Emerald is a form of Beryl and crystallizes in the structure of prismatic crystals, sometimes vertically striated and terminated by small pyramid-like faces. The name Emerald is the English name for the green chromian Beryl; the spelling dates from the Oxford English Dictionary of 1526; Unntil about the year 1800, the name only meant a green gemstone.
 
The chemical composition is: Be3Al2Si6O18
Hardness 7.5 - 8
 
It is said to have been one of the stones used in the breastplate of the high priest Aaron. In addition, the Piajo Indian Brujas (shamans) from Columbia, in South America, use the Emerald to "tell the tomorrows”.
 
“The Emerald held a very high place in the esteem of the Ancients, and no other precious stone has probably been the subject of so much regard and admiration, one of its principal charms being its brilliant green colour. Very curious are some of the traditions connected with it.
In the first place, it is interesting to note that this gem being the Zodiacal stone of Cancer, the Crab was thought to have the same healing powers as the Oculi Cancrorum, or Crab's Eye ointment, prescribed in olden days as a cure for ulcerous sores.
Emeralds are found in Siberia, India, the United States, and Mexico, and the belief that demons and griffins guarded the mines is said to be as strong amongst the Peruvians of the present time as in the days of the Romans.
The origin of the word Emerald is from a Sanskrit word meaning green, it being thought that there was nothing in Nature to equal its colour and brilliancy, and it was an old Hebrew belief that if a serpent fixed its eye upon the lustre of this stone it would become blind.
The cause of its beautiful green tint has been attributed by some scientists to the presence of oxide of chromium, by others to copper, whilst in 1848 an experimenter believed it to be derived from an organic matter called chlorophyl, similar to the colouring substance of the leaves of plants.
 
Pizarro, in his conquest of Mexico, found numerous Emeralds of surpassing beauty; but d’Acosta, a contemporary writer, states that many of the finest stones were ruined by the Spanish soldiers, who, being informed by a priest that to test their genuineness they should be placed upon an anvil and struck with a hammer, followed these instructions with most disastrous results.
 
The Incas possessed some wonderful Emeralds; one (described by de la Viga) as large as the egg of an ostrich, was believed to be inhabited by Esmeralda, the chief goddess of Peru. When sacking her temples the Spaniards discovered immense quantities of Emeralds, it being customary for her priests to obtain them by representing to the worshippers that these gems were esteemed by the goddess above all else, Emeralds being her own daughters.
 
Emeralds were known and esteemed in most remote times of the world's history, and are to be met with amongst even Egyptian and Etruscan remains. Faith in its virtues and qualities exist amongst the Orientals to the present time, representing to them hope in immortality, courage and exalted faith, and protection from pestilence, as well as a preserver of eyesight, its efficacy being increased by a verse from the Koran engraved upon it.
 
In India it is believed to confer the gift of memory and a knowledge of secrets and futureevents. Frequently used in the decorations of sacred images, and is endowed by the Indians with very high attributes.
 
The Romans believed that nothing evil could remain in the presence of this gem which discovered falsehood and treachery by changing colour and turning pale, and when powerless to avert misfortune would fall from its setting, giving rise to the belief that the falling of this gem is a bad omen.
 
This stone was also considered very beneficial to the eyes, on which account it was worn as a seal ring; in connection with this Pliny states: "If the sight hath been dimmed and wearied by intense poring upon anything, the beholding of this stone doth refresh and restore it again." It is also recorded that Nero, who was very shortsighted, used an emerald eye-glass to watch the gladiatorial contests. Probably from its connection with the Moon, which rules the House of Cancer (and was the goddess of midwives), it was considered particularly fortunate for women at childbirth, and was held to promote constancy and domestic felicity.
 
Worn in a ring it strengthens the memory and protects from giddiness; it was also said to guard sailors and fishermen from perils and mishaps at sea if suspended round the neck so as to lie upon the breast (the part of the body ruled by Cancer). It taught unknown secrets, and bestowed eloquence and renown, and, in the words of Miss Landon, an English poetess:
 
"It is a gem which hath the power to show
 If plighted lovers keep their faith or no.
 If faithful, it is like the leaves of Spring.
 If faithless, like those leaves when withering."
 
It should, however (being a very sensitive gem), be only worn by those whose birthdays fall between the 22nd of June and July 23rd, or by those with the Moon in good aspect.”
-The Book of Talismans, Amulets and Zodiacal Gemsby William Thomas and Kate Pavitt 1922