Geology of Gems banner

Chapter 1 - What Are Gems and Precious Stones?

There are hundreds of natural and man-made substances that are considered "gems" (or gem-like). This course focuses on the natural occurrence of these materials—how they form, and where they are found, and somewhat, how they are made. This chapter is an introduction to gems in relation to rocks and minerals. Following chapters expand on these concepts and introduce basic concepts fundamental to the science of geology—as related to the origin and occurrence of gems in the natural environment.

What is Gemology?
Gemology is the study of gems and precious stones, both natural and artificial (synthetic). Gemology is associated with the science of mineralogy (the study of the physical and chemical properties of minerals) and the technological manufacture of gemstones for jewelry and other uses.

"What are gems and precious stones?"

Simply defined, a gem is "a precious or semiprecious stone, especially when cut and polished or engraved." However, this definition is broad and falls apart when examined closely. Jewelers or people associated within the gem industry often use the terms gem, gemstone, and precious stone interchangeably. More specifically, gems consist of durable (hard) crystalline substances with unique chemical and physical properties. Gems are crystalline minerals that occur naturally, or in some examples can be man-made (synthetic). In this course, the word "gem" is used for select crystalline minerals whereas "precious stone" applies to everything else that are not of clearly definable as "a crystalline mineral" in composition and physical appearance. (These concepts are discussed below.)

Gems like diamonds, emeralds, and rubies are minerals whereas the term precious stone generally applies better to materials that can not be purely defined as "crystalline minerals" (having a crystalline atomic structure) (see Figure 1-1. Classic gems). Some gem minerals that occur as microcrystalline aggregates include opal, malachite, and jadeite. Commonly used precious stones in jewelry including amber, obsidian, jet, agate, shell, bone, and even glass, are not minerals. The word "precious" is also a vaguely or misleading term that applies some obscure meaning of "value." Not all "gem minerals" are "precious" or have great value. The term "precious" applies to classic "rare" gems like diamonds, rubies, and emeralds, whereas the word "semiprecious" is used for materials that either are not considered "rare" (hence considered not a valuable). Again, the application of these terms is subjective at best. For instance, the gemstone amethyst occurs in abundance in many locations around the and is considered only "semiprecious." In contrast, the rare occurrence of diamonds in the world and their relatively universal cultural mystique make them the icon of "precious gemstones."

Click on small images for a larger view throughout this website.

Precious gemstones
Fig. 1-1. Classic gems.
Most gems are minerals, however, some materials referred to as gemstones are actually rocks or natural organic compounds - there is a difference! Of the materials listed above, which is technically not a mineral? (Opal does not have a define crystalline structure!)
From the perspective of a gemologist a mineral is an exciting thing! Most gems are minerals. Even common minerals in their natural form can be quite beautiful and valuable, artistic if not jewelry. But what is a mineral, and how do we distinguish it from other substances?

What is a mineral?

A mineral is a naturally occurring, inorganic (never living) solid with a definite internal arrangement of atoms (crystal structure) and a chemical formula that only varies over a limited range that does not alter the crystal structure. On Earth, more than 4,000 minerals have been identified, of those fewer than 2 dozen are common in Earth's physical environment (Figure 1-2 shows common rock-forming minerals). In contrast, minerals considered "gems" are, mostly, exceedingly rare.

What is the difference between a rock and a mineral?

A rock is a relatively hard, naturally formed mineral or petrified matter; a naturally formed aggregate of mineral matter constituting a significant part of the earth's crust; stone is another common term used to describe rock. Rocks consist of one or more minerals. Figure 1-3 shows how minerals can be combined to form different kinds of rocks that form under different environmental conditions (discussed in more detail in Chapter 3).

Why aren't gems found everywhere?

The mineral composition of a rock reflects the physical environment and geologic history where a rock formed. Rock form in a variety of geologic setting ranging from locations on or near the earth surface, deep underground, or even in outer space. Most of the rocks we see on the surface of the planet formed by processes that happened long ago, but we can see these processes actively taking place in many places. Rapid rock formation can be seen happening such as lava cooling from a volcanic eruption in places like Hawaii or Iceland. However, most rocks we see around us form very slowly in settings that are not visible on the land surface. Slow processes creating rocks can be inferred by observing reefs growing in the oceans, or sediments being carried by flowing water in streams or moved by waves crashing on beaches. We can see sediments being deposited, but we cannot see them turning into stone because the process may take thousand or even millions of years. This is explained in more detail in Chapter 3 with an introduction to the "rock cycle".

Before examining gems and minerals further, a basic discussion of the fundamentals of chemistry are essential.

Rock Forming Minerals
Fig. 1-2. Rock-forming minerals are the most common (and abundant) minerals found on our planet Earth.
Minerals forming rocks
Fig. 1-3. Combinations of common minerals occur in different kinds of rocks. The kind of rock depends on the geologic setting where they form: igneous, sedimentary, or metamorphic.

Essential concepts of chemistry related to earth materials

Basic concepts of chemistry are essential to understanding the physical and chemical properties of earth materials (minerals, rocks, organic matter, etc.). The chemical characteristics of earth materials are reflect the environments how and where they are formed, they also determine their potential fate when exposed to chemical changes. For instance, rocks and minerals formed deep underground may not be stable in the surface environment where they are exposed to water, air, temperature changes, and other physical and chemical conditions.

All matter is made up of atoms, and atoms are made up of atomic particles (electrons, protons, and neutrons - see Figure 1-4). A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Common examples of elements are iron, copper, silver, gold, hydrogen, carbon, nitrogen, and oxygen. The Periodic Table is a list of 108 known elements arrange by atomic number (see Figure 1-5). Of these, 92 are naturally occurring (prior to development of artificial nuclear research and development). The lightest element, hydrogen, has one proton, whereas the heaviest naturally occurring element, uranium, has 92 protons.

Many elements have one or more isotopes. Isotopes are each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties. Some isotopes are not stable and ultimately break down or change in other elements. I this case, the isotope is considered a radioactive form of an element.

A molecule is a group of atoms bonded together, representing the smallest fundamental unit of a chemical compound that can take part in a chemical reaction

chemical compound
—a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together in a defined spatial arrangement by chemical bonds.

All minerals are chemical compounds, but by comparison relatively few compounds are naturally occurring minerals!

mixture—solid, liquid, or gas composed of two or more substances, but each keeps its original properties. Note that earth materials (rocks and sediments), magma (molten rock), seawater in oceans, and the atmosphere are all mixtures.
An atom of lithium is composed of a nucleus with 3 protons and several nuetron, and surrounded by a cloud of 3 spinning electrons
Fig. 1-4. Structure of an atom: this example is the element lithium composed of a nucleus of 3 protons, 4 neutrons, and an outer shell of 3 electrons spinning around the nucleus.
Periodic Table
Fig. 1-5. Periodic Table of the Elements
(Lawrence Berkeley National Lab)

Chemical Bonds

Molecular compounds are held together on an atomic level by chemical bonds. Three types of chemical bonds include ionic bonds, metallic bonds, and covalent bonds. The types of chemical bond influence the physical properties of the molecular compounds they form.

Molecular compounds held together by ionic bonds are salts. Salts readily precipitate from and dissolve in water. Natural salts like halite (NaCl) and gypsum (CaSO4) are soft minerals (not suitable for gems because they scratch or fracture easily, and can dissolve in water; see Figures 1-6 and 1-7).

Metals are held together by metallic bonds. Compounds with metallic bonds transmit electricity. Metalloids are intermediate between those of metals and solid nonmetals. Although most elements are metals (all those on the left and center parts of the Periodic Table), only a few elements occur naturally in metallic form including gold, platinum, copper, iron, and mercury (in liquid form). Some minerals are metalloid compounds including pyrite (FeS2), magnetite(Fe3O4), and galena (PbS)(see Figure 1-8).

Molecular compounds held together by covalent bonds and are non-metallic compounds. These materials can form crystal complexes and do not transmit electricity and tend to be durable compounds. Most gems are non-metallic compounds. The mineral quartz (SiO2) is a non-metallic crystalline compound (see Figure 1-9).
Salt dissolves in and precipitates from water Salt deposits in Death Valley
Fig.1-6. Salt crystals are held together by ionic bonds. Salt compounds dissolve in and precipitate from water. Fig. 1-7. This view shows salt crystals precipitating on a dry lakebed in Death Valley, California.
Metals (native copper and gold), magnetite and pyrite Quartz crystal
Fig. 1-8. Metallic bonds occur in metallic minerals (like native copper and gold) and metalloid minerals (like magnetite and pyrite). Fig. 1-9. Most minerals are non-metallic crystalline compounds held together by covalent bonds (and will not transmit electricity).

What Are Crystals?

A crystal is a piece of a homogeneous solid substance having a naturally geometrically regular form with symmetrically arranged plane faces. A crystalline substance has the structure and form of a crystal or is composed of crystals. In our world there are many crystalline substances; salt (NaCl, sodium chloride) or as geologists call it, halite, is usually precipitated from evaporating water without organic processes and is thus a mineral.
Sugar (C6H12O6, sucrose) also forms crystals when precipitated from water, but because it is "organic" and therefore it is not a mineral.

Very few things that are solid are not crystalline. However, because in our world much of what we see is formed by life processes, most observed solids are not minerals. You will quickly argue that rocks are all around us and that they are made of minerals, however in terms of variety only about a dozen minerals (the rock forming minerals) are abundant and in fact there is a great deal more variety of organic solids around us than minerals. We rarely spend much time observing minerals and really the average beginning student knows almost nothing about them and their properties. There are around 4,000 known species of minerals and most of them are extremely rare.

Of the few inorganic, non-crystalline solids dealt with in gemology, glass is the most important. Glass forms by rapid cooling of substances that have been melted to a liquid. There are natural and man-made glasses. Man made glass is often used as a gem substitute or simulates and should be suspected at all times. Fig. 1-10 shows an organized mineral structure with an ordered arrangement of atoms and a disorganized substance without a crystal structure. Both can be solid, but a disorganized solid is called non-crystalline or amorphous. Both are held together by chemical bonds, but crystalline solids have an ordered structure that fills space in 3 dimensions. With a crystalline structure you can predict where the next atom can be found in the structure.
crystalline versus noncrystalline atomic structural arrangements
Fig. 1-10. Minerals are made up of atoms arranged in a crystalline structure. The crystals may range in size from on a microscopic scale to full-sized visible masses. Non crystalline (amorphous) substances (like glass) have no orderly arrangement of atoms.
Crystallography is the scientific study of crystals!
Minerals are chemical substances composed of atoms arranged in unique crystal structures.

A crystal structure describes a highly ordered repeatable arrangement of atoms. Note that there is an important difference between the chemical formula of a mineral and the molecular crystal structure of a mineral! Only when molecules are arranged in an orderly, repeatable symmetric pattern will it be considered a mineral. For instance, water (H2O) is not a mineral but ice is! A crystal structure can be thought of as an infinitely repeating array of 3D 'boxes', known as unit-cells. The unit cell is calculated from the simplest possible representation of molecules to form a crystal structure.
cubic crystal forms hexagonal forms
Perhaps the simplest way to illustrate the arrangement of atoms into a geometric crystal structure is to use marbles stacked in different ways. The forms shown in Figures 1-11 and 1-12 show selected examples of some of the simplest forms created with vertically stacked marbles and marbles stack in an offset arrangement that is the most tightest possible with spheres of uniform size. Figure 1-11 shows cubic and rectangular cuboids and octagon (double pyramid) forms. Figure 1-12shows hexagonal prism and pyramidal forms that can be created by the same stacking arrangements sown in Figure 1-11. Figure 1-11. Stacked marbles illustrate atomic arrangement of crystal forms. Cubic, rectangular cuboid, and octahedral forms can form from the two arrangement of marbles. Figures 1-12. The same arrangement of stacked and offset marbles can produce hexagonal crystal forms (prisms and pyramids). Add more layers of marbles (atoms) and the crystal grows larger.
Minerals are chemical substances composed of atoms arranged in unique crystal structures.

A crystal structure describes a highly ordered repeatable arrangement of atoms. Note that there is an important difference between the chemical formula of a mineral and the molecular crystal structure of a mineral! , occurring due to the intrinsic nature of molecules to form symmetric patterns. A crystal structure can be thought of as an infinitely repeating array of 3D 'boxes', known as unit-cells. The unit cell is calculated from the simplest possible representation of molecules

The figures below illustrates the crystal structures of minerals (magnified and expanded millions of times from a molecular scale).

Figure 1-13 shows halite [or common salt (NaCl)] which consists of two elements sodium (Na) and chlorine (Cl) that when combined in a repeating arrangement in a crystalline structure (see Figure 1-14). The arrangement of atoms in a cubic structure of the mineral, halite, is repeatable whether on an atomic scale or a microscopic scale (as in table salt) or macroscopic (fist-sized chunk)(see Figure 1-14). Halite crystals grow from precipitating from water and is manufactured worldwide by evaporating seawater (see Figure 1-15 as an example where and how it is done).
halite crystal clusters Salt Molecule Rock salt has cubic crystals no matter what size the crystal Salt evaporation pond near Dead Sea (Jordon and Israel)
Fig. 1-13. The mineral halite is the raw material in the manufacture of table salt or, for melting ice on frozen walkways, rock salt. Fig. 1-14. Crystal structure of salt: the mineral halite
Chemical formula: NaCl
Crystal form: cubic
Fig. 1-15. Halite (salt) has the same cubic crystal shape no matter if the sample is fist-sized or ground up to table salt . Crystals show 90º corner angles. Fig. 1-16. Halite is mined or is manufactured by concentrating sea water or salty water, as shown here in these evaporation ponds located near the Dead Sea.
Figure 1-17 shows the crystalline structure of fluorite. Although the chemical formula of fluorite is CaF2, eight atoms of calcium (Ca) and sixteen atoms fluorine (F) are needed to make the minumum-sized unit cell of the crystal structure of mineral fluorite (see Figure 1-18). Billions of unit cells are required to combine to make a single small crystal you can hold in your hand! The geometric arrangements of unit cells on an atomic scale determine how a crystal appears on a macroscopic (visible) scale (Figure 1-19). Because minerals have repeating geometric arrangement of atoms in crystal lattices, crystals can be fashioned into a variety of shapes that are compatible with the crystal structure. In the case of fluorite, which usually exists in cubic crystals, it can be split and shaped into octahedral shaped crystal specimens (commonly sold in rock shops)(see Figure 1-20). The arrangement of molecules within a crystal structure determines how a mineral crystal can be split and cut into geometric shapes, including shapes used in finished gemstones (as illustrated in Figures 1-11 and 1-12). It is important to note that in most cases, the shape of a fashioned gemstone is nothing like the shape of a natural mineral crystal shape as they appear in nature. A gemologist cutting gemstones will closely examine the crystal structure of a mineral before faceting it into a gemstone.
Fluorite fluorite crystal structure Cubic crystal structure of fluorite fluorite octohedrons
Fig. 1-17. Cubic crystal masses of the purple mineral fluorite (yellow is calcite) Fig. 1-18. Unit cell of the cubic crystal structure of the mineral fluorite
Chemical formula:CaF2
Fig. 1-19. Unit cells of the mineral fluorite combine to form an extended crystal lattice in three directions. Fig. 1-20. Although the crystal structure of fluorite is cubic, chunk of fluorite crystals can be split along cleavage planes to form octahedral shaped crystals.
The mineral calcite is perhaps the most amazing mineral. It has many crystalline forms and can form in many geologic setting. It is also an exceeding important mineral resource - it is used in the manufacture of cement, and probably used somehow in the process of manufacturing of thousands of compounds used in industry ranging from the manufacture of steel to the production of medicines and food. Calcite consists of a crystalline structure composed of molecules of calcium carbonate (CaCO3). From a "point blank" science view, the Ca comes from the earth, and the CO3comes from the atmosphere, and nearly all the CaCO3 is deposited in the oceans and by water underground.

It is important to note that CaCO3 is a chemical formula representing a single molecule. It takes many molecules of CaCO3 to make the unit cell of "pure" mineral calcite (see Figure 1-21). A pure specimen of calcite (CaCO3) would be perfectly clear form called "iceland spar" (discussed more with Fig. 1-50 below). With pure calcite the unit cells will have 28 molecules of CaCO3, however, there can be a variety of other elements that can be substituted for a few of the calcium and carbon atoms with a unit cell, and it will keep the general crystal pattern of calcite. Elements including sodium, magnesium, iron, zinc, chromium, strontium, barium, and sulphur and can sneak into the structure of the unit cell and still maintain the general character of crystalline calcite. However, these differences can result in varieties calcite with some subtle differences in physical properties including color, crystal form, and special properties including fluorescence, phosphorescence, and thermoluminescence (discussed below). Calcite also fit the true definition of a "true" mineral because it can also be of biological origin—a product of respiration, excretion, and skeletal structures in plankton, microbial deposits, algal and coral reefs, and incorporated tissue of plants, invertebrate shells, and the shell of eggs.

The arrangement of unit cells can produce differently shaped crystals. For example, calcite can form several variation including "dogtooth spar," "nailhead spar," and combined forms of these crystal varieties (see Figure 1-22 and 1-23). This variation of crystal shapes is related to the physical conditions of where the mineral formed.
  Calcite crystal structure calcite crystal forms Variety of calcite crystals aragonite
  Fig. 1-21. Structure of the unit cell of the mineral calcite (calcium carbonate - chemical formula: CaCO3). It takes 28 molecules of CaCO3 to create the a single hexagonal shaped unit cell of calcite illustrated here on an atomic level. Fig. 1-22. Calcite crystals have a hexagonal crystal structure. The alignment of unit cells can form different crystal forms, all in hexagonal arrangement. Crystal forms of calcite include dogtooth spar, nailhead spar, and combined forms. Figure 1-23. Crystal forms of calcite: dogtooth spar, nailhead spar, and combined form. It takes billions of unit cells combined to form visible crystals. Crystals like these form in open cavities underground where the crystals grow slowly over time. Figure 1-24. The mineral aragonite is also composed of calcium carbonate (CaCO3), but the molecules are in a different crystalline structural arrangement than calcite. Calcite has a hexagonal crystal structure, whereas aragonite has an orthorhombic crystal structure (see crystal systems below).
calcite rhomb calcite rhomb with overlay of calcite mineral structure Calcite rhombahedral crystal structure dolomite crystals
Figure 1-25. Calcite crystals can be split along mineral cleavage planes to form blocks with perfect rhombohedral shape. Note that this rhombohedral shape still retains its internal hexagonal crystal structure! Figure 1-26. Cleavage planes are naturally weak zones within a crystal structure. This image illustrates how molecules of calcium carbonate line up in repeating arrangement forming the rhombohedral shape. Note the hexagonal shape of the crystal block. Figure 1-27. Calcium carbonate molecules arrange in the rhombohedral structure of the mineral calcite. When a crystal of calcite is crushed it tends to split into many small pieces that retain a rhombohedral shape. These "rhombs" can range in size from microscopic to large blocks. Figure 1-28. Another mineral, dolomite, has a chemical formula of CaMg(CO3)2. It has a trigonal-rhombohedral crystal form. The pink color comes from traces of iron within the crystal structure.

What Is Mineral Cleavage?

Mineral cleavage is the tendency of crystalline materials to split along definite crystallographic structural planes (or, for clarification, to break along smooth planes parallel to zones of weak bonding in crystalline substances). For instance, as illustrated in Figures 1-22 to 1-24, calcium carbonate forms crystalline forms, calcite and aragonite. However, when a mineral sample of calcite is crushed, the crystals shatter along planes of weakness in the crystal lattice. In the case of calcite, the crystals break along 3 planes of weakness within the crystal structure, forming rhombohedral blocks. These cleavage planes are always at the same angles (in 3 directions, the x, y and z dimensional axes)(see Figures 1-25 to 1-27). The rhombohedral shape of the calcite crystal fragments are always the same, whether as a hand-size specimen or crystal fragments on a microscopic level. (The same is true for halite illustrated in Figure 1-14, except the salt crystals are cubes instead of rhombs.)
Physical and Chemical Properties of Minerals (including gems) continues in Chapter 2.
Click below to proceed to: