Lets Talk Gemstones

By Edna B. Anthony, Gemologist
(Contact the author for permission to reproduce this article in any form.)

EMERALD

(A CYCLOSILICATE)

PART 2
PART 1 of my emerald article dealt with contact-metamorphic and hydrothermally-influenced contact-metamorphic emerald deposits located throughout the world. Columbia, South America is the only known source of emeralds created solely by hydrothermal conditions. The interior characteristics of emerald crystals, precipitated from nutrient-saturated hot water solutions into cavities and crevices, created by plate tectonic forces in the Andes mountains, differ markedly from those of emeralds formed by the other processes. Larger, cleaner, more transparent crystals with a more intense color tend to develop under hydrothermal conditions. Fewer solid and protogenetic materials are found as inclusions. A large majority of the finest emeralds marketed are recovered from these Columbian mines.

Despite a long, violent history, studded with acts of extreme cruelty and wanton destruction, the emerald supply seemed inexhaustible until recently. Today, many in the gem industry doubt that an orderly market can be achieved and maintained. The unstable political situation and a disarray in the Columbian industry have created turmoil. Exploration for new sources lags where primitive and hazardous mining conditions still prevail. Rampant smuggling and an "under the table" type dealing in the finest rough and finished gems continue. The lack of industry-wide accepted standards for treatments and disclosure causes much confusion. For the more knowledgeable and wary consumer, this presents short-term difficulties. The first concerted efforts to resolve the problems began only recently. Great changes must take place if the emeralds from Columbia are to remain readily available to those who appreciate these lovely green gems.

Long before the conquest of Peru by the Spaniards, emeralds from the Muzo and Somondoco (Chivor) areas were prized by the Indian cultures throughout Central and South America, including Indians in the Inca, Aztec, Toltec, and Mayan Indian tribes. Emeralds played an important role in the celebrations and religious rites of these cultures. Emeralds were also used extensively for personal adornment. Following the conquest, the insatiable greed of the conquistadors led to unsuccessful efforts by the natives to conceal and preserve their treasures and their sources. Treachery, torture, and murder became the order of the day.

The Somondoco Mine was discovered and seized by an expedition force led by Captain Pedro Fernandez Valenzuela. The mine was developed and operated under incredibly cruel conditions, until it was ordered closed by Charles II in 1675. Jungle growth reclaimed the area. The natives of Muzo fiercely defended their treasures against all attacks by the heavily armed Spaniards. An unsubstantiated report relates that only the release of vicious dogs by the invaders vanquished the defenders.

The Villa de Santisima Trinidad de los Muzos was established, and the exploitation of the prize and the natives soon began. An extensive chronology of the events following the conquest and capture of the mines at Muzo and Chivor is presented in a most excellent book by Dr. John Sinkankas, Emerald and Other Beryls, published by Geoscience Press.

The known emerald deposits in Columbia lie in a northwest-southeast belt that crosses the three northern ranges of the Andes, generally north and east of Bogota. Farthest north, the mines at Muzo, Cosquez, Pena Blanca, and the lesser known mines at Ramal, Amarilla, Alumbra, Cuincha, and Isabi are all situated just north of the equator, lying on the slopes of deep ravines of the westernmost range. The mines of Nemocon, Chirvaneque, and Raquira-Tinjaca (due east of Muzo) lie on a line northeast of Bogota between the Muzo and Chivor areas. The mines at Guateque, Chivor, and Gachala form a triangle that lies east and slightly north of Bogota in the southernmost region of the belt.

The geology and the mineralogy differ in each area. However, two characteristics are common to these emeralds of hydrothermal origin, in that they house few protogenetic mineral crystals. Three-phase inclusions frequently occupy the numerous internal fractures caused by great stress both during and after the formation of the emeralds. Emeralds free of inclusions are extremely rare. Primary inclusions develop during the growth period of a crystal and become trapped in the interior as the crystal grows. Acutely angled perimeters and jagged edges that often resemble saw blades are sure marks of their syngenetic development. Internal fractures that reach the surface of a developed crystal allow mineral-bearing liquids to seep onto cleavage planes and form secondary inclusions having smoother and softer contours. Both types may exhibit two- and three-phase characteristics. Sufficient numbers of tiny inclusions may cause emerald to be translucent or almost opaque. Such material is called "moralla." and is less dense than finer quality emerald. "Canutillos" denote uncut prismatic gem crystals too small and too thin to be faceted, but are sometimes set into jewelry. "Chispas" are tiny fragments of gemmy emerald rough.

In the Muzo area, the emerald occurs in calcite and dolomite veins of severely fractured and alternating layers of black shales and limestones. Dr. Sinkankas states that "in normal Muzo veins, albite is very uncommon", although it is found in minor quantities in the lower layers of the emerald-bearing formations. Upon exposure, the calcite and dolomite veins weather to a yellow-grey or brownish crumbled texture. Though usually short and prismatic, crystals as large as four by two inches have been found. Pyrite, apatite, fluorite, quartz, calcite, albite, and the rare and diagnostic parasite accompany and may be found as inclusions in the emerald crystals. Primary two- and three-phase inclusions, composed of liquid solutions that contain gas bubbles and crystallites of halite, calcite, dolomite, or other minerals, are also common. The yellow-green interiors of some of the finest Muzo emeralds exhibit a pebbled pattern called "oil drops", caused by the dense intergrowth of tiny emerald crystals. Such crystals exhibit fewer inclusions and have a slightly higher specific gravity.

The unusual trapiche emerald is found in both the Muzo Mine and the Pena Blanca Mine. Dr. Sinkankas makes a reference to R. Scheibe, who mentioned in 1926 the occurrence of trapiche emerald crystals in Banco Amarillo and also near Tambre Boliche, Muzo. Dr. Sinkankas also says that Fritz Klein believed Banco la Fragua was the source, but no other information is given. In some trapiche emeralds, inclusions consisting of albite, quartz, and a carbonaceous material outline a hexagonal beryl core, and they extend from it in "spokes" that divide the surrounding emerald material into six trapezoidal sectors. Often, the hexagonal beryl center is transparent and colorless, or it can be green. The cores appear black when they are heavily included by the carbonaceous material. More transparent brownish green cores can sometimes be obtained by heat treatment, which destroys much of the carbon in the crystals. In "coreless" trapiche crystals, the "spokes" intersect at the center of the crystal to create six triangular sectors.

Additional information, found in various references and obtained by personal investigation, is conflicting as well as confusing. On page 152 of The Internal World of Gemstones, (1983) Dr. E. Gubelin shows a "basal section through a trapiche emerald from Chivor with a bright central prism." In his book, Emerald and Other Beryls, (1989) Dr. Sinkankas tells of a study by Nassau and Jackson of crystals thought to be from Chivor. Later, it was learned that these had been recovered by a farmer on Pena Blanca land in 1963. Their drawings can be seen on page 263. Shown also is a drawing by F. Bernauer (1926) of a Muzo crystal with a tapering central core.

Excellent photographs of the two types of trapiche emeralds appear on page 253 of the PhotoAtlas of Inclusions in Gemstones by Dr. Gubelin and J. Koivula (1992, second, revised edition). One photograph shows a "coreless" specimen from the Muzo mine. The other depicts a dark rimmed, clear-cored crystal from Chivor. In 1996, I obtained two small trapiche cabochons, one "cored" and the other "coreless." Their yellow-green color coincides with the information that emeralds from the Muzo area exhibit yellow-green tones. At the 1998 Tucson Show, I discussed the subject with a dealer who claimed to be a partner in the group licensed to mine Muzo emeralds. He stated emphatically that trapiche emeralds were found only in the Muzo area. He believed that "coreless" crystals were found only in the Muzo Mine, and that the "cored" specimens were exclusive only to the Pena Blanca Mine. (The Summer 1998 issue of Gems and Gemology shows on page 138 a picture of a light greyish-green trapiche beryl crystal found at the miming area near Mananjary in Madagascar.)

The Muzo Mine experienced few interruptions of production, despite the presence of violent and cruel conditions following the Spanish conquest. Unlike the Muzo Mine, the Chivor Mine did not return to production until 1911. After Columbia attained its freedom from Spain, its government proclaimed in 1847 that all existing emerald deposits belonged to the nation. Private companies could operate the mines under lease with all production taxed. In 1896, the mining engineer, Don Francisco Restrepo learned through a study of documents, written by a priest during the Spanish operation, that the Orinoco plains were visible from the mine site through a gap in the surrounding mountains. This important information led to its location atop a mountain approximately 70 miles northeast of Bogota. In 1901, Restrepo and his associates concluded an agreement with the government that bestowed perpetual title of the area to him and his associates. The agreement conveyed title upon the payment of an amount equal to the sum of twenty years of taxes. The mine was then reopened, and operations commenced in 1911. The agreement was upheld by the Columbian Supreme Court in 1913. Further attempts to tax or obtain control of the property by the government were forbidden. The Indian name, Somondoco, meaning "god of the green stones", has been replaced by the company name.

Several sedimentary layers of hard, bluish calcareous shale and iron oxides (pyrite and altered pyrite that consist of limonite and hematite) cover the bands of emerald-bearing shales at Chivor. Emerald found in the upper layers is cemented by the limonite. Great care must be used to extract the crystals deposited in the isolated veins of albite. It is believed that these compact layers prevented the general upward disposition of the nutrient-saturated solutions. Crystallization of finer emerald material took place under more moderate temperatures and pressures in the lower shales and in soft albite-quartz-apatite veins.

Comparable size Chivor crystals usually house fewer inclusions than those from Muzo. Albite, pyrite, quartz, and goethite inhabit the interiors. Veils composed of myriads of tiny three-phase inclusions are characteristic of these emeralds. Color-zoning is common in Chivor beryl, which ranges from colorless through the deep blue-green of the finest crystals. Dark green centers are sometimes surrounded by light green or colorless beryl. The opposite configuration also exists.

The underground Buenavista group of mines adjacent to Chivor in Ubala possess the same mineralogic and geologic characteristics. Mina Buenavista Baja and the Buenos Aires, La Cueva, La Perla, and La Laguna claims all lie across the Rio Rucio. Recovery of the emerald crystals from the argilitic (hard clay) material embedded in slate has proved very difficult.

Mining takes place in situ at several of the Gachala deposits, although the first emerald crystals were found in alluvial debris. Slate beds surround the emerald-bearing ferruginous (containing iron) shale at Mina el Reten. Mina Rio Batatas is a minor source of emeralds. The crystals from this source are found in sandy clay shales in hard sandstone formations. Numerous other sites in the area produce crystals of little commercial importance.

The Nemocon Mine is located on the outskirts of Bogota in a salt column covered by black shale heavily impregnated with pyrite. A search for lime deposits in 1915 led to the discovery of small, poor quality emerald prisms found in fibrous calcite veins. The emerald is accompanied by crystals of pyrite, smoky quartz, and the occasional dolomite.

There are many other little known mines in the very forbidding mountain ranges of the Andes. The possibility exists that more untold treasures wait to be discovered there in those majestic mountains. The incomparable gem emeralds of hydrothermal origin from Columbia are wonderful treasures that are sought after allover the world. These emeralds may become more dear if the recent efforts to bring order to the market prove unsuccessful. Emeralds are fragile stones that require great care to preserve their magnificent beauty. It is a wise emerald owner who learns their characteristics and entrusts their care only to those highly qualified persons of impeccable integrity.


 
Gemstone Properties
SPECIE
emerald
Composition:
beryllium aluminum silicate
Be 3 Al 2 (Si 6 O 18 )+Cr, Fe
Class:
silicate; cyclosilicate
Group
beryl
Species:
emerald
Crystal System:
hexagonal; per Schumann, trigonal
Variety:
emerald of hydrothermal origin
Colors:
green
Phenomena:
unusual trapiche patterns; chatoyancy and asterism are rare
Streak:
white
Diaphaneity:
transparent, translucent, semi-translucent, and opaque
Habit:
prismatic columnar
Cleavage:
difficult for basal, brittle
Fracture:
conchoidal to uneven
Fracture Lustre:
vitreous to resinous
Lustre:
vitreous
Specific Gravity
varies from 2.67 to 2.78; Muzo moralla is 2.56
Hardness
7.50 to 8.0
Toughness:
poor; brittle; a very delicate gem
Refractive Index
varies from o=1.575 to 1.602; varies from e= 1.570 to 1.592
Birefringence:
varies from 0.005 to 0.010
Optic Character
uniaxial negative
Dispersion:
0.014
Pleochroism
varies; is distinct in strong colors
Ultraviolet 
Fluorescence
SW usually inert, at times weak yellow or green; LW rare weak red or orange; iron content quenches fluorescence
Spectra
diagnostic; thin lines in red; weak lines in blue; broad band in violet
Color Filter
no information
Aqua Filter
no reaction 
Chelsea Filter
varies; weak to moderate red induced by chrome
Solubility
affected only by hydrofluoric acid
Thermal Traits
avoid thermal shock; very fragile; remove stone during jewelry repairs; avoid ultrasonic cleaners
Treatments
nearly all material oiled at mine sites; cedarwood oil; palm oil; Opticon; colored oils, waxes, and resins; recently patented Gematrat process
Inclusions
(see preceeding text)