Gems

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 continent. Emerald has been found primarily on the South American continent, particularly near Bogotá, Colombia. 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 societal figures wore these ornaments. In modern society anyone who can afford to purchase the jewels may acquire them. Synthetic gems are used in electronics, drills, and cutting tools.

Technical Definition

A gemstone is a gem that has been cut, ground, or polished from a large rock. Attributes that impart magnificent beauty to a gem include flawless crystalline structure, uniformity and intensity of color (or uncommon color), hardness, durability, and extent of transparency and refractivity.

All minerals of the Earth, including gems, come from rock. The geological events that yield igneousrock produce all the precious gems and most semiprecious gems. Igneous rock is formed upon the cooling of hot molten lava (magma). During this cooling process, the liquid rock solidifies.

Description, Distribution, and Forms

Gems are minerals of beauty, rarity, and durability. Like all minerals, gems have a definite chemical composition in which the atoms are arranged in a specific pattern. Repetition of this pattern generates a crystalline shape that imparts a characteristic color, luster, hardness, and transparency to each gem. The traditional precious gems are diamond, emerald, ruby, and sapphire, but any gem may be considered precious 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). Organic gems are derived from living or once living organisms. Aside from the traditional precious stones—diamond, emerald, ruby, and sapphire—familiar mineral gems include aquamarine, garnet, jade, olivine, 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 remain 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 mineral of emerald, 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 quality 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. Transition metal ions, when present in trace amounts within the crystalline structure, impart a light color to an otherwise colorless diamond. Colored diamonds range from blue, blue-white, and blue-green to red and yellow. Because the cut of a gem is critical to maximizing both beauty and value, cutting must be done by experts 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 addition to color, “fire,” and size, the degree of flawlessness and the hardness of a stone are important in determining its final market value. The other precious gems are assessed for market value in a manner similar 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 colors of green to brown-green. Garnet occurs commonly in the Earth’s crust in metamorphic rock (formed by the action of heat and pressure) rather than in igneous (volcanic) rock, the origin of most gems.

Colorless diamond is made exclusively of carbon atoms arranged in rigid tetrahedrons. 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 corundum (Al₂O₃) is an oxide of aluminum, a hard, white substance. The presence of transition metal impurities 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 yellow color. Finally, oriental amethyst acquires its violet color from chromium and titanium cations within the corundum lattice.

Similarly, silicon dioxide (SiO₂), or quartz, is a clear, colorless crystal unless transition metal impurities are present. Then rose, purple, or smoke-gray colors may be produced. Another example is found in the simple arrangement of sulfate (SiO₄) tetrahedrons 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 Antarctica. (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 emeralds, Sri Lanka has rich alluvial deposits.

The African continent has been the main source of all known diamonds. Aside from India, other minimally productive diamond sources have been Brazil and the Democratic Republic of the Congo. Within the United States, Arkansas has yielded the most diamonds, although the number is very low in comparison with the other regions mentioned. Even less productive 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 number 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, Zimbabwe, 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 Northern Ireland, Italy, Russia, Namibia, and Brazil, where the largest (110.5 kilograms) aquamarine stone was found in 1910. Other semiprecious stones are as diverse in their distribution as the precious gems are, but most of these stones are more common.

Synthetic garnets are useful in industry. One synthetic 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, synthetic 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 variety 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. Emeralds 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 fractures 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 conquers 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 derived from adamas, a Greek term meaning “invincible” or “unconquerable.” The earliest recorded reference 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 diamonds. 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 recently, chemists have been able to synthesize diamonds for industrial use. The General Electric Company 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 diamond). This material can be synthesized in bulk and at a low cost. Although the durability, refractivity, and transparency of the stone resemble diamond, zirconium 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. African 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 alluvial 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 geology, 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 lasers that emit coherent light and in microwave devices. Ranking just below diamond on the Mohs hardness scale, ruby and sapphire gems are also used in cutting, grinding, and drilling tools.

Bibliography

Bonewitz, Ronald Louis. Rock and Gem. New York: DK, 2005.

Chatterjee, Kaulir Kisor. “Gemstones—Miscellaneous.” In Uses of Industrial Minerals, Rocks, and Freshwater. New York: Nova Science, 2009.

Hall, Cally. Gemstones. 2d American ed. Photography by Harry Taylor. New York: Dorling Kindersley, 2002.

Maillard, Robert, Ronne Peltsman, and Neil Grant, eds. Diamonds: Myth, Magic, and Reality. New rev. ed. New York: Bonanza Books, 1984.

O’Donaghue, Michael. Gems: Their Sources, Descriptions, and Identification. 6th ed. Oxford, England: Butterworth-Heinemann, 2006.

Read, P. G. Gemmology. 3d ed. Boston: Elsevier/Butterworth-Heinemann, 2005.

Schumann, Walter. Gemstones of the World. 3d rev. and expanded ed. New York: Sterling, 2007.

‗‗‗‗‗‗‗. Handbook of Rocks, Minerals, and Gemstones. Translated by R. Bradshaw and K. A. G. Mills. Boston: Houghton Mifflin, 1993.

White, John Sampson. Minerals and Gems. Special photography by Chip Clark. Washington, D.C.: Smithsonian Institution Press, 1991.

Zim, Herbert S., and Paul R. Shaffer. Rocks, Gems, and Minerals: A Guide to Familiar Minerals, Gems, Ores, and Rocks. Rev. and updated ed. Revised by Jonathan P. Latimer et al., illustrated by Raymond Perlman. New York: St. Martin’s Press, 2001.

U.S. Geological Survey. Gemstones: Statistics and Information. http://minerals.usgs.gov/minerals/pubs/commodity/gemstones/index.html#mcs

U.S. Geological Survey. An Overview of Production of Specific U.S. Gemstones. http://minerals.usgs.gov/minerals/pubs/commodity/gemstones/sp14-95/contents.html