Read an Excerpt

Gems and Gemstones

THE UNIVERSITY OF CHICAGO PRESS

Chapter One

How are gemstones formed in nature? We can attempt to answer this question through a combination of direct observations of nature, synthesis of gemstones in the laboratory, and reconstruction of the natural process through scientific theory. We can also distinguish between two very different ways in which gemstones form in nature: organically—from biological processes; and inorganically—from geological and chemical processes. The formation of organically derived gems is discussed further on page 259.

INORGANIC GEMS make up the vast majority of gemstone varieties. Inorganic gems are created by physical chemistry, geological processes, and extreme forces of nature. Ultimately, the growth of an inorganic gemstone's external shape is a product of its internal arrangement of atoms, chemical composition, and other factors of physical chemistry. Our understanding of the natural growth process of gemstones is largely theoretical and based partly on the study of synthetic gemstones created in laboratories, and partly on our knowledge of geology and physical chemistry. We can observe the complete formation of synthetic gemstones because they are created in a lab over a period of days or weeks. We cannot observe the complete formation of natural gemstones in nature because they may develop over longer periods of time (although we are not sure how long) and usually under enormous heat and pressure. Additionally, the process generally occurs deep underground where we cannot observe it.

What we do know is that inorganic gemstones originally form from a liquid source and their formation is influenced by three factors: temperature, pressure, and chemical ingredients. The principal crystal growth processes that lead to gemstone formation are (1) freezing from molten rock, and (2) precipitation from mineral-rich HYDROTHERMAL SOLUTIONS. We can use some simple analogies to help us visualize the processes.

Crystal growth through freezing from molten rock (MAGMA), in its simplest form, can be compared to the freezing of ice in water as the temperature cools, although the freezing point of gemstone crystals is at a much higher temperature than that of ice. Freezing is simply the transformation of a substance from liquid to solid as the temperature falls. The freezing point at which solid ice forms from water is generally 32°F (at normal surface pressure). In contrast, the freezing point at which most gemstones form from magma is hundreds or thousands of degrees Fahrenheit. Deep in the earth's crust and mantle (fig. 8), temperatures and pressures are much greater than on the surface of the earth. For example, at the crust/upper-mantle boundary, which is thought to average about 30 kilometers deep, the temperature is estimated to be about 930° to 1,650°F. At the mantle/outer-core boundary, which is thought to be about 2,930 kilometers deep, the temperature is estimated to be about 7,200°F. At 7,200°F and enormous pressure, most gemstone minerals would still be in a molten liquid state. As the magma moves upward through the crust, it begins to cool, and various minerals crystallize from the molten rock as they reach their respective freezing points. Many gemstones, such as Diamonds, were formed billions of years ago, when the upper subsurface temperatures of the earth were much greater than they are today, and the mantle was closer to the surface. The overall temperature of the earth has cooled and the crust has thickened considerably since the earth's early formation.

Crystal growth through precipitation from solution, in its simplest form, can be observed through a simple experiment to grow crystals of sugar, or rock candy. Take a quart-size container of water at room temperature and stir in sugar by the tablespoon to the point at which no more sugar will dissolve even with vigorous stirring. This makes a transparent solution of water that is saturated with sugar. Next put the clear solution into a large saucepan and bring it to a boil. Once the solution is boiling, remove it from the fire and continue to dissolve tablespoons of sugar until it is saturated once again at the higher temperature. The heated water holds much more dissolved sugar than the room-temperature solution does, because the saturation capacity of water increase as temperature rises. (Increasing the pressure on the solution will also increase the saturation capacity of water, although this is not so easy to achieve in a simple kitchen experiment.) As the solution cools, the saturation point for dissolved solids will fall, and it will become SUPERSATURATED with sugar. This will cause solid sugar crystals to form out of the solution. If you hang a string in the solution for 20 to 30 minutes while it is cooling, you will eventually see crystals form (it helps to keep a weight on the end of the string to keep it straight).

Gemstone crystals can form in much the same way, but with much higher temperatures and pressures. It might be hard to imagine Quartz crystals like amethyst or citrine resulting from a process similar to the one that produces sugar crystals, because the Silicates that we observe from day to day, such as Quartz or glass, do not noticeably dissolve in water at normal pressure (i.e., the water in your drinking glass does not dissolve the glass, nor do the waves of the surf dissolve the beach sand). But even Silicates such as Quartz will dissolve into solution at very high pressures and temperatures of the sort found deep underground and will crystallize back out of solution as it cools. We know this because we can approximate similar conditions of pressure and heat in a laboratory pressure chamber. In fact, this is part of the process of growing industrial Quartz crystals, which are used to make everything from optical Quartz for microscopes and cameras to semiconductors for computers and cell phones. The conditions in laboratories where Quartz crystals are made duplicate the pressures that exist deep under the earth's surface. As you go deeper and deeper under the surface of the earth into the lower crust, the pressure and heat continue to increase. Very deep beneath the surface, water flowing through rock under great heat and pressure becomes rich in dissolved minerals. This super-hot, mineral-rich water is called a HYDROTHERMAL SOLUTION, and it collects in rock cavities called hydrothermal vents (fig. 9). Just as dissolved sugar crystallizes as water cools and evaporates from the pan of sugar-saturated water, Quartz or other minerals crystallize out of hydrothermal solutions underground within rock cavities as the temperature and pressure decrease. It is believed that many of the finest crystals in nature have grown from these solutions.

To better understand crystal growth and where inorganic gemstones form, we need to understand some general geology. There are three basic rock types produced on our planet—IGNEOUS, SEDIMENTARY, and METAMORPHIC—and gemstones can be found in all three.

IGNEOUS ROCKS ("fire-formed" rocks) form from molten rock (MAGMA or LAVA) either aboveground as EXTRUSIVE IGNEOUS ROCKS, or underground as INTRUSIVE IGNEOUS ROCKS. Extrusive igneous rocks form from volcanic eruptions and typically consist of dark-colored, opaque basalts and ash. In general, extrusive igneous rocks are not usually a good source of quality gemstones. Exposure to surface temperature and pressure during solidification of the rock causes rapid chilling and does not generally allow for segregation and growth of large crystals. One exception is in basaltic rocks containing empty cavities from gas that was trapped in the magma before it solidified. Sometimes these cavities later fill with solutions rich in silica that create linings of amethyst, as well as other varieties of Quartz and other mineral crystals. A common example of such a crystal-lined cavity is called a GEODE or DRUSY CAVITY (see fig. 10). Drusy cavities are also occasionally found within sedimentary rocks where fossils have been dissolved away, leaving cavities that are later filled with crystals.

Intrusive igneous rocks are the product of much slower cooling and solidification processes than extrusive rocks and are formed within large reservoirs of molten rock deep beneath the earth's surface. These rocks typically consist of lighter-colored granites and are the source of most species of inorganic gems. Although often formed deep underground, over time erosion carves away overlying rock, bringing the intrusive rock to the surface or close enough to the surface to mine. The type of granite that is the most productive for a variety of large colored gemstones is called a PEGMATITE (fig. 11). Gems form in pegmatites through a combination of solidifying from molten rock and, more importantly, from precipitation from mineral-rich hydrothermal solutions in cavities and open seams and wide cracks within the pegmatite. Different chemical combinations result in different gemstone varieties. When the pegmatite and hydrothermal solution is rich in the element Boron, Tourmaline crystals can result. If it is rich in the element Beryllium, varieties of Beryl crystals can form. When the gemstone crystals form completely within the rock matrix of the pegmatite through solidification of magma, they are commonly highly fractured and filled with inclusions. But when the gemstones, or at least the growing ends of gemstones, form as free crystals within the hydrothermal cavities, they can form beautiful gem-quality crystals with smooth geometrically constructed faces. Hydrothermal solution-filled cavities allow gemstones to develop undisturbed, sometimes growing into large, transparent, spectacular crystals. Gemstone miners blast and excavate through rock in search of such pockets, or VUGS, which are sometimes also referred to as "nature's jewel boxes."

Sometimes pipes of magma moving to the surface transport gems formed deep beneath the earth's surface, as long as the gems have high-enough melting points not to be melted or otherwise destroyed by the great heat of the magma. One prime example of such a transported gem is Diamond. Diamonds that are mined today are thought to have been formed about 3 billion years ago around 100 to 150 miles below the surface of the earth in the upper mantle. These gems were transported by deep pipes of magma extending from the upper mantle to near the earth's surface. These carrot-shaped igneous pipes of now-solidified rock are called KIMBERLITE pipes. Kimberlite is the main type of rock mined for Diamonds. Only about 1 of every 200 kimberlite pipes is minable for Diamonds, and within the minable kimberlite pipes, it can take as much as 100 tons of ore to recover 2 carats of gem-quality Diamond. Other gems that form deep in the earth and are transported for long distances to the earth's surface include peridot and Pyrope Garnet.

SEDIMENTARY ROCKS form from eroded fragments or chemicals leached from other rocks. Most sedimentary rocks start out as layers of sediments that accumulate within a body of water. As these layers accumulate to great thicknesses, the overlying sediments create huge pressure and heat on the lower layers. Eventually the heat and pressure cause the lower layers of sediments to consolidate to form rock through a number of processes, starting with compaction. The compaction of the sediments eventually squeezes all water out of the sediments in a process called DEWATERING and chemically binds the particles of sediment together in a process called CEMENTATION. The overall process of sediments becoming sedimentary rock is called LITHIFICATION. It is believed that this process can sometimes take thousands of years. Few gemstones form from sedimentary rocks, but one exception is Opal. Opal forms through circulation of silica-rich water through sedimentary rocks, cavities in rocks, or hard CARBONATE organic materials such as fossilized bone or shells. Opal is consequently found in veins, fissures, and as opalized fossils. Although the source of silica-rich water that forms Opal is often from surface groundwater, it can also come from hydrothermal solutions rising from below the surface of the earth.

Gemstones may also be discovered in sedimentary deposits weathered from pegmatites and kimberlites. Over millions of years, erosion can reduce even pegmatites or other rocks to piles of rubble or gravel in a streambed. Because gemstones are generally very durable, they often survive the processes of weathering and erosion intact and are found as water-worn pieces of gravel. If you screen-wash stream sediment of eroded igneous and metamorphic rocks, you can occasionally find gemstones among the Quartz pebbles and other wear-resistant rock (fig. 285). Such gem-bearing gravel deposits are called PLACER DEPOSITS. Quick note: Because the sedimentary process did not actually create the gemstone itself, gemstones eroded from igneous or metamorphic rocks cannot really be considered gemstones of sedimentary rock origin. It is simply nature's way of mining gems from mostly non-sedimentary sources.

METAMORPHIC ROCKS, or "changed" rocks, form when igneous, sedimentary, or previously formed metamorphic rocks are altered by heat and pressure, turning them into a different type of rock. Most metamorphic rocks are derived from sedimentary rocks and fall into one of two categories: (1) those derived from sedimentary rocks rich in silica, and (2) those derived from Carbonates such as limestone. Metamorphic rocks derived from silica-rich sources may contain such gemstones as almandite Garnet, Tourmaline, Corundum, and spessartite Garnet. Garnets, for example, are sometimes found in mica schists, which are metamorphosed from mudstone and clay. Metamorphic rocks derived from Carbonates may contain Corundum, Grossular Garnet, Quartz, Spinel, Tourmaline, Zircon, or Zoisite. Rubies, for example, are sometimes formed from metamorphosed clay within a limestone that has been metamorphosed into marble.

Whether organic or inorganic, there are many ways in which nature produces a gemstone. The process of gemstone formation is an artifact of the evolution of the earth itself, and it has been in progress for billions of years. Next we will look at the CLASSIFICATION and diversity of gemstones. Just as there are shared special characteristics between species of organisms that help us to understand their organization and interrelationships, there are also shared special characteristics between varieties of gemstones that can help us better understand their organization and interrelationships.

the CLASSIFICATION of INORGANIC GEMS

You ask what is the use of classification, arrangement, systematization? I answer you: order and simplification are the first steps toward mastery of a subject ... THOMAS MANN, The Magic Mountain (1927)

In order to better understand the diversity of gemstone varieties, we must start with some sort of classification system. When we organize objects created by nature, science strives to develop logical, natural classifications. The differences and special similarities between gemstone varieties are numerous, wide-ranging, and often confusing. Why do some varieties have similar names? Why are all rubies Corundum, but not all Corundum rubies? Why do we group gemstones the way we do? How does it all fit together? To help answer these questions, we will explain the rationale and makeup of our organizational system and illustrate the interrelationships of gem varieties with a series of tree diagrams like the one in figure 12.

With natural history objects, whether living or inorganic, we classify and organize varieties of objects based on their relationships and natural characteristics. Whether we are classifying living species, whose structure and characteristics are derived from their DNA, or inorganic mineral species, whose structure and characteristics are derived from physical chemistry, we group them according to unique features. Just as we group all animal varieties with hair and mammary glands together in a class called Mammalia, we group all gemstone varieties whose basic chemical composition is Be₃Al₂ (SiO₃)₆ (also known as Beryllium Aluminum Silicate) in a group called Beryl. The gray wolf and the domestic dog are classified as subgroups of the biological species Canis lupus much like emerald and aquamarine are classified as subgroups of the mineral species Beryl. Species and other unambiguous groups are defined by special similarities shared by all of their members. And because we group these varieties based on what appear to be uniquely derived natural features (hair, chemistry, etc.), we consider these groups to be "natural" groups that express natural interrelationships between varieties. Our natural gemstone groups imply that an emerald is more closely related to an aquamarine than it is to a Pearl, just as a mouse is more closely related to a dog than it is to a plant. Thus, in addition to providing an organized classification of gemstone variety names, our classification system is an organized presentation of many properties associated with particular gemstone varieties. For example, all Beryl varieties—such as emerald, aquamarine, morganite, and heliodor—have (1) the same basic chemical composition, (2) a hexagonal crystal structure, and (3) a Mohs hardness of 7½–8. If an emerald is a Beryl and an aquamarine is also a Beryl, what makes them different? The Beryl varieties are distinguished from each other primarily by trace elements whose concentrations are generally too small to be included in the basic chemical formula. These trace elements are nevertheless responsible for the color differences between emerald, aquamarine, and other Beryl varieties.

(Continues...)

---

Excerpted from Gems and Gemstones by Lance Grande Allison Augustyn Copyright © 2009 by Lance Grande and The Field Museum. Excerpted by permission of THE UNIVERSITY OF CHICAGO PRESS. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

---