A Primer of Gemstones | NOVA

 

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Agate

Agate, along with carnelian, chrysoprase, jasper, and flint, is a variety of chalcedony, a type of quartz made up of tiny crystalline fibers.

Class: semiprecious
Origin of Name: from the Latin achates, the former name of Sicily’s river Drillo, along which agate was found in ancient times
Color: green, yellow, red, reddish-brown, white, bluish-white
Chemical Composition: silica
Crystal System: trigonal (microcrystalline)
Hardness: 6.5
Specific Gravity: 2.57-2.64
Geographic Origins: worldwide but chiefly Brazil and Uruguay

 

Amethyst

Amethyst is a transparent variety of quartz that contains trace amounts of iron, an impurity that lends amethyst its lovely violet hues.

Class: semiprecious
Origin of Name: possibly from the Greek amethustos (“not drunken”), perhaps given to the stone in the belief that the wearer would not suffer greatly from drinking too much alcohol
Color: various shades of purple
Chemical Composition: silica
Crystal System: trigonal
Hardness: 7
Specific Gravity: 2.65
Geographic Origins: Brazil, Mexico, Ontario

 

Aquamarine

Aquamarine is a variety of the mineral beryl, which also includes emerald and lesser known gems such as heliodor, goshenite, and the beautiful rose-pink morganite.

Class: semiprecious
Origin of Name: from the Latin aqua marina (sea water)
Color: blue, blue-green, green
Chemical Composition: beryllium aluminum silicate
Crystal System: hexagonal
Hardness: 7.5
Specific Gravity: 2.63-2.91
Geographic Origins: Madagascar, Sri Lanka, Russia, Brazil, Colorado, North Carolina

 

Chrysoberyl

Harder than all other gemstones save diamond and corundum, chrysoberyl is strongly pleochroic, meaning different colors—such as red, orange-yellow, and green—appear depending on which angle you view the gem from.

Class: sometimes considered precious
Origin of Name: from the Greek chrysoberyllos (golden beryl)
Color: green, greenish-yellow, brown
Chemical Composition: beryllium aluminum oxide
Crystal System: orthorhombic
Hardness: 8.5
Specific Gravity: 3.68-3.78
Geographic Origins: Sri Lanka, India, Burma, Brazil

 

Diamond

While tiny diamonds are the stuff of dental drills, large and rare specimins are the most valued of gems. In 1995, Sotheby’s sold a 100.1-carat, pear-shaped “D” flawless diamond to a Saudi Arabian sheikh for $16,548,750, the highest price to date paid for a diamond. (One carat equals one-fifth of a gram.)

Class: precious
Origin of Name: from the Greek adamas (unconquerable), a nod to its unequaled hardness
Color: clear, yellow, brown, green, blue, pink, and rarely, red
Chemical Composition: carbon
Crystal System: cubic
Hardness: 10
Specific Gravity: 3.515
Geographic Origins: South Africa, India, Indonesia, China, Russia, Australia, Brazil, California, Colorado, Canada

 

Emerald

Emerald, like aquamarine, is a variety of the mineral beryl. The largest gem-quality emerald crystal, found in Colombia’s Cruces Mine in 1969, weighed 7,025 carats, or about three pounds.

Class: precious
Origin of Name: from a Persian word that later appeared in the Greek as smaragdos, from which the form esmeralde and later emerald were derived
Color: blue-green to green
Chemical Composition: beryllium aluminum silicate
Crystal System: hexagonal
Hardness: 7.5
Specific Gravity: 2.63-2.91
Geographic Origins: Colombia, Zimbabwe, Zambia, Pakistan, Russia, Australia, North Carolina

 

Garnet

Garnet has a number of varietal names depending on color and composition. Examples include almandite, pyrope, rhodolite, grossular, uvarovite, spessartine, andradite, and demantoid. The latter is the most expensive kind of garnet, usually more expensive than emerald.

Class: semiprecious
Origin of Name: probably from Latin granatum (pomegranate), presumably for similarity in color between the gem and the fruit
Color: red, orange, yellow, green, clear
Chemical Composition: magnesium, iron, or calcium aluminum silicates
Crystal System: cubic
Hardness: 6.5-7.5
Specific Gravity: 3.58-4.32
Geographic Origins: numerous sources worldwide

 

Jade

Jade comes in two varieties: jadeite, originally found in Guatemala and carved by early Indian civilizations, and nephrite, most famously carved by the Chinese. The largest piece of jade ever found was a 636-ton lens of nephrite jade unearthed in Canada’s Yukon Territory in 1992.

Class: semiprecious
Origin of Name: from the Spanish piedra de hijada, name given for the jadeite carved by Indian civilizations of Central America
Color: white to green, orange, brown, lilac (jadeite); green to creamy-white (nephrite)
Chemical Composition: sodium aluminum silicate (jadeite), calcium magnesium aluminum silicate, with some iron (nephrite)
Crystal System: monoclinic (jadeite and nephrite)
Hardness: 7 (jadeite), 6.5 (nephrite)
Specific Gravity: 3.3-3.36 (jadeite), 2.9-3.1 (nephrite)
Geographic Origins: Burma (jadeite); Siberia, New Zealand, Taiwan, British Columbia

 

Lapis lazuli

The best lapis lazuli still comes from the Sar-e-Sang mines in Afghanistan, where the vivid blue rock was traded to Egypt and Sumer (Iraq) in ancient times, and later throughout the East and Europe.

Class: semiprecious
Origin of Name: from the Persian lazhward (blue)
Color: dark blue to greenish-blue to purple-blue
Chemical Composition: aggregate of several minerals, including hauyne (which lends the stone its color), sodalite, nosean, and lazurite (a combination of hauyne and sodalite)
Crystal System: the four minerals above belong to the cubic system
Hardness: 5.5
Specific Gravity: 2.7-2.9
Geographic Origins: Afghanistan, Siberia, Chile

 

Moonstone

Moonstone is a member of the feldspars, the most abundant mineral group in the Earth’s crust.

Class: semiprecious
Origin of Name: named for its pearly luster, reminiscent of the Moon
Color: colorless, white to yellowish grey, reddish to bluish grey
Chemical Composition: potassium aluminum silicate
Crystal System: monoclinic
Hardness: 6
Specific Gravity: 2.56-2.59
Geographic Origins: Sri Lanka, India, Madagascar, Burma, Tanzania, Colorado, Indiana, New Mexico, New York, North Carolina, Pennsylvania, Virginia, Wisconsin

 

Opal

Australia boasts the largest opals: a 26,350-carat gem-quality white opal found in 1989, and a 1,982.5-carat gem-quality uncut black opal unearthed in 1986. (One carat equals one-fifth of a gram.)

Class: sometimes considered precious
Origin of Name: probably derives from the Sanskrit word upala (precious stone)
Color: pale (white opal); clear (water opal); black, grey, or brown (black opal); yellow, orange, red (fire opal)
Chemical Composition: silica with up to 10 percent water (in precious opal)
Crystal System: non-crystalline or only poorly crystalline
Hardness: 5.5-6.5
Specific Gravity: 1.98-2.20
Geographic Origins: Australia (white and black opal), Mexico (fire and water opal), Brazil, Guatemala, Honduras, Nevada, Oregon, Idaho

 

Pearl

Although pearl is not an inorganic mineral but rather the organic product of oysters, it is considered by many to be a precious gem, and indeed, the pearl known as La Régente, weighing in at 302.68 grains (or a little over half an ounce), was sold in 1988 for $859,280.

Class: sometimes considered precious
Origin of Name: perhaps from the Latin perna, a kind of sea mussel
Color: creamy white to silver-white; also yellow, pink, green, blue, black
Chemical Composition: mostly calcium carbonate, with some conchiolin and water
Crystal System: n/a
Hardness: n/a
Specific Gravity: 2.60-2.78
Geographic Origins: Japan, Australia, Persian Gulf, Venezuela, California, Florida

 

Peridot

So precious was peridot to ancient Egyptians that they enslaved residents of St. John’s Island (present-day Zebirget) in the Red Sea, forcing them to mine this rich “oily” green mineral for use in jewelry.

Class: semiprecious
Origin of Name: a French word possibly derived from the Arabic faridat (gem)
Color: pale yellowish green to brownish green
Chemical Composition: magnesium iron silicate
Crystal System: orthorhombic
Hardness: 6.5-7
Specific Gravity: 3.22-3.40
Geographic Origins: Zebirget Island (Red Sea), Burma, Pakistan, Norway, Brazil, Arizona, New Mexico

 

Ruby

Rubies, like sapphires, are a variety of the mineral corundum, which is known for its remarkably rich color. The Eminent Star ruby, the largest known, weighs 6,465 carats, or almost three pounds.

Class: precious
Origin of Name: from the Latin ruber (red)
Color: red, orange-red, purple-red
Chemical Composition: aluminum oxide
Crystal System: trigonal
Hardness: 9
Specific Gravity: 3.96-4.05
Geographic Origins: Burma, Thailand, Cambodia, Kenya, Tanzania, Zimbabwe

 

Sapphire

Sapphire, like ruby, is a variety of corundum. The Lone Star, a star sapphire cut in England in 1889, weighs 9,719.5 carats, or about four and a half pounds.

Class: precious
Origin of Name: from the Latin sapphirus (blue)
Color: blue, white, yellow, orange, green, purple, pink
Chemical Composition: aluminum oxide
Crystal System: trigonal
Hardness: 9
Specific Gravity: 3.96-4.05
Geographic Origins: Kashmir (India), Sri Lanka, Burma, Australia, Montana, North Carolina

 

Spinel

Next to ruby and the rare red diamond, spinel is the most expensive of all red gems, commanding prices as high as $2,000 per carat for stones above one carat in weight. (One carat equals one-fifth of a gram.)

Class: semiprecious
Origin of Name: probably from the Latin spina (thorn)
Color: red, orange, brown, green, blue, violet, clear
Chemical Composition: magnesium aluminum oxide
Crystal System: cubic
Hardness: 8
Specific Gravity: 3.58-4.06
Geographic Origins: Sri Lanka, Burma, Brazil

 

Topaz

Today, the many colors of topaz—from classic yellow to pink—are well-known. Since 1988, the Smithsonian Institution has displayed the 22,892.5-carat (or roughly 10-pound) American Golden Topaz.

Class: sometimes considered precious
Origin of Name: reputedly named after Topazius, the ancient Greek name for the Red Sea island of Zebirget (though only the gem peridot is found there); alternatively, may derive from the Sanskrit tapas (fire)
Color: clear to pale blue to yellow, orange, brown, and pink
Chemical Composition: aluminum fluorosilicate with some hydroxyl
Crystal System: orthorhombic
Hardness: 8
Specific Gravity: 3.49-3.57
Geographic Origins: Brazil, Russia, Mexico, California, Colorado, Utah, Maine, New Hampshire

 

Tourmaline

Tourmaline shows the greatest range of colors of any gem and its varieties of composition led the Victorian thinker John Ruskin to write that “the chemistry of it is more like a medieval doctor’s prescription than the making of a respectable mineral.”

Class: semiprecious
Origin of Name: from the Sinhalese tourmali (mixed stones)
Color: most colors, most popularly pink and green
Chemical Composition: complex silicate of boron and aluminum
Crystal System: trigonal
Hardness: 7-7.5
Specific Gravity: 3.0-3.25
Geographic Origins: Namibia, Zambia, Nigeria, Russia, Brazil, Maine, North Carolina, California

 

Turqoise

Egyptians and their predecessors mined turquoise in Syria as far back as 4000 B.C., and ancient civilizations of Mexico also prized it for its superb color.

Class: semiprecious
Origin of Name: probably from the French pierre turquoise (Turkish stone)
Color: sky blue
Chemical Composition: hydrous phosphate of copper and aluminum
Crystal System: triclinic (cryptocrystalline)
Hardness: 5-6
Specific Gravity: 2.6-2.9
Geographic Origins: Iran, southwestern U.S.

 

Zircon

Zircon resembles diamond in its luster and fire, yet it has a tendency to chip out at the facet junctions over the years, so it not prized as highly. (Cubic zirconia, a well-known diamond simulant, is an altered form of the mineral baddeleyite, whose chemical composition is zirconium oxide.)

Class: semiprecious
Origin of Name: from the word zargoon, meaning vermilion in Arabic or golden-colored in Persian
Color: vermilion, yellow, green, brown, blue, clear
Chemical Composition: zirconium silicate
Crystal System: tetragonal
Hardness: 7.5
Specific Gravity: 4.6-4.7
Geographic Origins: Burma, Sri Lanka, Cambodia, Vietnam

 

Guide to Properties

Class

In their classic book Gemology (Wiley, 1979), the mineralogists Cornelius Hurlbut of Harvard and George Switzer of the Smithsonian Institution claim, “Since there is no rigid set of criteria that separate gems of great value from those of less value, the term semiprecious should be abandoned and all gems referred to as precious.” However, since many people still divide gems between the two types, we offer here definitions of precious and semiprecious stones from The Glossary of Geology, 3rd Edition, published by the American Geological Institute:

Precious stone:

“A gemstone that, owing to its beauty, rarity, durability, and hardness, has the highest commercial value and traditionally has enjoyed the highest esteem since antiquity; specifically, diamond, ruby, sapphire, and emerald (and sometimes pearl, opal, topaz, and chrysoberyl).”

Semiprecious stone:

“Any gemstones other than a precious stone, or any gemstone of lower commercial value than a precious stone; specifically, a mineral that is less than 8 on the Mohs scale of hardness. A gemstone may also be regarded as semiprecious because of its comparative abundance, inferior brilliance, or unfamiliarity to the public, or owing to the whims of fashion. This arbitrary classification is misleading, as it does not recognize, for example, that a ruby of poor quality may be far less costly than a fine specimen of jadeite.”

Color

Gemstones gain their color from the way they affect light as it passes through them. Like a prism, a clear diamond splits incoming light into its constituent wavelengths, creating a pleasing rainbow of colors. Many other gems get their distinctive colors by absorbing one or more wavelengths, because of their chemical compositions. Rubies and sapphires, for example, are both varieties of the mineral corundum, identical in almost every respect. The difference lies in the trace amounts of other elements present. A dash of chromium makes a vivid red ruby; a bit of iron and titanium results in a deep blue sapphire.

Chemical composition

Gemstones have specific chemical compositions that serve to identify them. Diamond consists of carbon, for instance, while emerald is a beryllium aluminum silicate and peridot is a magnesium iron silicate. Some gemstones, such as jade, are aggregates of one or more minerals.

Crystal system

All crystalline minerals consist of atoms packed in geometric arrays; the arrays, called crystal structures, are divided into seven crystal systems based on the symmetry of their atomic geometries. Mineralogists distinguish among the different crystal systems by drawing imaginary lines called crystallographic axes between the points, joints, or planes of the crystals. These lines intersect in characteristic ways at a point within the crystal called the origin. Crystals in the cubic system, for example, have three crystallographic axes, all of equal length and all at right angles to one another. (Note that cube-shaped crystals are not the only kind in the cubic system; other shapes, including octahedrons and icositetrahedrons, also qualify, because they have crystallographic axes that meet the definition.)

A second type of crystal system, known as tetragonal, also has three crystallographic axes at right angles to one another. However, while two of the axes are of equal length, the third is either shorter or longer. The bipyramid, for example, is an elongated octahedron.

The five other systems are hexagonal, trigonal, orthorhombic, monoclinic, and triclinic. Along with composition, crystal structure determines a mineral’s properties. It also influences what happens to light when it enters a mineral and where planes of weakness may lie, which helps gemcutters.

Hardness

Hardness, which mineralogists can use to help identify minerals, is a measure of a gemstone’s resistance to abrasion. In 1822, the Austrian mineralogist Friedrich Mohs developed the Mohs scale, a standard scale using 10 familiar minerals against which all minerals are measured. A mineral (or, on the scale below, an everyday object) will scratch all minerals of a lower number on the scale. The intervals between minerals on the scale are not uniform, however. For instance, the difference in hardness between diamond and sapphire is much greater than that between sapphire and topaz. The minerals, from least to most hard and interspersed with several everyday objects for comparison, are as follows:

1. 
   talc
2. 
   gypsum (fingernail)
3. 
   calcite (copper penny)
4. 
   fluorite
5. 
   apatite (pocket knife)
6. 
   feldspar
7. 
   quartz
8. 
   topaz
9. 
   sapphire
10. 
    diamond

Origin of name

The origins of the names of many gemstones have come down to us from antiquity and in many cases are somewhat obscure, but in those cases we offer the best guesses of historians.

Specific gravity

Specific gravity, a property that mineralogists use to identify minerals, is the number of times heavier a gemstone of any volume is than an equal volume of water. That is, it is the ratio of the density of the gemstone to the density of water.