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.
|
|
BERYL
