Encyclopedia of Global Resources part 54 potx

Although argon has been found in certain igneous rocks with helium, and all the gases have been found in water from hot springs, the atmosphere is still the major source of the noble gas

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entry and therefore is not covered here Radon gas,

which is ubiquitous, is an end product of uranium

de-cay that is radioactive and emanates from soil, rocks,

and hot springs in areas where uranium and thorium

are found

Primary Uses

The primary uses of these gases are in arc welding,

neon lights, fluorescent lights, and lasers They are

also used as Geiger counters and inert atmospheres

Technical Definition

The inert or noble gases are Group VIIIA of the

peri-odic table of the elements They are colorless,

taste-less, and odorless monoatomic gases

Description, Distribution, and Forms

Neon (abbreviated Ne), atomic number 10, has three

naturally occurring stable isotopes: neon 20 (90.51

percent), neon 21 (0.27 percent), and neon 22 (9.22

percent) The atomic weight is 20.183, with a boiling

point of−246° Celsius and a melting point of −249°

Celsius Argon (Ar), atomic number 18, has three

nat-urally occurring stable isotopes: argon 40 (99.600

per-cent), argon 38 (0.0632 perper-cent), and argon 36

(0.3364 percent) The atomic weight is 39.944, with

a boiling point of−186° Celsius and a melting point of

−189° Celsius

Krypton (Kr), atomic number 36, has six naturally

occurring stable isotopes (78, 80, 82, 83, 84, and 86),

of which 84 is the most abundant (57.0 percent) The

atomic weight is 83.80, with a boiling point of−157°

Celsius and a melting point of−153° Celsius One

iso-tope that has been studied, krypton 85, is mainly

gen-erated in uranium reprocessing plants but also in

nu-clear reactors and as a product of spontaneous fission

The study concluded that the concentration could

grow to the point that krypton 85 could produce as

much radiation exposure for humans as is the natural

background radiation The outcome of this could be

an increase in skin cancer

Xenon (Xe), atomic number 54, has nine naturally

occurring stable isotopes The atomic weight is

131.30, with a boiling point at −112° Celsius and a

melting point of−107° Celsius

Although argon has been found in certain igneous

rocks with helium, and all the gases have been found

in water from hot springs, the atmosphere is still the

major source of the noble gases Dry air contains

0.937 percent (9,370 parts per million) argon, 18

parts per million neon, 1.1 part per million krypton, and 0.086 part per million xenon The higher concen-tration of argon is thought to be because radioactive potassium 40 decays to argon The group has been called rare gases or inert gases, but since the atmo-sphere is almost 1 percent argon, and because kryp-ton and xenon are not totally inert, the name “noble gases” has gained favor The noble gases are always found as inert, monoatomic gases Although com-pounds of xenon and krypton have been formed, they can be formed only under extreme conditions;

no compounds occur naturally

Radon (Rn), atomic number 86, is a decay product

of radium and occurs in nature as a very dense, odor-less, colorodor-less, and highly radioactive gas Its radioac-tivity, ubiquity, and tendency to accumulate in homes makes it a health hazard and a major contributor to lung cancer

History

In 1785, Henry Cavendish found that a very small portion (less than 1/120) of the air could not be re-acted in the experiments that rere-acted oxygen and ni-trogen This clue was not followed, however, and it was 1882 before a noble gas was discovered by Lord Rayleigh and Sir William Ramsay In experiments to measure the density of gases, Rayleigh found that the density of nitrogen from ammonia and that from air with the oxygen removed were not the same Ramsay then studied atmospheric nitrogen By reacting the nitrogen with red-hot magnesium, he isolated a small amount of much denser gas When its spectrum was examined there were lines that did not match any known element This new element was named argon, from the Greek word for idle or lazy, because of its in-ert nature

Ramsay suspected that another element might ex-ist between argon and helium (which had been dis-covered in the Sun in 1868), as their atomic weights of

40 and 4 were so different In May of 1898, Ramsay and Morris William Travers allowed liquid air to boil away gradually until only a small amount was left They removed the nitrogen and oxygen with red-hot copper and magnesium When they examined the spectrum, there were new lines This new element was named krypton, from the Greek word for hidden Krypton was a new element of the group, but it was not the one for which they had searched In June,

1898, Ramsay and Travers liquefied and solidified an argon sample Instead of keeping the last gas to boil

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away (which had led them to krypton), they kept the

first fraction When they examined the spectrum it

produced, they found a blaze of crimson light unlike

that of any other element The new element was

named neon, from the Greek word for new

Ramsay and Travers continued their search for

ele-ments using a new liquid-air machine supplied by

Ludwig Mond By repeated fractionation of krypton,

a still heavier gas was extracted in July, 1898 The

spec-trum identified it as a new element, which they called

xenon, from the Greek for stranger It has been known

for some time that clathrates, organic hydroxy

com-pounds with large cavities, would contain (but not

bond to) the larger noble gases (argon, krypton, and

xenon), but it was not until 1962 that compounds of

the noble gases were first made by Neil Bartlett Most

of the compounds are xenon, but a few are krypton

with fluorine or oxygen No compounds of neon or

argon have been prepared

Obtaining the Noble Gases

The noble gases are obtained as a by-product of the

liquefaction and separation of air Dr y

carbon-dioxide-free air is liquefied and distilled The volatile

fraction contains nitrogen, neon, and helium The

remaining liquid of oxygen, argon, krypton, and

xe-non is fractionated to yield argon contaminated with

oxygen The oxygen is removed by reaction with hot

copper-copper oxide Further separation of the gases

is achieved by selective adsorption and desorption

with charcoal Some argon is obtained as a by-product

in the production of ammonia (NH3) The argon is

an impurity in the nitrogen and hydrogen gases

About 635,000 metric tons of argon are obtained

an-nually Smaller amounts of the other gases are

col-lected

Uses of Noble Gases

The main use of argon is as an inert atmosphere for

high-temperature metallurgical work It is also used to

fill incandescent lamps The inert atmosphere allows

the filament to burn for a long period of time before it

burns out Argon is also used in lasers and Geiger

counters (radiation counters) The naturally

occur-ring presence of argon isotopes is used to date

geolog-ical formations There are two methods that use the

amount of argon isotopes to date materials in the

mil-lions of years range One method uses the argon 40 to

argon 39 ratio; the other uses the argon 40 to

potas-sium 40 ratio

All the noble gases are used in discharge tubes (neon lights) Each gas produces a particular color— for example, red by neon and blue by xenon Other colors can be produced by a combination of gases The neon-light industry was started by Georges Claude in the early 1900’s and grew into a large indus-try Fluorescent tubes are also filled with the noble gases, but the color of the tube depends on the phos-phor coat on the inside of the tube The denser noble gases, especially argon, have been used to fill the space between layers of glass in thermal insulating windows Neon is also used in fog lights, television tubes, lasers, and voltage detectors Krypton is used in flashbulbs and ultraviolet lasers The wavelength of one isotope of krypton is the standard for the metric system Xenon is also used in ultraviolet lamps, sun-lamps, paint testers, projection sun-lamps, and electronic flashes Radon has been used in radiation therapy to treat cancers but for the most part has been super-seded by radionuclides It also has some uses in re-search

C Alton Hassell

Further Reading Greenwood, N N., and A Earnshaw “The Noble Gases: Helium, Neon, Argon, Krypton, Xenon,

and Radon.” In Chemistry of the Elements 2d ed

Bos-ton: Butterworth-Heinemann, 1997

Henderson, William “The Group 18 (Noble Gas) Ele-ments: Helium, Neon, Argon, Krypton, Xenon,

and Radon.” In Main Group Chemistry Cambridge,

England: Royal Society of Chemistry, 2000

Isrặl, H., and G W Isrặl Trace Elements in the Atmo-sphere Ann Arbor, Mich.: Ann Arbor Science, 1974 Krebs, Robert E The History and Use of Our Earth’s Chemical Elements: A Reference Guide Illustrations by

Rae Déjur 2d ed Westport, Conn.: Greenwood Press, 2006

Ojima, Minoru, and Frank A Podosek Noble Gas Geo-chemistry 2d ed New York: Cambridge University

Press, 2002

Porcelli, Donald, Chris J Ballentine, and Rainer

Wieler, eds Noble Gases in Geochemistry and Cosmo-chemistry Columbus, Ohio: Geochemical Society,

2002

Stern, Rudi The New Let There Be Neon Enlarged and

updated ed Cincinnati, Ohio: ST, 1996

Weeks, Mary Elvira Discovery of the Elements 7th ed.

New material added by Henry M Leicester Easton, Pa.: Journal of Chemical Education, 1968

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Web Sites

Universal Industrial Gases, Inc

Argon (Ar) Properties, Uses, Applications: Argon

Gas and Liquid Argon

http://www.uigi.com/argon.html

Universal Industrial Gases, Inc

Properties, Applications and Uses of the “Rare

Gases”: Neon, Krypton, and Xenon

http://www.uigi.com/rare_gases.html

See also: American Gas Association; Atmosphere;

Haber-Bosch process; Helium; Hydrogen; Nitrogen

and ammonia; Oxygen

Gasoline and other petroleum fuels

Categories: Energy resources; products from

resources

Gasoline is the most important product from petroleum

and is the dominant transportation fuel in the world.

Other petroleum products with important fuel uses

in-clude kerosene (usually refined to jet fuel), diesel oil for

railway locomotives and trucks, and heating oils.

Background

Petroleum is the source of nearly all the world’s

trans-portation fuels: gasoline for automobiles, light trucks,

and light aircraft; jet fuel for airplanes; and diesel fuel

for locomotives, heavy trucks, and agricultural

vehi-cles Heating oils (also called fuel oils or furnace oils)

are used for domestic heating and industrial process

heat; they are also used in oil-fired electric generating

plants Petroleum fuels are a vital component of the

energy economies of industrialized nations

The first step in making all petroleum fuels is

distil-lation of the petroleum or crude oil Kerosene, diesel

oil, and heating oils require comparatively little

refin-ing thereafter to be ready for marketrefin-ing

Consider-able effort is put into gasoline production both to

en-sure adequate engine performance and to guarantee

that sufficient quantities will be available to meet

mar-ket requirements

Gasoline

The most important characteristic of gasoline is its

combustion performance When gasoline is ignited

in the cylinder, the pressure rises as combustion pro-ceeds The pressure can, potentially, get so high that the remaining unburned gasoline-air mixture deto-nates rather than continuing to burn smoothly The explosion, which can readily be heard, is usually called “engine knock.” Engine knock puts undue me-chanical stresses on the engine components, is waste-ful of fuel (which the driver will experience as re-duced mileage), and reduces engine performance, such as acceleration Several factors contribute to gine knock One is the compression ratio of the en-gine—the ratio of volumes of the cylinder when the piston is at the upward and downward limits of its stroke Generally, the higher the compression ratio, the more powerful the engine and the greater the ac-celeration and top speed of the car A higher compres-sion ratio results in higher pressures inside the cylin-der at the start of combustion If the cylincylin-der pressure

is higher to begin with, the engine is more likely to knock

A second characteristic affecting knocking ten-dency is the nature of the fuel The dominant family

of chemical components of most gasolines is the paffins These compounds contain carbon atoms ar-ranged in chains, either straight (the normal par-affins) or with branches (isoparpar-affins) Normal paraffins have a great tendency to knock, whereas branched paraffins do not An octane rating scale was established by assigning the normal paraffin hep-tane the value 0 and the isoparaffin “iso-ochep-tane” (2,2,4-trimethylpentane) the value 100 The octane rating of a gasoline is found by comparing its knock-ing characteristics (in a carefully calibrated and stan-dardized test engine) to the behavior of a heptane/ iso-octane blend The percentage of iso-octane in a blend having the same knocking behavior of the oline being tested is the octane number of the gas-oline Gasoline is sold in three grades, a regular gaso-line with octane number 87, a premium gasogaso-line of about 93 octane, and a medium grade of about 89 oc-tane

Another important property of gasoline is its ability

to vaporize in the engine, measured by the vapor sure of the gasoline Gasoline with high vapor pres-sure contains a large number of components that va-porize easily This is desirable for wintertime driving

in cold climates, since easy vaporization helps starting when the engine is cold It is not desirable for driving

in hot weather, because the gasoline could vaporize in the fuel system before it gets to the engine, leading to

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the problem of vapor lock, which temporarily shuts

down the engine Oil companies adjust the vapor

pressure of their gasolines depending on the region

of the country, the local climate, and the season of the

year

Many process streams within a refinery are blended

to produce the gasolines that actually appear on the

market Gaseous molecules that would be by-products

of refining can be recombined to produce gasoline

in processes called alkylation or polymerization Some

gasoline, called straight-run gasoline, comes directly

from distillation of the petroleum Refinery streams

of little value can be converted into high-octane

gaso-line by catalytic cracking The octane numbers of

straight-run gasoline, or a related product called

straight-run naphtha, can be enhanced by catalytic

re-forming Other refinery operations can also yield

small amounts of material boiling in the gasoline

range Various of these streams are blended to make

products of desired octane, vapor pressure, and other

characteristics

Environmental concerns about gasoline have

cen-tered on the emission of unburned hydrocarbons

(in-cluding evaporation from fuel tanks), carbon

monox-ide and nitrogen oxmonox-ide emissions from combustion,

and the presence of aromatic compounds, some of

which are suspected carcinogens and contribute to

smoke or soot formation These concerns have led

to the development of reformulated gasolines One

aspect of production of reformulated gasoline is

in-creased vapor pressure, which retards evaporation

A second is removal of aromatic compounds;

re-moval actually complicates formulation because

aro-matics have desirably high octane numbers A third

step is the addition of oxygen-containing compounds,

oxygenates, which serve several purposes: They

re-duce the flame temperature, for example, and change

the combustion chemistry to reduce formation of

carbon monoxide and nitrogen oxides Oxygenates

also have high octane numbers, so they can make

up for the loss of aromatics An example of an

oxygen-ate useful in reformuloxygen-ated gasoline is methyl

tertiary-butyl ether

Jet Fuel

Jet fuel is produced by refining and purifying

kero-sene Kerosene is a useful fuel, particularly for some

agricultural vehicles, but the most important fuel use

of kerosene today is for jet aircraft engines Because

many jet planes fly at high altitudes, where the outside

air temperature is well below zero, the flow character-istics of the fuel at very low temperature are critical When the fuel is cooled, large molecules of paraffins settle out from the fuel as a waxy deposit The temper-ature at which the formation of this wax first begins, noticeable as a cloudy appearance, is called the cloud point Eventually a fuel can be cooled to an extent where it cannot even flow, not even to pour from an open container This characteristic temperature is the pour point

Smoke emissions from jet engines are an environ-mental concern The “smoke point” measures an im-portant property of jet fuel combustion Aromatics are the most likely compounds to produce smoke, while paraffins have the least tendency A jet fuel with

a low smoke point will have a high proportion of par-affins relative to aromatics The sulfur content of jet fuel can be important, both to limit emissions of sul-fur oxide to the atmosphere and because some sulsul-fur compounds are corrosive Both sulfur and aromatics contents of jet fuel can be reduced by treating with hy-drogen in the presence of catalysts containing cobalt,

or nickel, and molybdenum

Diesel Fuel

A familiar automobile engine operates by igniting the gasoline-air mixture with a spark plug Diesel engines operate differently: They have no spark plugs, but rely

on compression heating of the air in the cylinder to ignite the fuel A diesel engine has a much higher compression ratio than a comparable spark-ignition engine In a crude sense, a diesel engine actually oper-ates by knocking The desirable composition for die-sel fuel is essentially the inverse of that for gasoline: Normal paraffins are ideal components, while iso-paraffins and aromatics are not The combustion behavior of a diesel fuel is measured by the cetane number, based on a blend of cetane (hexadecane), as-signed a value of 100, and alpha-methylnaphthalene, assigned 0, as the test components A typical diesel fuel for automobile and light truck engines would have a cetane rating of about 50

Many of the physical property characteristics of jet fuel are also important for diesel fuel, including the cloud and pour points and the flow characteristics (viscosity) at low temperature Sulfur and aromatic compounds are a concern Aromatics are particularly undesirable because they are the precursors to the formation of soot As environmental regulations con-tinue to become more stringent, refiners will face

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ditional challenges to reduce the levels of these

com-ponents in diesel fuels

Heating Oils

Heating oils, also called furnace oils or fuel oils, are

of-ten graded and sold on the basis of viscosity The

grades are based on a numerical classification from

number 1 to number 6 (though there is no number 3

oil) As the number increases, so do the pour point,

the sulfur content, and the viscosity Number 1 oil is

comparable to kerosene Number 2 is an oil

com-monly used for domestic and industrial heating Both

have low pour points and sulfur contents and are

pro-duced from the distillation of petroleum The other

oils (numbers 4-6) are obtained by treating the

resid-uum from the distillation process They are

some-times called bunker oils because they have such high

viscosities that they may have to be heated to have

them flow up, from the storage tank, or bunker, and

into the burners in the combustion equipment

Harold H Schobert

Further Reading

Berger, Bill D., and Kenneth E Anderson Modern

Pe-troleum: A Basic Primer of the Industry 3d ed Tulsa,

Okla.: PennWell Books, 1992

Black, Edwin Internal Combustion: How Corporations

and Governments Addicted the World to Oil and Derailed

the Alternatives New York: St Martin’s Press, 2006.

Conaway, Charles F The Petroleum Industry: A

Nontech-nical Guide Tulsa, Okla.: PennWell Books, 1999.

Kunstler, James Howard The Long Emergency:

Sur-viving the Converging Catastrophes of the Twenty-first

Century New York: Atlantic Monthly Press, 2005.

Middleton, Paul A Brief Guide to the End of Oil

Lon-don: Constable and Robinson, 2007

Mushrush, George W., and James G Speight

Petro-leum Products: Instability and Incompatibility

Wash-ington, D.C.: Taylor & Francis, 1995

Royal Dutch Shell, comp The Petroleum Handbook 6th

ed New York: Elsevier,1983

Speight, James G The Chemistry and Technology of

Petro-leum 4th ed Boca Raton, Fla.: CRC Press/Taylor &

Francis, 2007

Yergin, Daniel The Prize: The Epic Quest for Oil, Money,

and Power New ed New York: The Free Press, 2008.

See also: Oil and natural gas chemistry; Oil industry;

Petroleum refining and processing; Propane;

Trans-portation, energy use in

Gems

Category: Mineral and other nonliving resources

Where Found Mineral gems occur within the Earth’s crust and are widely distributed on the planet The most important source of the world’s diamonds is the African conti-nent Emerald has been found primarily on the South American continent, particularly near Bogotá, Co-lombia Because sapphire and ruby are color varieties

of the same mineral, corundum, they frequently occur

in the same regions Historically, rubies and sapphires have been found in Sri Lanka, Burma (Myanmar), Thailand, and Cambodia

Primary Uses All naturally occurring gems are primarily used for jewelry, ornamentation, or decorative purposes The most beautiful, durable, and uncommon gems are frequently embedded in or dangle from works of gold, silver, or platinum Historically, kings and queens, aristocrats, popes, and other important soci-etal figures wore these ornaments In modern society anyone who can afford to purchase the jewels may ac-quire them Synthetic gems are used in electronics, drills, and cutting tools

Technical Definition

A gemstone is a gem that has been cut, ground, or pol-ished from a large rock Attributes that impart mag-nificent beauty to a gem include flawless crystalline structure, uniformity and intensity of color (or un-common color), hardness, durability, and extent of transparency and refractivity

All minerals of the Earth, including gems, come from rock The geological events that yield igneous rock produce all the precious gems and most semipre-cious gems Igneous rock is formed upon the cooling

of hot molten lava (magma) During this cooling pro-cess, the liquid rock solidifies

Description, Distribution, and Forms Gems are minerals of beauty, rarity, and durability Like all minerals, gems have a definite chemical com-position in which the atoms are arranged in a specific pattern Repetition of this pattern generates a crystal-line shape that imparts a characteristic color, luster, hardness, and transparency to each gem The

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tional precious gems are diamond, emerald, ruby,

and sapphire, but any gem may be considered

pre-cious if it is uncommonly beautiful

Gems are subdivided into two categories: organic

gems (such as pearls, amber, and coral) and mineral

gems (from rock or of other geological origin)

Or-ganic gems are derived from living or once living

or-ganisms Aside from the traditional precious stones—

diamond, emerald, ruby, and sapphire—familiar

mineral gems include aquamarine, garnet, jade,

oliv-ine, topaz, turquoise, and many forms of quartz These

“semiprecious” gems have a combination of beauty

and affordability that makes them both desirable and

marketable

Diamond is found in ultrabasic rock and alluvial

deposits (Alluvial deposits are the deposits that

re-main after the physical wearing of rock.) Ultrabasic

rock is igneous rock (volcanic) that is essentially made

of silicate minerals or ferromagnesian minerals such

as olivine, hornblende, augitite, and biotite (mica) Also found in igneous rock are corundum, the min-eral of emmin-erald, commonly in hexagonal, elongated, and broad crystals, as well as beryl, the mineral of ruby and sapphire, often shaped as bipyramids or barrel-shaped hexagons

Diamonds are valued based upon the overall qual-ity of the diamond, which is assessed by the “four C’s”: color, cut, carat, and clarity Most gemologists agree that a colored diamond is the rarest gem of all Transi-tion metal ions, when present in trace amounts within the crystalline structure, impart a light color to an oth-erwise colorless diamond Colored diamonds range from blue, blue-white, and blue-green to red and yel-low Because the cut of a gem is critical to maximizing both beauty and value, cutting must be done by ex-perts who know exactly where and how to cut a stone

to reveal the optimal brilliance (or “fire”)

of the crystal’s refractive planes The size

of a diamond is expressed in carats, where one carat weighs 200 milligrams In addi-tion to color, “fire,” and size, the degree of flawlessness and the hardness of a stone are important in determining its final mar-ket value The other precious gems are as-sessed for market value in a manner simi-lar to that of diamond

Two of the semiprecious gems, garnet and olivine, are silicate minerals Garnet stones are often rhombo-dodecahedral (twelve-sided) or hexaoctahedral shapes

in nature; these intriguing crystals occur

in colors of red, green, and black Olivine crystals are perfect small cubes with col-ors of green to brown-green Garnet oc-curs commonly in the Earth’s crust in metamorphic rock (formed by the action

of heat and pressure) rather than in igne-ous (volcanic) rock, the origin of most gems

Colorless diamond is made exclusively

of carbon atoms arranged in rigid tetra-hedrons Yet blue, yellow, and other colors

of diamond also occur in nature Chemists discovered that the trapping of certain transition metal ions in the crystal as it forms can result in coloration of the stone The same fact holds for other precious gems For example, the mineral

Properties of Gem Minerals

Gem Material Hardness

Specific Gravity

Refractive Index

*Average for a variety of types.

Source: Data are derived from Sybil P Parker, ed., McGraw-Hill Concise

Encyclopedia of Science and Technology, 2d ed., 1989.

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dum (Al2O3) is an oxide of aluminum, a hard, white

substance The presence of transition metal

impuri-ties within the corundum crystal results in colorful

gems Specifically, ruby is corundum with chromium

cations, which give the crystal a rich red color Iron

and titanium cations cause the brilliant blue of the sapphire, while iron cations give oriental topaz its yel-low color Finally, oriental amethyst acquires its violet color from chromium and titanium cations within the corundum lattice

Data from the U.S Geological Survey, U.S Government Printing Office, 2009.

Source: Mineral Commodity Summaries, 2009

720,000 1,100,000 350,000 2,200,000

23,300,000

600,000

6,100,000

230,000 210,000

Carats

30,000,000

20,000,000

15,000,000 10,000,000

5,000,000 Tanzania

Russia

Namibia

Guyana

Guinea

Ghana

Sierra Leone

South Africa

Other countries

10,000,000

230,000

25,000,000

200,000

18,000,000

470,000 100,000

5,400,000

210,000

Congo, Democratic

Republic of the

Canada

Brazil

Botswana

Australia

Angola

Central African

Republic

China

Côte d’Ivoire

25,000,000

Top Nations Producing Gem Diamonds, 2008

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Similarly, silicon dioxide (SiO2), or quartz, is a

clear, colorless crystal unless transition metal

impuri-ties are present Then rose, purple, or smoke-gray

col-ors may be produced Another example is found in

the simple arrangement of sulfate (SiO4)

tetrahe-drons about a metal cation; when they are around

a magnesium cation, olivine (Mg2SiO4) results, but

when around a zirconium cation, zircon (ZrSiO4) is

formed

Mineral gems are widely distributed on the Earth

Gems have been found on all continents except

Ant-arctica (Antarctic exploration for this purpose has

not yet occurred.) A country that has yielded a great

variety and abundance of gems is Sri Lanka This

small island has yielded more than a dozen different

types of gemstones Located just south of the tip of

India, which itself is famous for its diamonds and

em-eralds, Sri Lanka has rich alluvial deposits

The African continent has been the main source of

all known diamonds Aside from India, other

mini-mally productive diamond sources have been Brazil

and the Democratic Republic of the Congo Within

the United States, Arkansas has yielded the most

dia-monds, although the number is very low in

compari-son with the other regions mentioned Even less

pro-ductive mines have been found on the eastern slopes

of the Appalachian Mountains

Rubies are found in Sri Lanka, Burma, Thailand,

and Cambodia; sapphires occur in the same regions as

rubies, since both are color varieties of corundum

Both ruby and sapphire stones are found in Russia,

China, Germany, India, and Australia as well as on the

African continent In the United States, a small

num-ber of rubies have been found in North Carolina,

while Montana has provided a mining site for small

but exceptionally brilliant blue sapphires

Emerald has primarily been found on the South

American continent near Bogotá, Colombia Another

important source of emeralds is Siberia; emeralds

have also been mined in Brazil, Egypt, Austria,

Zimba-bwe, Mozambique, Tanzania, and South Africa

Within the United States pale, muted green emeralds

with many flaws have been found in North Carolina

These crystals are poor-quality gems and have little

value Emerald is a color variety of the mineral beryl;

another color variety of beryl is the semiprecious gem

aquamarine Aquamarine is more abundant in the

Earth’s crust than emerald and is plentiful in

North-ern Ireland, Italy, Russia, Namibia, and Brazil, where

the largest (110.5 kilograms) aquamarine stone was

found in 1910 Other semiprecious stones are as di-verse in their distribution as the precious gems are, but most of these stones are more common

Synthetic garnets are useful in industry One syn-thetic gem, yttrium iron garnet, is used in microwave devices Another, yttrium aluminum garnet (YAG), is used in lasers as a source of coherent light; it is also used as an artificial gemstone On the molecular level, the crystals of synthetic gems are subtly different from those of natural stones These differences, however, typically evade the untrained eye For this reason, syn-thetic gemstones are used in jewelry but cannot be sold as fine jewels

History Since ancient times, gems have been used to adorn the human body and create artwork Although no one knows when gems were first discovered, desired, or used, there is archaeological evidence that beads of garnet were worn by people of the Bronze Age five thousand years ago The Old Testament refers to a va-riety of gems, including amethyst, diamond, emerald, malachite, and cinnabar It is known that, in the first century b.c.e., emerald was the preferred stone of Cleopatra VII, the last queen of ancient Egypt Emeralds represented regeneration and spring in some ancient societies The Incan civilization used the rich green stones to guard sacred temples Emer-alds and rubies are among the rarest of gems, and carat per carat their monetary value often exceeds that of most colorless diamonds There is virtually no such thing as a truly flawless emerald, as internal frac-tures mar the interior of the crystals True ruby (or

“oriental ruby”) is the only subclass of corundum to have a distinct category of its own When asterism (the appearance of a six-rayed star) is found in a stone, it is coveted even more; legend has it that asterism con-quers evil forces Excluding red ruby, all corundum is classified as sapphire, which may range in color from clear yellow, green, and lavender to the traditional cornflower blue Other corundum gems may also have asterism; the Star of India, a 563-carat, blue-gray stone, is the largest sapphire known

Historically, diamond has been the most important

of the precious stones The word “diamond” is

de-rived from adamas, a Greek term meaning

“invinci-ble” or “unconquerable.” The earliest recorded refer-ence to diamond comes from a civilization in India during the fourth century b.c.e Until the eighteenth century, India was believed to be the only source of

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amonds Then, in the early 1800’s, small, productive

diamond mines were discovered in Brazil In 1867,

the first of the rich South African mines was found

India and Africa have produced the largest and most

famous diamonds known, including the Hope, the

Victoria-Transvaal, the Cullinan, and the Koh-i-Noor

diamonds

In 1902, Auguste Verneuil, a researcher in Paris,

was able to grow red crystals of beryl in the laboratory

Thus, the first synthetic gem was a ruby More

re-cently, chemists have been able to synthesize

dia-monds for industrial use The General Electric

Com-pany achieved laboratory synthesis of diamond in

1955 Because events in nature cannot be exactly

mimicked in the laboratory, synthetic diamonds lack

the aesthetic appeal of naturally occurring stones;

therefore they are not used in fine jewelry making A

synthetic substance used to mimic diamond in jewelry

is cubic zirconium (imitation diamond or faux

dia-mond) This material can be synthesized in bulk and

at a low cost Although the durability, refractivity, and

transparency of the stone resemble diamond,

zirco-nium lacks its hardness and durability

Obtaining Gems

Traditional mining methods have been used to mine

gems Historically, mines were operated through the

exploitation of imported slaves or local natives

Typically the laborers were used to dig pits deep into

the Earth The removed earth was pulverized and

sifted through, either using water to flush away the

gravel or using dry sifting methods These techniques

were used in the mines of Africa, Brazil, and Colombia

during the eighteenth and nineteenth centuries

Afri-can mines have become more mechanized, using

large drills and other equipment, but mining with

manual labor continues in Colombia As in the past,

there are risks of mine collapse and suffocation

Gems can also be collected by sifting through

allu-vial deposits along the edges of streams and rivers

The gems of India and Sri Lanka have mostly been

collected by this method Miners in Sri Lanka still use

the bottoms of their feet to feel for gems within the

stones under running river water In Thailand miners

continue to take their boats out in low tide to dredge

the mud for gems

Uses of Gems

Aside from having aesthetic appeal, some gems are

useful in industry and instrumentation For example,

diamond, the hardest substance known, has been used as a cutting tool In mining and exploratory geol-ogy, diamond drills are used to cut through stones and layers of rock Additionally, finely powdered diamond

is used to grind, shape, and polish large diamond stones as well as other gemstones Synthetic diamond has replaced the natural gem for industrial tools, while synthetic ruby and sapphire are used to make la-sers that emit coherent light and in microwave de-vices Ranking just below diamond on the Mohs hard-ness scale, ruby and sapphire gems are also used in cutting, grinding, and drilling tools

Mary C Fields

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