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Feature Articles
Gemstones
The rarity of these glittering jewels makes them valuable to geologists studying
conditions deep in the Earth
Lee A. Groat
T
he sparkle and luster of gemstones
has made them prized objects for
thousands of years. Gems are valued
for their color, luster, transparency, durability and high value-to-volume ratio.
Because many gems are produced from
relatively small, low-cost operations in
remote regions of developing countries,
it is difficult to obtain accurate statistics
regarding their production and value.
However, world production of uncut diamonds was worth $12.7 billion in 2008,
and in 2001 the trade journal Colored
Stone calculated that the world coloredgem trade was worth about $6 billion
per year. Although synthetic forms of
many gems now exist, they have yet to
have a serious impact on the international gemstone market.
Part of the reason that gemstones
reach such high values is their rarity. A
typical diamond deposit yields 5 grams
of gems per million grams of mined material, with only 20 percent of the gems
being of jewelry quality. Like oil, gems
can take an immense stretch of geologic
time to form. Radioactive-decay dating
of microscopic inclusions in diamonds
has found these gems to be 970 million
to 3.2 billion years old. Thus high-quality
gems can be mined out much faster than
they are produced, essentially making
them a finite resource. For instance,
one emerald mine established in 1981
in Santa Terezinha, Brazil, produced a
peak of 25 tons of rough stones valued
at $9 million in 1988; the same tonnage
of stones mined in 2000 sold for only
$898,000. This scarcity also makes gemstones highly valuable to geologists.
Exceptional geological conditions are
required to produce gem deposits. The
desire to unravel the history of such unusual circumstances is drawing increasing numbers of Earth scientists to the
study of gems and their origins.
Although there are dozens of different
types of gems, among the best known
and most important are diamond, ruby
and sapphire, emerald and other gem
forms of the mineral beryl, chrysoberyl,
tanzanite, tsavorite, topaz and jade.
(Common gem materials not addressed
in this article include amber, amethyst,
chalcedony, garnet, lazurite, malachite,
opals, peridot, rhodonite, spinel, tourmaline, turquoise and zircon.)
Lee A. Groat earned a Ph.D. in geology from the
University of Manitoba in 1988. He has been at the
University of British Columbia since 1989, where
he is now a professor of Earth and ocean sciences
and director of the Integrated Sciences program. In
1999 he was awarded the Young Scientist Medal
of the Mineralogical Association of Canada and
in 2002 he was awarded a Killam Prize for Excellence in Teaching. He is a past editor of American
Mineralogist and current coeditor of Canadian
Mineralogist. In 2009 the new mineral groatite,
NaCaMn2+2(PO4)[PO3(OH)] 2, was named in his
honor. He is on the board of directors and an adviser
to several Canadian mineral exploration companies,
and co-runs a geological consulting company. Address: Department of Earth and Ocean Sciences,
University of British Columbia, Vancouver, BC
V6T 1Z4 Canada. E-mail: [email protected]
Diamond
Diamond is the crystalline phase of
carbon formed at very high pressures.
It is the most highly valued gem; exceptional stones can fetch upward of
$500,000 per carat (1 carat = 0.2 grams)
and individual pieces can be valued at
more than $20 million.
The Golkonda region in south-central
India was the original source of diamonds for hundreds of years, until discoveries were made in Brazil during the
18th century and at Kimberley, South
Africa, in 1866. Today, the top three
diamond-producing nations by value
are Botswana, Russia and Canada, with
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American Scientist, Volume 100
significant production from Angola,
Australia, Congo, Lesotho, Namibia,
Sierra Leone and South Africa.
Diamond crystallizes in the cubic system, meaning that its constituent carbon
atoms are arranged in cells with axes
of the same length (specifically 0.356
nanometers) at right angles to each other. This formation results in a number
of shapes, including octahedra, cubes,
cubo-octahedra and less regular aggregates. Diamond can exhibit a number
of distinct physical properties, such as
emitting a glow under ultraviolet (UV)
light or x-rays. Such x-ray fluorescence
is exploited when processing ore to distinguish diamonds from the waste rock.
Gems emit visible light when hit with
UV or x-rays because defects in the crystal structure absorb the radiation, causing their constituent electrons to vibrate
between energy levels, and the energy is
released as light. (Gemstones that have
been treated with heat or radiation, or
are synthetic or fakes, will often fluoresce
at different wavelengths, so this property
can also be used to verify real stones.)
Diamonds are divided into types
according to the presence or absence
of nitrogen and boron, as well as the
structural organization of these impurities within the crystal lattice. Type I
diamonds are described as containing
significant nitrogen that is detectable by
infrared absorption spectroscopy (a process that detects which wavelengths are
absorbed or transmitted by a stone, each
element being associated with typical
wavelengths). Type II diamonds do not
contain significant nitrogen.
Color in natural diamond is related
primarily to the substitution of other elements for nitrogen and other defects
caused by physical deformation in the
crystal lattice; there are often multiple
color-causing defects in a single sample.
© 2010 Sigma Xi, The Scientific Research Society. Reproduction
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Joel Arem/Photo Researchers, Inc.
Figure 1. A diamond in the rough is still attached to the host rock, called kimberlite, that brought it to the surface in South Africa, but it formed
deep in the Earth millions to billions of years ago. Gemstones of all types are rare, and therefore valuable, because their formation requires a highly
unusual set of geological circumstances. Earth scientists value the information that gemstones can impart about the inner workings of the planet.
Type I diamonds in which the nitrogen
impurities are clustered are generally
colorless, brown or yellow; when the impurities are more widely diffused the
diamonds are yellow, orange or brown.
Pink, red and purple diamonds are also
of Type I, and the coloration has been
tied to deformation of the impurity-laden
part of the crystal lattice after the gem finished forming. Type II diamonds contain
very few or no nitrogen impurities but
may have boron impurities, which typically render the diamond blue to gray.
When they are virtually devoid of all impurities, they are colorless or brown.
Mineral inclusions within diamonds
permit calculation of pressures and
temperatures of the environment in
which they formed. Diamonds generally crystallize at depths of 135 to 200 kilometers and at temperatures of 1,100 to
1,200 degrees Celsius. The vast majority
originates from within the lithosphere
(the rigid crust and upper mantle of
the Earth) below very old, stable parts
of the continental crust called cratons—
areas toward the center of tectonic
plates that are far from areas of growth
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or subduction. The rest originate largely from sublithospheric sources, which
can be as deep as the lower mantle.
Such sources are generally described
as being within deep keels of ancient
cratons, where geothermal energy is
suppressed by these relatively cold
masses, thus allowing crystallization to
occur. This setting is low in silica, and is
dominated by rocks such as peridotite
or eclogite, both of which are high in
magnesium and iron.
Well-formed diamond crystals most
likely result from two processes. One
is the reduction (the gain of electrons)
of oxidized carbonate (CO3) in its solid
state, or dissolved within a melted rock
or chemical-rich fluid. The other is the
oxidation (the loss of electrons) of reduced carbon in the form of methane.
Crystallization in either a molten-rock or
a fluid-dominated setting allows physically unconstrained crystal growth.
Carbonate that is reduced to diamond would likely be in the form of
the minerals dolomite [CaMg(CO3)2]
or magnesite (MgCO3). The carbonate
component has been hypothesized to
originate from carbon introduced into
the mantle when volatile chemicals,
such as CO2, escape oceanic crust as it
subducts beneath another tectonic plate
and enter a region of molten rock.
Methane that is oxidized to diamond
is thought to originate from reduced fluids in the upper mantle. The release of
water by this reaction would aid in fluiddriven, or metasomatic, reactions at the
site of diamond growth in the subcratonic lithosphere. The ultimate origin of
carbon in diamonds, however, is ambiguous and a subject of ongoing research.
Models for mantle evolution suggest
that diamonds older than 2.5 billion years
were most likely generated from methane oxidation, whereas those younger
than 2.5 billion years were mainly
formed through carbonate reduction.
Radiometric dating determines age
by measuring the percentage of different
isotopes (variants of elements that differ only in the number of neutrons) in a
material that has naturally decayed over
geologic time. Such analysis of diamond
is achieved by studying its silicate and
sulfide mineral inclusions that crystal-
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2012
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129
EURASIAN PLATE
NORTH AMERICAN PLATE
PHILLIPINE
PLATE
CARIBBEAN
PLATE
ARABIAN
PLATE
COCOS
PLATE
PACIFIC PLATE
AFRICAN PLATE
PACIFIC
PLATE
INDIAN
PLATE
SOUTH AMERICAN PLATE
NAZCA PLATE
INDO-AUSTRALIAN
PLATE
SCOTIA PLATE
rubies
ANTARCTIC PLATE
sapphires emeralds beryls
jade
topaz
diamonds
Figure 2. Gemstones of all types can be found worldwide. The size of each symbol signifies the economic importance of the gems from a particular
region. Gems are often found in areas of tectonic or volcanic activity, but some deposits seem to be located where there is no evidence of magmatism.
lized at the same time as the host diamond. However, analyses of minerals
along exposed planes where the crystal
has cleaved have been shown to record
the date of the eruption that brought the
diamond to the surface, not necessarily
the date of its formation.
Regardless of depth of formation,
diamonds are transported quickly to
the surface via rapidly rising bodies of
molten rock—called kimberlite or lamproite magmas—that originate from the
growth areas themselves or from greater
Kimberly pipe
depths. The transporting magma may be
corrosive to diamond and thus requires a
speedy ascent to preserve the gems. Kimberlites erupt at average speeds of 10 to
30 kilometers per hour through the rapid
release of carbon dioxide and water, creating buoyancy. However, the mechanism of this release was not clear until
recent work by James K. Russell and his
colleagues at the University of British
Columbia. In high-temperature experiments, Russell’s group showed that it is
not a decrease in pressure that causes the
De Beers pipe Bultfontein pipe Dutoitspan pipe Wesselton pipe
0
depth (meters)
200
diatreme
zone
diatreme
zone
400
600
root
zone
diatreme
zone
diatreme
zone
diatreme
zone
root
zone
root
zone
800
root
zone
1,000
1,200
kimberlite
karoo
dolerite
shale
ventersdorp
andesitic lavas
conglomerate
quartzite
archaean basement
gneiss,
amphibolite, schist
Figure 3. The kimberlite pipes that delivered diamonds to the surface, and the types of rock
they have traveled through, have been mapped for five mines in Kimberley, South Africa.
The diatreme zone is where diamonds can be found. (Image adapted from M. Field et al., Ore
Geology Reviews 34:33, with permission of Elsevier.)
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American Scientist, Volume 100
release, but the magma’s travel upward
from a region that is carbon-rich to one
that is silica-laden (a property found in
cratons, which could help explain diamond’s association with those regions).
An increase in silica content in the magma causes a quick drop in its solubility of
carbon dioxide, causing continuous and
vigorous expulsion of the gas and driving the ascent of the kimberlitic magma.
Kimberlites are usually about 65 to
135 million years old, but some are as old
as 1.1 billion years. They are not as aged,
however, as the diamonds they end up
transporting. With few exceptions, diamond formed well before kimberlite
or lamproite eruption—on the order of
hundreds of millions to billions of years.
Although eruption ages of diamondiferous kimberlite are variable, those kimberlites younger than 1.6 billion years
comprise the majority of economically
significant bodies. Radiometric dating
indicates that kimberlite eruptions have
grown more frequent over time, with
some of the youngest dated ones belonging to the Eocene (56 to 34 million years
ago) kimberlite clusters of the Lac de
Gras area in Canada’s Northwest Territories. Diamond deposits older than
about 1.6 billion years are in the form of
paleoplacers (sediments created by gravity separation that have been compacted
into rock), lamprophyric dikes (sheets
of low-silica rock that cut through other
geological layers) and breccias (broken
rock fragments that have been cemented
© 2010 Sigma Xi, The Scientific Research Society. Reproduction
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into a solid), as well as rare diamondiferous but uneconomic kimberlite.
Besides its beauty, diamond’s exceptional physical properties, unusual
formation and value have prompted
abundant research. Studies on the origin of host structures and global distribution of the gemstone have facilitated
research and understanding of the
deep Earth and led to various methods
for laboratory synthesis of diamonds.
Ruby and Sapphire
Ruby and sapphire are gem varieties
of the mineral corundum, essentially
an oxide of aluminum that has the
general formula Al2O3. Ruby and sapphire are perhaps the world’s most
widely sold colored gemstones, accounting for approximately one-third
of sales by value. They can command
some of the highest prices paid for any
gem: In 2006 an 8.62-carat Burmese
ruby sold for $3,640,000, and in 2009
a 16.65-carat Kashmir sapphire was
purchased for $2,396,000.
Corundum crystallizes in the hexagonal system—the crystal’s three axes
on the horizontal plane intersect at
60-degree angles, and the fourth, vertical axis intersects at 90 degrees. Ruby is
red and sapphire is blue; all other colors
are referred to as sapphire with a modifier (such as “yellow sapphire”). Both
ruby and sapphire can exhibit asterism
or “stars” on the surface of round-cut
stones, called cabochons. These are
caused by light reflecting from needlelike inclusions of a titanium-oxide mineral called rutile, or other iron or irontitanium oxide phases, aligned along
crystallographic planes and parallel to
the hexagonal faces at 60 degrees.
Wikimedia
Figure 4. Uncut natural ruby crystals, about 2
centimeters long, from Winza, Tanzania.
The color of ruby is due to chromium replacing aluminum in the crystal
structure. Chromium is also responsible
for the green color of emerald, and the
reason for the difference in color between emeralds and rubies is still unresolved. In both gems, the chromium
is surrounded by six oxygen atoms, but
absorbs light differently in each crystal.
One theory for the color variation is that
it is caused by the electrostatic potential
imposed by the rest of the lattice ions on
the active electrons of the chromiumoxygen unit. The main effects are
thought to be from the electric field
generated in the neighborhood of the
chromium-ion site in ruby, which is
absent in emerald because of the symmetry of its lattice. This charge results
in the absorption features being shifted
to higher energies in ruby such that the
gem has two large bands of visible light
absorbed at wavelengths of approximately 400 and 550 nanometers, and
two transmission windows at 480 nanometers (blue) and 610 nanometers (red).
Ruby appears red because the human
eye is more sensitive to red above 610
nanometers than to blue. Red fluorescence under ultraviolet light and sometimes daylight, combined with the red
color of ruby, is the cause of the fire effect seen in many rubies from Myanmar
and Vietnam.
The blue color of sapphire results
from electron transfer between less than
0.01 percent of iron (Fe2+) and titanium
(Ti4+) ions replacing aluminum ions
(Al3+) in the crystal structure. This charge
transfer uses specific amounts of energy
from light at certain wavelengths; the
wavelength used is absorbed and not
seen. In sapphire, light from the red end
of the spectrum is used as energy for the
charge transfer between iron and titanium atoms, making the gem look blue.
Colorless “gueda” sapphire is commonly heated to achieve greater transparency and blue colors by melting inclusions
to release iron and titanium.
Orange-pink gem corundum is called
padparadscha, from the Sanskrit term
for the color of the lotus flower. Aluminum in the crystal structure is replaced
by a combination of chromium ions
(Cr3+), which create a pink hue, and
iron ions (Fe3+), which undergo a charge
transfer with oxygen ions (O2-) to produce a yellow hue.
Most gem corundum is produced
from placer deposits that are classified as alluvial (water transport), colluvial (gravity transport) and eluvial
(weathering). Gem corundum is also
produced from paleoplacers.
The global distribution of corundum
deposits is linked to collision, rift and
Figure 5. Sapphires were discovered by an Inuit hunter on Baffin Island in Canada in 2002 (left). A microphotograph taken with cross-polarized
light shows a polished thin section (30 microns thick) of sapphire-bearing rock from the same locale (middle). The gold in the center is pyroxene;
it is rimmed by a mixture of green mica and gray feldspar, which is in turn circled by purple scapolite and gray nepheline. The sapphire is usually found in the nepheline (although there isn’t any in this image). A method used to find sapphire on Baffin Island is to use ultraviolet light to
excite fluorescence from scapolite, as this mineral often occurs with sapphire. However, at this location in the summer, it is only dark enough to
use this method for two to four hours a night. (Unless otherwise noted, all photos are courtesy of the author.)
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2012
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subduction geodynamics. Three main
periods of corundum formation are
recognized: the Pan-African orogeny
(750 to 450 million years ago), which
produced primary gem corundum
deposits in Africa, India, Madagascar
and Sri Lanka; the Himalayan orogeny
(45 to 5 million years ago), which produced the marble-hosted ruby deposits in Asia; and Cenozoic alkali-basalt
extrusions (65 to 1 million years ago).
The traditional sources are Kashmir,
Myanmar, Sri Lanka and Thailand.
Newer major producers include Australia, Madagascar and Vietnam.
The finest rubies and sapphires come
from thick marble layers composed of
calcite (CaCO3). But how did the aluminum needed to form corundum get
into the marble? How about the chromium, titanium and iron needed to
impart color? Why is there no silica,
which is three times more common in
the Earth’s crust? If silica were present,
it would bind with aluminum and prevent the formation of corundum. My
colleagues and I are currently studying
a ruby and pink-sapphire deposit in
central British Columbia to investigate
these questions. Preliminary results
suggest that the material precursor to
the marble was limestone deposited
in thin interlayers of mudstone. When
the limestone metamorphosed to form
marble, the minerals in the mudstone
(primarily mica, containing silica and
many other trace minerals) underwent
a complicated series of reactions to ultimately form corundum.
Emerald
Emerald is the green gem variety of the
beryllium-based mineral beryl with general formula Be3Al2Si6O18. Like corundum, beryl crystallizes in the hexagonal
system. The color of emerald is due to
trace amounts of chromium, vanadium
or both elements replacing aluminum in
the crystal structure. In the beryl crystal,
rings of silicon and oxygen are stacked,
leaving channels in the center that can
trap water or other impurities.
Emerald is one of the most valuable
gemstones. The highest price ever paid
for an emerald was $1,149,850 for a
10.11-carat Colombian stone in 2000. The
pricing of emeralds is unique in the colored gemstone market because a greater
importance is placed on color than on
clarity, brilliance or other characteristics.
Colombia is thought to supply an
estimated 60 percent of the world’s
emeralds. Official production in 2001
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American Scientist, Volume 100
Figure 6. Gemstones are often found in remote, desolate locations. An emerald deposit in
this area of Canada’s Northwest Territories is located in the white outcrop. The foreground
is granite, the rust-colored rock is Rabbitkettle limestone, and the black is Earn Group shale.
was 5.5 million carats, worth more
than $500 million. Zambia is considered to be the world’s second most important source of emeralds by value.
Beryl may contain significant amounts
of water at the channel sites. When heated above 400 degrees, the trapped water
breaks into gaseous molecules that are
confined to the channel voids. The channel water is liberated at temperatures
of about 800 degrees, without much
effect on the natural ratio of hydrogen
isotopes. Thus, channel water may represent the original fluid composition from
the time of formation, and the measurement of the change in ratio of hydrogen
isotopes in water released from beryl
may permit determination of the source
of the fluids from which the beryl grew.
Figure 7. An emerald found by the author
and his colleagues in Ontario, Canada.
Isotopic compositions of historical
emerald artifacts have been used to
show that during historical times, artisans worked with emeralds originating
from deposits that were supposedly
discovered only in the 20th century.
Research by Gaston Giuliani and his
colleagues at the Center for Petrographical and Geochemical Research
in France found that most of the highquality emeralds cut in the 18th century in India were transported there from
Colombia, whereas previous studies
had assumed a closer origin.
Beryl and chrysoberyl (described in
the following section) are relatively rare
because there is very little beryllium in
the upper continental crust. Beryllium
tends to be concentrated in rocks such
as granites, pegmatites (formed from
water-rich magmas) and black shales
(a sedimentary rock), as well as rocks
that these materials can metamorphose
into. Chromium and vanadium are
more common in the upper continental
crust and are concentrated in igneous
rocks such as peridotites, dunites (a
type of peridotite) and basalts (quickly
cooled magma) of the oceanic crust and
upper mantle and their metamorphic
equivalents. However, high concentrations of chromium and vanadium can
also occur in sedimentary rocks, particularly black shales.
Thus, unusual geologic and geochemical conditions are required for
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crust
pegmatite
magma
beryllium
crust
chromium-rich
hydrothermal fluid
granite
Figure 8. Pegmatites are a major source of emeralds (although they are not the only one). Magma
rises and cools, and the last portion of the molten rock concentrates water and minerals such as
beryllium. This enriched magma flows into a crack caused by the heating of surrounding rock, expanding the crack and forming a pegmatite. In areas conducive to emerald formation, hydrothermal fluid rich in chromium (blue drops) percolates through the rock and mixes with the pegmatite
lava. As it cools, the pegmatite forms small voids where emeralds can crystallize.
tiple growth stages, such as augen (eyeshaped) textures in country rocks and
emeralds with curved inclusion trails.
Critics of this interpretation say the classic pegmatite model could still apply. Although pegmatitic sources of beryllium
may not be apparent, fluids can travel
far from pegmatites, especially along intensely sheared rocks, and pegmatites do
occur in the region.
Recently, boron isotopes of tourmaline coexisting with emeralds were used
to study the origin of the Habachtal deposit. The boron isotope values found
suggest that two separate fluids were
channeled and partially mixed in a
shear zone during tourmaline-emerald
mineralization, but neither of these
fluids is associated with pegmatites. A
regional-metamorphic fluid carried isotopically light boron, as observed in the
country rocks. The other fluid derived
from interlayered serpentinites (mafic
rocks oxidized by water) and has isotopically heavier boron that is typical
for mid-ocean ridge basalts or altered
oceanic crust.
The same method was applied to
the Tsa da Glisza emerald occurrence
in Yukon, Canada. Here, the principal
occurrence of emerald is along contacts between quartz-tourmaline veins
and mafic country rocks. In this case,
the isotope values were reported to be
consistent with a dominantly granitic
source of boron, with a contribution of
isotopically heavy boron from maficultramafic formations.
Other Beryls
Other members of the beryl family that
have been used as gems include aquamarine and maxixe (blue), golden beryl
(yellow), heliodor (greenish yellow),
goshenite (colorless), morganite (pink)
and red beryl. All except red beryl are
ordinarily found in pegmatites and certain metamorphic rocks.
Our group has suggested that the
color of blue beryl can be attributed to
charge transfer between iron ions in
different oxidation states. Iron cations
(Fe2+) at the aluminum sites exchange
electrons with small amounts (about
0.04 atom per formula unit for darkblue material) of Fe3+ cations. Aqua-
Chris Ralph/Wikimedia
beryllium (found mostly in rocks that
form within continents) to meet with
chromium or vanadium (concentrated
in volcanic rocks associated with ocean
ridges). In the classic model, berylliumbearing pegmatites, in their magma
state, interact with chromium-bearing
ultramafic or mafic rocks (those high
in magnesium and iron). However,
researchers are recognizing that other
geological events involving tectonics
may play a significant role in certain
emerald deposits.
A case that does not fit the classic model is the Colombian deposits,
where there is no evidence of magmatic
activity. The more than 200 emerald deposits and occurrences in Colombia are
located in two narrow bands on both
sides of the Cordillera Oriental mountain range. The Colombian emeralds
formed as a result of hydrothermal
growth associated with tectonic activity.
A number of studies have pointed to an
evaporitic origin for the parent hydrothermal fluids. The fluids are thought
to have formed from water interacting
with salt beds in black shales, which
were buried to depths of at least 7 kilometers and reached temperatures of at
least 250 degrees. The highly alkaline
briny fluids migrated upward through
the sedimentary layers along décollement thrust planes (gliding planes between two rock masses that allow independent types of deformation in the
rocks above and below the fault) and
then interacted with the black shales.
It is generally agreed that organic
matter, and sulfur derived from it, is
important in the formation of the Colombian emerald deposits, but debate
exists over the exact nature of the associated reaction.
A similar origin has been described
for very rare emerald mineralization in
the Neoproterozoic (1 billion to 542 million years ago) Red Pine Shale of the
southwestern Uinta Mountains in Utah:
Sulfate-bearing brines from the Uinta
Basin migrated upward along the South
Flank Fault Zone and interacted with
the carbon-rich shale. The sulfate was
reduced by organic carbon to form sulfides and emerald.
There has been considerable debate
over the role that tectonic processes play
in the formation of some emerald deposits. For example, a regional-metamorphic
origin has been suggested for emeralds
at Habachtal in Austria. This conclusion
is supported by the physical evidence
of metamorphism, shearing and mul-
Figure 9. Three types of beryl—morganite,
aquamarine and heliodor—have the same base
mineral but are colored by different impurities
in their crystal matrices. Aquamarine (middle)
and heliodor (right) are both colored by iron
ions, but in a different oxidation state. Morganite (left) is colored by manganese ions, which
in a different oxidation state create red beryl.
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2012
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133
leagues at Vilnius University in Lithuania proposed the new gem beryl variety
“chromaquamarine” with composition
close to emerald but with considerably
more iron (0.48 to 1.11 percent) than
chromium (0.08 to 0.15 percent) or vanadium (less than 0.02 percent).
Figure 10. The crystal structure of beryl shows SiO4 groups (blue tetrahedra) where a silicon atom
is surrounded by four oxygen atoms at the corners of a tetrahedron. The groups link together into
rings, forming channels that trap water molecules at their centers (red is oxygen and white is hydrogen). The green distorted tetrahedra are BeO4, and the dark green octahedra are AlO6 groups.
marine is produced from mines in
Brazil, Colombia, Kenya, Madagascar,
Malawi, Russia, Tanzania and Zambia,
and from placers in Sri Lanka.
Maxixe is a dark-blue gem beryl from
Brazil in which the color results from
the inclusion of nitrate (NO3) and carbonate (CO3-) at the centers of the crystal ring structure. The color fades with
prolonged exposure to light as a result
of hydrogen atoms decaying in the crystals. A recent study has suggested that
the nitrate is created by a natural process, whereas the carbonate is due to
irradiation from surrounding rock.
The color of golden beryl and heliodor is attributed to iron cations (Fe3+).
The pink color of morganite (named for
financier J. P. Morgan) is attributed to
manganese (Mn2+) ions. Morganite occurs in Afghanistan, Madagascar and at
Pala in California.
The gem value of goshenite is relatively low; to increase value it can be
colored by irradiation with high-energy
particles or by electrolysis (splitting materials with an electric current), with the
resulting color depending on the impurities. In the past, goshenite was used
for manufacturing eyeglasses and lenses
owing to its transparency.
The color of red beryl is due to manganese (Mn3+) ions. Red beryl has only
been reported from topaz-bearing rhyolites (a type of volcanic rock high in
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silicon and low in iron and magnesium)
in Mexico, New Mexico and Utah. The
beryllium necessary to form the red
beryl is thought to be derived from the
host rhyolite. Mobilization of the beryllium, in the form of beryllium-fluorine
complexes, was promoted by the very
low calcium contents of the host rock,
which inhibited the formation of fluorite
(CaF2). This movement took place when
fluorine-rich gases from the cooling
rhyolite mixed with vapors from heated
ground water to produce a supercritical fluid—its temperature and pressure
were above a threshold point where it
can seep through solids like a gas but
still dissolve materials like a liquid. The
absence of clay mineral inclusions in the
red beryl suggests that it formed at temperatures below those required for the
crystallization of rhyolite magma (less
than 650 degrees) but above those of
clay alteration (200 to 300 degrees).
The only known commercial occurrence of gem-quality red beryl is the
Ruby Violet (or Red Beryl) mine in the
Wah Wah Mountains of Beaver County,
Utah. Total production for the 25 years
prior to 2003 was more than 60,000 carats, of which about 10 percent was facetable. Prices can be as high as $10,000
per carat for faceted stones.
Based on a study of material from
Poona, Australia, and Kifubu, Zambia,
in 2006 Arūnas Kleišmantas and his col-
Chrysoberyl
Another beryllium-based gem, distinct
from the beryls, is chrysoberyl, which
has the general formula BeAl2O4. It is
generally a golden-yellow, to a greenishor brownish-yellow color. The color is
due to iron cations (Fe3+) substituting
for aluminum in the crystal structure.
The bulk of gem chrysoberyl available
in recent years has come from alluvial
deposits in the Bahia, Espírito Santo and
Minas Gerais states of Brazil.
The two most valued varieties of
chrysoberyl are alexandrite and cymophane. Alexandrite ranges in color from
greenish blue in natural light to deep red
under incandescent light. The effect is
from small amounts of chromium substituting for aluminum in the crystal
structure. Cymophane is translucent
chatoyant or “cat’s eye” chrysoberyl. In
this variety, needlelike inclusions of rutile
produce an effect visible as a bright band
of light that moves across the stone as it
is rotated. This effect is best seen in gemstones cut in cabochon form. Cat’s eye
material is found as a small percentage
of the overall chrysoberyl production.
There has been much debate about
the origin of chrysoberyl deposits. Most
are associated in some way with pegmatites, but in many cases the stone is
associated with aluminum-rich minerals
absent in most pegmatites. Some studies
have concluded that under conditions of
high temperature and pressure, the assemblage of beryl and aluminum-silicate
is unstable and decomposes to the assemblage of chrysoberyl and quartz.
Chrysoberyl was discovered in association with emerald and phenakite
(another beryllium-silica mineral) at
Franqueira in northwestern Spain in
1968–1969. The chrysoberyl is often cyc­
lic twinned (nonparallel crystals share
some of their crystal-lattice points), and
many crystals show the alexandrite
effect of color shifting under different
types of light. The beryllium minerals
occur in zones rich in phlogopite (a silicate mineral) in a dunite (an ultramafic
rock) intruded by a pegmatite associated with a type of granite that is high
in aluminum oxide and potassium. The
chrysoberyl could have formed by the
© 2010 Sigma Xi, The Scientific Research Society. Reproduction
with permission only. Contact [email protected].
David Weinberg/Wikimedia
Figure 11. Alexandrite, a chrysoberyl, shows an effect where it is green under natural light
(which contains a lot of blue wavelengths) and red under incandescent light (which contains
a lot of red wavelengths). Small amounts of chromium in the crystal structure cause the effect.
buildup of thin layers on existing olivine crystals (a magnesium-iron silicate
mineral), because the two minerals share
features in their crystal structures.
Study of additional samples resulted in the conclusion that chrysoberyl
formed from the breakdown of another
mineral called sapphirine (a silicate of
magnesium and aluminum with a high
beryllium content, named for its color
being similar to sapphire) during metamorphism after it was formed.
Textural and compositional evidence
suggests that chrysoberyl formed during
the regional metamorphism of granulite
facies (medium or coarse-grained metamorphic rock bodies that underwent
intense pressure and temperature changes), a process that could have released beryllium from the host sedimentary rock.
Tanzanite and Tsavorite
Tanzanite is the dark-blue gem variety
of the mineral zoisite, with the general
formula Ca2Al3(SiO4)(Si2O7)O(OH). The
blue color is due to vanadium, which
substitutes for aluminum in the crystal
structure. Most uncut tanzanite is grayish brown, grayish purple, brownish
purple, bluish and greenish brown, and
must be heat treated to alter impurities
and remove undesirable hues.
Tanzanite was discovered in 1967
near Merelani in northeastern Tanzania, which remains its only known
source. The deposits are located in
the western flank of a series of folded
and metamorphically deformed rocks
called the Lelatema Fold Belt. The tanzanite crystals are commonly found either in cavities at the hinges of folds in
quartz veins that have been stretched
by geologic processes after their formation, or bedded in hydrothermally
altered gneisses (a metamorphic rock
with visible layering), marbles and
calcium-silicates. Mineral composiwww.americanscientist.org
tional and textural data suggest that
chromium and vanadium were leached
from black shale undergoing prograde
metamorphism (where increasing pressure and temperature drives off volatile chemicals), and were concentrated
during a retrograde hydrothermal
metamorphic episode (when the rock
was cooled and could reincorporate
volatiles) to form tanzanite and other
minerals. The tanzanite-forming fluids
are estimated to have circulated along
fold hinges at temperatures between
390 and 450 degrees and at a pressure
of about 3 kilobars. Fission track dating
(a technique that examines the damage
trails left by naturally decaying materials) of tanzanite suggests a crystallization age of 585 million years ago.
The famous jewelry retailer Tiffany
& Co. introduced the name of the gem
in 1969. The worldwide market for
rough tanzanite is estimated to be worth
around $100 million per year. For extremely fine stones of less than 50 carats,
prices can attain $1,000 per carat.
Tiffany & Co. also named the gem
tsavorite, after Tsavo National Park in
Kenya. Tsavorite is a green gem variety of a calcium-aluminum type of garnet called grossular (named after the
gooseberry, Ribes grossularia, of simi-
lar color) that has the general formula
Ca3Al2(SiO4)3. The green color of tsavorite is due to vanadium substituting for
aluminum in the crystal structure. Faceted tsavorite gems of three carats or
more are very rare.
The green-gem garnet that became
known as tsavorite was discovered in
1967 in northeastern Tanzania. Most
of the world’s tsavorite mines and significant deposits are in eastern Tanzania and southeast Kenya, near the
Tanzanian border. All of these deposits
are hosted in vanadium-rich graphitic
gneisses associated with marbles in
formations that are metasedimentary
(sedimentary rocks that show significant signs of metamorphism).
All of the Merelani, Lemshuku and
Namalulu mining districts in Tanzania are on the western flank of the
Lelatema Fold Belt. The best tsavorite
crystals occur in the Merelani district,
where they are found with tanzanite
and chromium-bearing tourmaline in
hydrothermally altered graphite gneiss.
As with tanzanite, the gem crystals are
concentrated in quartz veins, fold hinges and stretched structures.
At the Komolo and Lemshuku
mines in the Lemshuku mining district,
tsavorite occurs in quartz veinlets and
gashes that cut vanadium-bearing graphitic gneiss, and in potato-like nodules. The nodules generally produce
crystals with visible fractures. A deposit
formed via erosion and redeposition of
the crystals is located downslope from
the primary occurrences.
It has been suggested that tsavorite forms under pressures in excess of
5 kilobars and temperatures above 750
degrees. However, the tanzanite in the
Merelani district has been shown to
have formed at lower temperatures and
pressures, and presumably much later,
from hydrothermal fluids rather than
from regional metamorphism.
Wikimedia
Parent Géry/Wikimedia
Figure 12. Tanzanite, both uncut and faceted, is blue (left) and tsavorite (right) is green.
© 2010 Sigma Xi, The Scientific Research Society. Reproduction
with permission only. Contact [email protected].
2012
March–April
135
Wikimedia
Figure 13. Most topaz is naturally colorless,
but it can often have pink or brown hues.
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American Scientist, Volume 100
Neither greisens nor veins are important sources of gem topaz, although one
exception may be Ouro Preto in Minas
Gerais, Brazil. Much gem topaz is produced from this region, especially the
chromium-rich red “imperial” variety.
A number of hypotheses have been proposed for the origin of these deposits,
ranging from pegmatitic to a hydrothermal vein. The latter is supported by high
concentrations of hydroxyl in the topaz.
Two Kinds of Jade
The term “jade” refers to two rock types:
jadeitite rock consisting almost entirely
of jadeite (NaAlSi2O6), and nephrite, a
variety of tremolite-actinolite mineral
[Ca2(Mg,Fe)5Si8O22(OH)2] with extremely tiny crystals—so small that they are
vague even when viewed microscopically. Jadeitite is harder than nephrite, but
the fracture toughness and surface energy of nephrite is approximately twice
that of jadeitite. Jadeitite is less common
and more valuable than nephrite and is
used more in jewelry than in sculptures.
Pure jadeite is white. Green and blue
colors are attributed to iron substituting
for aluminum in the jadeite crystal structure. The “imperial” emerald-green color is due to chromium replacing as little
as 2 to 3 percent of the aluminum sites. A
mauve color is attributed to manganese
when iron contents are very low. Nephrite rarely displays an intense emeraldgreen color from chromium substitution.
There are only about 14 documented
jadeitite occurrences. The most important jadeitite district is the “Jade Tract”
in Kachin State, northern Myanmar,
where the jadeitite occurs as intrusions
or fragments in serpentine-dominated
conglomerations of rock, and in alluvial
deposits. Another important source of
jadeitite is the middle Motagua Valley
in Guatemala, where two belts associated with serpentinite oppose each other
across the boundary between the North
American and Caribbean plates.
The evidence suggests that all jadeite
(and many nephrite) deposits form at
the edges when fluids interact with serpentinizing peridotites and surrounding
rocks, from a depth of 50 kilometers up
to the near surface.
Most nephrite is produced by contact or infiltration metasomatism in two
different ways: the replacement of serpentinite by calcium at contacts with
more silicic rock, and the replacement
of dolomite by silicic fluids associated
with granitic magmas. The serpentinereplacement deposits are larger and
John Hill/Wikimedia
Topaz
Topaz has the general formula
Al2SiO4(F,OH)2. Most topaz is colorless, but it can also be pale yellow,
pink, orange, brown, blue, green or
gray. The pink and reddish hues result from chromium, manganese and
iron substitution for aluminum. The
brownish and blue colors are primarily due to a variety of defects called
color centers within the stones and are
greatly enhanced by heat treatment. Irradiated sky-blue topaz began appearing on the market in the mid-1980s; the
color is thought to be due to hydroxyl
(OH) substituting for oxygen anions at
fluorine sites prior to irradiation.
More than 80 world deposits of wellcrystallized topaz are known. Pegmatites, especially those in Minas Gerais,
Brazil, produce the bulk of gem topaz.
Such pegmatites are typically shallow
and enriched in rare-earth elements.
Topaz also occurs as a primary mineral within rhyolite flows. A fluorineenriched belt of Cenozoic rhyolite units
rich in topaz occurs in the western
United States and Mexico. The topaz
occurs within lithophysae (small gaps
in volcanic rocks) and more rarely in
fractures or within the rhyolite. Textures indicate that the topaz formed
over a range of time from early in the
extrusive events to later in the process, and at temperatures ranging from
650 to 850 degrees, with most crystals
forming at the lower end of this range.
However, colorless topaz from Cerro
El Gato in the San Luis Potosí area of
Mexico has been reported to crystallize at temperatures above 500 degrees from fluids enriched in elements
leached from the lava, whereas ambercolored topaz crystallized below 500
degrees from fluids richer in volatile
elements, including arsenic.
Figure 14. A piece of carved Mayan jade is
the rock jadeitite. Jade can also be the mineral nephrite.
more abundant than the dolomitereplacement types. The mineral assemblages for nephrite associated with
serpentinite suggest that they underwent metamorphism and metasomatism at temperatures from about 300 to
350 degrees down to perhaps 200 to 100
degrees. The most important serpentine-replacement deposits are found at
mines in northern British Columbia and
in the East Sayan Mountains in Siberia.
Dolomite-replacement mines occur in
the Kunlan Mountains of China and the
Cowell province in southern Australia.
High-Tech Gem Hunting
Many gems are still mined in remote
places by individuals, but the gem
industry has become modern and
systematic in its methods of discovering new deposits. Since the 1980s,
when images from the Landsat Earthobserving satellite were declassified
and made cheaply available, high-tech
prospectors have been able to use its
visible-spectrum photos, as well as
spectroscopic images of areas without vegetation to directly map minerals. Newer satellites have greatly improved image resolution.
In Canada, satellite and aerial images
have been used to search for potential
sites of diamond-containing kimberlites, as these volcanic formations have
a different chemical and magnetic reading than that of the surrounding rock.
Searching for magnetic signatures helps
particularly in the discovery of kimberlites that are covered over or underwater. In 2002, 300 million acres of possible
kimberlite area in Canada were identified. This region was narrowed down
to 8 million acres by direct sampling.
© 2010 Sigma Xi, The Scientific Research Society. Reproduction
with permission only. Contact [email protected].
mid-Cretaceous
Anvil-Cassar suite
plutonic rocks
Dawson
City
ultramafic rocks
volcanic rocks
Mayo
Earn group
Faro
Carmacks
60
kilometers
0
60
120
Ross River
Bibliography
Haines
Junction Whitehorse
Watson Lake
Figure 15. A map of the Yukon territory in Canada shows regions high in beryllium (found
in mid-Cretaceous Anvil-Cassar suite rocks and plutonic rocks) and others high in chromium
and vanadium (found in ultramafic rocks, volcanic rocks and Earn group rocks). Regions
where all these deposits come together are potentially good sites to search for emeralds. (Image courtesy of Don Murphy, Yukon Geological Survey.)
Exploration companies have now identified 22 kimberlites that are expected to
be high in diamond content.
Other physical and chemical properties of rocks can be used for exploration. Cédric Simonet and his colleagues
at Akili Mineral Services in Kenya have
found that rocks associated with ruby deposits in that country have a lower electrical resistance and radioactivity when
compared to the surrounding host rocks.
Another approach is geochemistry,
looking for telltale mineral compositions that are the signature of certain
gem deposits. In Colombia, host rocks
that are high in sodium but depleted in
lithium, potassium, beryllium and molybdenum have been found to be good
indicators of emerald deposits. Surveys
have tested the streams and sediments
that drain from known emerald deposits
and found them to be good indicators of
the composition of the gem-laden rock,
demonstrating that streambeds could
be tested to find nearby emerald deposits. Anomalous sodium content in sediments has been successfully used to find
several new emerald occurrences.
Berylometers are helpful instruments
for emerald and beryl searches. These
machines use antimony-sourced gamma radiation to excite a response from
beryllium atoms, so its content can be
mapped in host rocks. There are two
models currently available; one weighs
only about five pounds but takes several minutes to read results and has a
relatively high detection limit. The other
weighs 38 pounds, but is much faster
and more sensitive. Their respective
www.americanscientist.org
As technology improves, it may become easier to locate regions where gemstones could be found. Will this make
them less rare, and perhaps less valuable? It’s hard to say what the market
outcome may be, but an increase in discovery will only make gems more precious to geologists, who value above all
the information the crystals can impart
about the inner workings of the Earth.
drawbacks currently limit the application of these machines to field studies.
The vast expanses of black shale in
northwestern Canada would seem to
have potential for Colombian-style emerald mineralization. Our group is currently studying how to go about exploring
for such deposits, starting with geochemical analyses that look for regions high in
sodium and low in beryllium and potassium. We are also looking at structural
geology—regions that have tear faults
and associated thrusting upward of rock
that is linked to décollement planes.
Other studies have shown that for
pegmatite sources of emerald, the pegmatites must be fractionated (their constituent elements separated based on
their different solubilities) to become enriched enough to reach beryllium saturation. Ratios of potassium to rubidium
and rubidium to strontium are some of
the geochemical signatures that can be
used to find such pegmatites. Satellite
imagery may help to identify plutons
that have been buried and overlooked.
However, there must be some caution used in applying technology to the
search for gems. Our group has found
that some commercial analyses of samples that are looking for beryllium and
chromium may not adequately dissolve
the mineral phases containing these elements, leading to inaccurate readings. In
addition, we have found that the use of
mass spectrometry to look for beryllium,
which is a very light element, in an analytical program that includes numerous
heavy elements may decrease the sensitivity of the beryllium analyses.
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Forrest, M. 2006. Tanzanite comes of age. Materials World 14:28–30.
García-Lastra, J. M., M. T. Barriuso, J. A. Aramburu and M. Moreno. 2005. Origin of the
different color of ruby and emerald. Physical Review B 72:113–104.
Giuliani, G., et al. 2000. Oxygen isotopes and
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Groat, L. A. (ed). 2007. The Geology of Gem Deposits. Mineralogical Association of Canada,
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Groat, L. A., G. Giuliani, D. D. Marshall and
D. J. Turner. 2008. Emerald deposits and
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dark blue aquamarine from the True Blue
showing, Yukon Territory, Canada. Canadian Mineralogist 48:597–613.
Kievlenko, E. 2003. Geology of Gems. Moscow:
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Kleišmantas, A. 2006. Identification of beryl
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Zaitsev, A. M. 2001. Optical Properties of Diamond. Berlin: Springer.
© 2010 Sigma Xi, The Scientific Research Society. Reproduction
with permission only. Contact [email protected].
For relevant Web links, consult this
­issue of American Scientist Online:
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