Emerald in Calcite, Muzo  BERYL Aquamarine
(EMERALD, AQUAMARINE, HELIODOR, MORGANITE, GOSHENITE)

  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 denotes hues that are emerald green. Other 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 inclusions, 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). Such synthetic emeralds are easily distinguished from naturals by having somewhat lower R.I.s and S.G.s, and by inclusions. There are several other current and former manufacturers; Biron is perhaps the best known one that employs hydrothermal synthesis.

Sources

  • 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.
      • 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.
      • S.G.=2.71; R.I.E=1.578, R.I.O=1.584.
    • 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.

Shaping and Treatment

  • Usually step-cuts. Name Emerald Cut is synonymous with step-cut.
  • Heavily flawed stones sometimes cut en cabochon; nearly all cutting done this way prior to 19th century.
  • Unaffected by heat treatment. Most(?) natural stones are "oiled" to improve clarity. Oil fills cracks and cavities, and if oil with R.I. close to stone is used then cracks are nearly invisible. "Traditional" oils are Canada Balsam (a light yellowish resin that liquefies at low temperature; R.I.= 1.537) and cedarwood oil.  A plethora of other crack-filling substances ("fillers"), some more permanent than others, are also now in use.
  • "Traditional" oils can decrepitate with time and are thus not considered a permanent treatment, though gemstones can be cleaned and re-oiled if desired. Some treated cracks in oiled stones are known to fluoresce a pale yellow in u.v. light; untreated won't. Heavily oiled stones may visibly "sweat" when warmed by a strong light.
  • Rather sophisticated techniques are required to confidently identifying fillers. The identity of the filling agent can be important when trying to assess long-term effects on clarity and durability.
  • Perhaps the most popular of all emerald simulants is green glass with an R.I. and S.G. that has been adjusted to match emerald, and that may contain artificial inclusions that mimic naturals. Though these stones may look convincing in casual observation, they are isotropic and at 10X magnification the inclusions are clearly unlike anything seen in natural emerald.

Pricing and Valuation

  • Priced on a per carat basis, according to color, clarity and size of stone.
  • Exceptional stones of five carats or more have sold for as much as $25,000/carat!
  • Good stones in the 1-2 carat range command on the order of $1500-$4500/carat. At 5-10 carats, finer stones can start at $10,000/carat.  Recent price trends indicate a decline in wholesale prices for emerald of all quality.  This trend has continued.  Compare these trends with a table of  Colombian Emerald prices.
  • "Commercial quality" stones (light to dark green, poor clarity) sell for $50-$800/carat in the 1-2 carat range, depending on clarity and color.
  • Synthetic (a.k.a. "created" or "cultured") flux-growth emeralds made by Gilson and Chatham sold in 3, inclusion-based, grades (Gem, Fine, A) are priced per carat according to weight. "Gem" grade wholesale prices range from $245/carat for 0.12-1.5 carat stones, to $450/carat for 9-9.9 carat stones. Grade A stones of the same size are about half to a third of these prices. Flux-growth does not involve the use of water or a vapor phase and thus these synthetics do not contain the fluid inclusions that are common in natural emerald.
  • Hydrothermally grown synthetic emeralds once manufactured by Union Carbide (1965-1970; "Linde-Created Emeralds") and presently being manufactured by Vacuum Ventures ("Regency-Created Emerald") and Biron ("Kimberly-Created Emerald) can, however, contain fluid inclusions but lack alkalis usually present in naturals, which can be detected with a spectroscope.

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. (best-known near Haddam Neck, Connecticut).

Pricing and Valuation

  •     Fine gem quality aqua (no gray) of 10 carat size was selling for about $500/carat in 1986. Smaller stones, those with a gray overtone, or those of pale color sell for appreciably less. Stones with flaws visible to the naked eye are highly undesirable, and should be cheap to purchase. In the last few years the buying public in this country has discovered inexpensive treated deep blue topaz, which has resulted in a decline in aqua. prices. There is still a reportedly strong demand for fine aqua. in Europe and Japan, and finer, deeper blue (higher priced) gems are more frequently available there than in this country.  Extra fine aquamarine gemstones of 1 carat sold for roughly $250/ct (wholesale) in 1997.  Lower grade gemstones of the same size sold for substantially less.

        Interestingly, one source states that aqua. prices prior to 1982 were maintained in large part by demand by Brazilian buyers, who purchased it as an inflationary hedge. Prices for Brazilian aqua. were at an all time high in 1982 when the market collapsed, in part because of the wide availability of fine African material at lower prices, and because of the aforementioned surge in the demand for treated blue topaz.


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.  Red Beryl has recently been synthesized and may soon be commercially available.

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.

Pricing and Valuation

    Very little info.; $100-$200/carat for very best stones, lighter stones $10-15/ct. Deeper colors command a higher price. Because depth of color in all but the very best stones is weak, color tends to wash-out in smaller gems. One source states a gem of 15-20 carats is needed to preserve the best body color. Alas, stones of this size are rare, but for a superb example see the one in the Vargas collection in the display room on the first floor of the Geology building.

    Red beryl rare, expensive; $4000-$10,000/carat. Gemstones of a full carat or larger are extremely rare.


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.


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1998
Updated 08/20/09
Comments and questions to helper@mail.utexas.edu
Department of Geological Sciences
The University of Texas at Austin