BERYL : Mineralogy and Gemology http://www.geo.utexas.edu/courses/347k/redesign/gem_notes/Beryl/beryl_triple_frame.htm The mineral beryl (pronounced barrel) is a principle store of beryllium in the earth's crust. In its rare gem form it is notable for a variety of vivid allochromatic colors, each of which have different gem names. All are nearly chemically identical, with the formula Be3Al2Si6O18, but contain differing trace quantities of Cr (Emerald - med. to dark green), Fe (Aquamarine - blue; Heliodor or Golden Beryl - yellow), or Mn (Morganite - pink, apricot; Red Beryl - red, of course!) that substitute for Al in the crystal lattice and act as chromophores. Indeed, because of a relatively low refractive index and low dispersion (=diminished brilliance), color is the prime factor that accounts for the popularity of most gem beryl. Gem beryl is found nearly exclusively in hydrothermal veins, pegmatites, or at the contacts of larger igneous intrusions that invade aluminous schist, shale or impure limestone. The volatile fraction of the vein liquid or magma provides Be and the host rock the requisite Al. Properties Crystal System: Hexagonal Habit: Usually well formed hexagonal prisms, either flattened or elongate, with pinacoidal terminations. Hardness: 7.5-8 Cleavage: 1; Weak, basal, perpendicular to prism faces. Toughness: Fair; inclusions (e.g. most Emeralds) reduce toughness. Fracture: Conchoidal Specific Gravity: Emerald 2.71 (2.67-2.78) Synthetic Emerald (Chatham,Gilson) 2.65 Aquamarine and Heliodor 2.68 (2.68-2.73) Morganite 2.80-2.90 (2.71-2.90) R.I.: Most Beryl has R.I.s of 1.57-1.59: Emerald: usually 1.571-1.584; varies with place of origin from 1.565-1.599 (see below) Synthetic emerald (Chatham, Gilson) has a lower R.I. of 1.561-1.564. Aquamarine and Heliodor 1.570-1.586 (1.567-1.590) Morganite 1.580-1.600 (1.572-1.600) Goshenite 1.566-1.594 Birefringence: Low (0.005-0.009) Dispersion: Low (0.014) Pleochroism: Emerald - Weak; green to blue green. Aquamarine - Weak; blue to darker blue. Morganite - Weak; light red to light violet Heliodor - Weak; greenish yellow to yellow Color: See above. Goshenite is colorless beryl. The name Emerald is reserved for emerald green beryl. Light green varieties should be referred to as green beryl. A rare red variety (Red Beryl) resembling fine ruby, from Utah, and a rare deep blue variety (Maxixe-Brazil) are also known. U.V. Fluorescence: Emerald - none to weak orange-red or green. Others very weak to none. Synthetic emerald may fluoresce a weak dull red and appear opaque under long u.v. light. Distinguishing Properties Low R.I. Low S.G. Dichroism Low Birefringence -------------------------------------------------------------------------------- Emerald The most popular of colored stones. Birthstone for May. Color and clarity highly variable, major factors in valuation. The soft, velvety appearance and "lime Jell-O" color of the best emeralds is unique among all natural gems, hard to confuse with anything else. Characteristically, nearly all stones are included, with the best colored stones sometimes being the most included. The term jardin (meaning garden) is used for mossy-appearing, densely included stones. Good stones of high clarity and color are extremely rare in sizes above 2 to 3 carats. Color is highly prized; weakly colored, light to medium green stone of excellent clarity up to 5 carats in size do not command the price of smaller, more included stones of better color. Unlike other gem beryl, emeralds are nearly always mined in situ; they seldom survive fluvial transport, perhaps due to an abundance of inclusion which decreases their toughness. Emeralds can be synthesized by both flux growth and hydrothermal processes. The two current major manufacturers, who both employ the flux growth technique, are Chatham Research Laboratories in San Francisco (Chatham Created Emerald) and Ets. Ceramiques Pierre Gilson (Gilson Emerald). Synthetic emerald from these to firms are easily distinguished by testing from naturals by their somewhat lower R.I.s and S.G.s, and by inclusions. There are several other current and former manufacturer. Sources A. Colombia - once 80-90% of world production; currently about 60%? Traditionally considered the worlds finest emeralds. Most from 2 mining districts northeast and east of Bogota, Muzo and Chivor. Ancient mines originally worked by Aztecs; "rediscovered" by Spanish in 1537 (Chivor) and 1559 (Muzo). Long history of intermittent production continuing to present-day. 1) Muzo - Emerald in calcite veins that invade black shale. [S.G.=2.71; R.I.E=1.578, R.I.O=1.584] Rough is often color zoned with paler core. Both Muzo and Chivor emeralds are characterized by three-phase inclusions - trapped fluid containing gas, fluid, and crystals of halite. Muzo emeralds often contain inclusions of calcite and yellow-brown needles of the mineral parisite. Mines owned by government since 1871; have been leased to various operators since then. Five year leases instituted in 1977 discontinued in 1982(?) due in part to poor recovery brought about by rapid mining techniques (dynamite and bulldozers). 10 year leases were most recently held by two companies, Tecminas and Coesminas, who developed underground workings. Once the most prolific emerald mines in the world. 2) Chivor - Emerald in quartz-albite-apatite veins that invade a gray calcareous shale. Chivor emerald has a lower S.G. and lower R.I. (S.G.=2.69, R.I.E=1.571; R.I.O=1.577) than Muzo emerald. Crystals tend to be more elongate than those from Muzo. Color is said to tend toward a bluer green ("cool green") as opposed to the slightly yellower green ("warm green") of Muzo. Characteristic inclusions are pyrite and albite. The Chivor mines are privately owned; owners pay a 25% royalty on all production to the Colombian government. Recent reports indicate little production, but the possibility of new ownership renewed hope that these mines would once again become productive. 3) Also mining areas at Cosquez (near Muzo) and Gachala (near Chivor). B. Zimbabwe- Sandawana Valley (S.G.=2.755; R.I.E=1.586, R.I.O=1.593) Mostly small stones (0.5 ct or smaller), heavily flawed in larger sizes Excellent color In schists invaded by pegmatites and quartz veins Noted for inclusions of acicular tremolite, and for somewhat higher R.I. than Colombian and Brazilian emeralds. C. Zambia - Miku deposit, near Kitwe (S.G.=2.75; R.I.E=1.583, R.I.O=1.590) Once a major producer;40% of all emeralds sold in U.S. in 1989 where from this source. In schists adjacent to pegmatites; can contain tourmaline and biotite inclusions. Excellent clarity, but often darker, with a more noticeably bluish cast that Colombian emerald. Source of significant production in 1980's; has redefined notion that best emeralds are invariably included. D. Brazil - Major producer. Salininha and Carnaibu Districts, Bahia; Santa Terezinha District, Goias; Nova Era and Itabira Districts, Minas Gerias. Most in alluvial deposits, not veins Majority of stones are too pale to be called emerald. Chromophore is vanadium in some stones, not Cr as in most other emeralds. Brazilian emeralds were once thought of as typically lighter-toned and much yellower compared to other sources. As for all generalities, there are many exceptions; Santa Terezinha has produced emeralds of exceptional color, though mostly of small size. E. Others - minor production USSR (Ural Mts., yellowish, heavily flawed), India, Tanzania (similar in quality to Colombia), Australia, Pakistan (Swat Valley), Afghanistan (Panjshir Valley), N.Carolina (near Hiddenite), South Africa, Austria, Madagascar. -------------------------------------------------------------------------------- Aquamarine Birthstone for the month of March. Name is applied to blue to greenish-blue to bluish-green ("sea green") beryl. Most common untreated material is greenish-blue, although very light to medium blue varieties are by no means rare. Heat treatment is used to deepen the color and to drive off green overtones; common, acceptable, permanent. Most prized colors are a fine greenish-blue and an intense, deep, sky blue. Those with a grayish overtone or of light tone are quite common in almost any size. Flawless to slightly flawed material is widely available. Aqua can be distinguished from blue topaz by hardness, R.I. and S.G.. Blue topaz has a decidedly higher R.I. (1.62); blue glass is, of course, singly refractive and not dichroic. Although laboratory (hydrothermal) synthesis of aquamarine has been achieved, synthetic aqua is not yet (c.1998) widely available. Sources Exclusively in pegmatites, or as water worn, frosted pebbles or cobbles in stream gravels. Single crystals weighing up to 243 pounds are known. A) Brazil - most important source, mostly from the state of Minas Gerais, near the town of Teofilo Otoni. Famous but now depleted deposit of Santa Maria noted for exceptional deep "electric" blue aqua. Common lighter blue aqua shows deepest color only in stones of 5-10 cts. or more. Blue-green stones sometimes referred to "Brazilian Aqua." B) Madagascar - usually medium dark blue. Darker aqua. sometimes referred to as "Madagascar Aqua." C) Africa (Namibia, Nigeria, Zambia, Zimbabwe) - mostly since 1982. Noted for untreated, deeper blue color that persists in even the smallest cut stones. 1-2 carat gems retain a "spectacular" deep blue. Most Nigerian stones show a slight green overtone that cannot be removed by heat treatment (Cr is chromophore, rather than Fe present in treated Brazilian aqua.). D) Others - Afghanistan, Northern Ireland, Namibia, USSR, Sri Lanka, Pakistan, India, U.S.A. (best-known near Haddam Neck, Connecticut). -------------------------------------------------------------------------------- Morganite Pink to peach-colored beryl, named after J. P. Morgan, the famous financier and banker. Does not exist in sufficient quantities to create an international trading market, mostly a collectors stone. A deep red beryl, known only from in Utah, is not called morganite, but red beryl. Heat treatment of some specimens renders them colorless, in others heating may be used to drive off a yellow overtone (produced by a yellow color center) to yield a nice pink. Sources Nearly all from 3 sources: Pala and Mesa Grande districts in San Diego Co., California; Minas Gerias, Brazil (#1 source, Urucum and Bananal Mines) and Mt. Bity region in Madagascar. Stones of a rich apricot or peach color are known to fade to pink. All are pegmatite occurrences. -------------------------------------------------------------------------------- Heliodor Golden yellow beryl, also known as yellow beryl if golden tint lacking. Good colored material is relatively rare. Found in pegmatites at a number of places in Brazil; also in Ural Mt., USSR, and Namibia. $100-$200/carat for very best stones; $1-1.50/carat for paler stones or with brown overtones. Deeper, golden colors command a higher price. Most will turn blue or colorless after heat treatment, though some Brazilian material is said to be heated to yield Morganite. -------------------------------------------------------------------------------- Goshenite Colorless Beryl. Collectors stones can be purchased for a few dollars/carat. Unaffected by heat treatment. Some can be (and are) irradiated to produce golden beryl or heliodor. -------------------------------------------------------------------------------- EMERALD FROM COLOMBIA http://www.gemtec.com/ MINING There are two basic methods used for extracting emeralds all over the world: by means of tunnels that follow the gem bearing veins and by open pit or open cut mining. In Colombia, emeralds were originally mined by the Spanish through the selective excavation of tunnels that allowed following attractive leads. However, tunneling presented a major waste removal problem, in addition to the fact that it could also mean bypassing extremely rich 'pockets' that could be only a few inches away from the tunnel's wall. This method therefore proved unprofitable and was gradually replaced for open pit systems. The Colombia 'emerald rectangle', an area of 250 kilometers long by 50 wide, in which the most important mines of Muzo and Chivor are located, presents a steep slope topography and a plentiful supply of water that allow a more efficient waste disposal. The most common system used here is the so-called 'benching' of the working face consisting of narrow horizontal steps across the slope, the exposed friable rock being easily dislodged with steel bars, and then carefully inspected by men using tools like sheath knives or even their bare hands. When this step has run out of emeralds, another one is traced above, dumping the waste on the exhausted step below. In most sites, bulldozing is mainly used to strip off the vegetation and topsoil that cover the hillsides and to remove waste, which is also done by flushing the debris away with powerful streams of water. Very mechanized exploration has proven itself risky, since it does not eliminate the hit-and-miss element, making necessary further inspections so as not to discard valuable material along with waste. Benching can only be accomplished where the slopes are sufficiently steep, there is enough water supply and room to accumulate dump waste at the base of the hill. Otherwise it becomes necessary to apply other methods, such as tunneling, extensive bulldozing and even the use of explosives, which might end up fracturing large valuable crystals. In recent years with the availability of better prospecting techniques a return to the method of tunneling is evident due to its advantages of operation, reduced personnel and machinery and more control over the production. Beryl Deposits http://www.trinityminerals.com/articles/beryl.htm Deposits Beryl occurs in many different geological environments. Probably the most common are the many pegmatites throughout the world which have produced gem quality beryl crystals. The pegmatite districts of southern California, Colorado, Brazil, Madagascar, and Pakistan are just a few of the classic areas which produce aquamarine, morganite and heliodor from pegmatites. Beryl may also be found in miarolitic cavities within granite. Examples of such occurrences are Mt. Antero, Colorado which is famous for aquamarine and the Ural Mountains of Siberia. Unlike pegmatites which form after the host rock has solidified, miarolitic cavities form during the same period that the host rock itself solidifies. Granite is an intrusive, igneous rock. Beryl has also been discovered in rock formations which are of volcanic origin. These lava flows form an extrusive, igneous environment. The rhyolites of western Utah and a small area of New Mexico are the only known examples. It is these areas which are famous for the bright red beryl crystals which form in lithophysae within the rhyolite. Beryl may also be found in certain metamorphic environments. These deposits of schist often contain emerald and are the product of granitic intrusions on one side and silica-poor rocks on the other. The basic elements which form the beryl come from the intruding granitic stock, but pass into the adjacent, silica-poor rock formations where they crystallize. Some of the most notable occurrences of this type of beryl deposit occur in Egypt, Africa, Austria, and the Ural Mountains of the USSR. Greisens are another type of rock in which beryl may be found. Greisens are formed when extremely hot, mineralized water passes through fractures in granitic. This superheated mineral water alters the granite host rock in a process known as hydrothermal alteration. The most notable occurrence of beryl in this environment is Sherlovaya Gora Mountain in eastern Transbaikalia, USSR. The emerald deposits of Columbia are unique, but are also of hydrothermal origin. The emeralds in these carbonate veins were formed when minerals dissolved in hot water originated from an unknown igneous source and passed into neighboring calcite-rich rock formations.
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