
Chemical formula	Beryllium aluminium silicate with chromium, Be₃Al₂(SiO₃)₆::Cr
Identification
Color	Green
Crystal habit	Hexagonal Crystals
Crystal system	Hexagonal
Cleavage	Poor Basal Cleavage (Seldom Visible)
Fracture	Conchoidal
Mohs Scale hardness	7.5 - 8.0
Luster	Vitreous
Refractive index	1.576 - 1.582
Pleochroism	Distinct, Blue-Green/Yellow-Green
Streak	White
Specific gravity	2.70 - 2.78

Emerald (Be₃Al₂(SiO₃)₆) 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.

Emerald showing its hexagonal structure

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.

Flux-grown synthetic emeralds fluoresce a dull red with long wave UV 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 (FTC) 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."[link]

Furthermore, all natural emeralds, with the exception of the red emerald 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 (this also accounts for flux-grown emeralds being more stable when subjected to high temperatures). Hydrothermally-grown emeralds, however, contain water molecules.

Wispy inclusions are common in flux-grown synthetic emeralds.

The Gachala Emerald is one of the largest emeralds in the world at 858 carats. This stone was found in 1967 at La Vega de San Juan mine in Gachalá, Colombia.

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.

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.

References

Cooper, J.C. (Ed.) (1992). Brewer's Myth and Legend. New York: Cassell Publishers Ltd. ISBN 0-304-34084-7.
Sinkankas, John (1994). Emerald & Other Beryls. Geoscience Press. ISBN 0801971144
Hurlbut, Cornelius S.; Klein, Cornelis (1985). Manual of Mineralogy (20th ed.). New York: John Wiley and Sons. ISBN 0471805807
Weinstein, Michael (1958). The World of Jewel Stones. Sheriden House.

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