Emerald
Based on this article, it is said that emerald is a rare and valuable gemstone that can be seen clearly under a gemological microscope and, as such, it has provided the incentive for developing synthetic emeralds. Emerald is one of the most highly esteemed precious stones, known to ancient Egyptians, Hindus, Greeks, and Romans. In India emeralds were used to adorn images in temples, and Moslems used emeralds as amulets, inscribed with verses from the Koran. Emeralds were believed to change color when surrounded by deception and treachery. They were also believed to be preservatives against decay, dysentery, and the bites of venomous creatures and to promote easy childbirth.
Emerald is a grass-green variety of beryl that is highly valued as a gemstone. Its physical properties are those of beryl. Its refractive and dispersive powers are not high, so cut stones display little brilliancy or fire. The colour that gives this gem its value is due to the presence of small amounts of chromium. The most important production of fine quality gem material is from Colombia; emeralds are also mined in Russia, Australia, South Africa, and Zimbabwe. Synthetic emeralds are identical to natural crystals and may rival them in colour and beauty.
An emerald is medium- to dark-green gem variety of the mineral beryl, crystallizing in the hexagonal system. A flawless emerald with good color is one of the most sought after and highly prized of all precious gems. Emerald is restricted in its occurrence, and only infrequently are exceptional stones found; most emeralds are flawed and cloudy, and few stones command high prices.
In contradistinction to beryl and its other gem varieties, emeralds have only been found in mica schists or metasomalized limestones that can be seen clearly under a gemological microscope. The most outstanding occurrences include the Muzo and El Chivor mines in Colombia. Noteworthy occurrences in mica schists include Tokovoja in the Ural Mountains, where emerald occurs with the beryllium minerals chrysoberyl (and its gem variety alexandrite) and phenakite; Habachtal, Austria; Transvaal, South Africa; and Kaliguman, India. The ultimate source of an emerald can often be assessed by a study of its inclusions.
A hydrothermal synthetic emeralds have been attributed to IG Farben, Nacken, Tairus and others, but the first satisfactory commercial product was that of Johann Lechleitner of Inbruck, Austria, which appeared on the market in the 1960s. These stones were initially sold under the names “Emerita” and “Symeralds”, and they were grown 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 this article, acidic conditions are essential to prevent the chromium (which is used as the colorant) 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 500 to 600°C with a temperature gradient of 10 to 25°C. Growth rates as fast as 1/3 mm per day can be attained.
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 Chatham. Because Chatham’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 that can be seen clearly under a gemological microscope 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 emerald crystals of 7 mm of thickness. Original article
