precious metal elements (e.g., Au, Pt, Ag, and Rh) occur very rarely in the Earth’s crust. Figure 1.4 is a graphic showing the relative abundance of the elements (vertical axis) against their atomic number (horizontal axis). Note the highlighting of the top eight rock‐forming elements, the rarest metals, and the Rare Earth Elements (also known as the Lanthanide Series). Because of the large variability in abundance of elements, the vertical scale in Figure 1.4 is logarithmic.
Table 1.2 Approximate abundance of dominant elements in the Earth’s crust. Data from Mason and Moore (1982).
| Element, Symbol | Abundance in Earth’s crust (weight %) |
|---|---|
| Oxygen, O | 46.60 |
| Silicon, Si | 27.72 |
| Aluminum, Al | 8.13 |
| Iron, Fe | 5.00 |
| Calcium, Ca | 3.63 |
| Sodium, Na | 2.83 |
| Potassium, K | 2.59 |
| Magnesium, Mg | 2.09 |
| All others | 1.41 |
Figure 1.4 Relative abundance of the elements in the Earth's crust as compared to silicon atoms. Haxel et al. (2002) / U.S. Geological Survey / Public domain.
Most elements are generally very rare; their concentrations are therefore commonly reported in parts per million, or ppm. The value of “1 ppm” indicates that there will be one gram for every million grams of the material (i.e., 1 gram in every tonne). A value of 10,000 ppm is equivalent to 1% (10,000 parts for every million). The level of concentration in the Earth's upper crust for gold is approximately 0.002 ppm. This is the same as 2 parts per billion (ppb), meaning for every billion atoms counted, only two will be gold!
1.3.7 Compounds and Mixtures
Elements combine and interact through chemical bonds. When two or more elements join together they form a compound. As with an element, a compound is represented by symbols called chemical formula. Examples of common compounds and their formulae are water (H2O), composed of hydrogen (H) and oxygen (O), and common table salt, (NaCl) composed of sodium (Na) and chlorine (Cl). Most gemstones are compounds, such as sapphire (Al2O3), composed of aluminum (Al) and oxygen, and emerald (Be3Al2Si6O18), composed of beryllium (Be), aluminum, silicon (Si), and oxygen.
Mixtures differ from compounds in that a mixture is comprised of two or more compounds that are not interacting through chemical bonding. The world of rocks and minerals is a perfect example of this. Minerals, like sapphires, are compounds that are held together through chemical bonding. Rocks, on the other hand, can be thought of as bulk mixtures of minerals held together through an interlocking physical network of mineral grains not through chemical bonding. This is similar to how furniture can be made whole with joints, nails, and screws that physically hold the pieces together while the individual pieces are independently held together through chemical bonding that make up the wood itself.
1.3.8 Chemical Bonds
Chemical bonds are attractive forces between atoms. In a simplistic view, they form when outer electrons from two or more atoms interact, resulting in their atoms becoming “joined”. The two main types of bonding seen in nature are ionic and covalent.
Ionic bonding occurs between two atoms, one with a strong tendency to gain electrons (the anion) and the other with a strong tendency to lose electrons (the cation). Here, the cation can be thought of as having “donated” electrons (therefore becoming positively charged with less electrons than it started with) to the anion (which then becomes negatively charged with the extra electron). Normal table salt (NaCl) is a good example of ionic bonding where sodium, which usually has a valence of +1, combines with chlorine, which usually has a valence of −1, in a one‐to‐one ratio. This is a simple case and in reality most compounds are more complicated than this. In the mineral world, most bonding that occurs is ionic bonding, where electrons are donated from cation to anion.
Covalent bonding occurs when atoms “share” valence (or outermost) electrons between them. Covalent bonding is much more common in organic compounds (those that form living matter). However, in the gem world, this type of bonding is best observed in diamond. Diamond is a compound made up of carbon (C). The carbon atoms share electrons between them in a tight 3D network forming “molecules” of interconnected carbon atoms. These covalent bonds are very strong and give diamond its hardness and strength.
Another type of bonding that is less common in nature, but commonly studied by scientists, is metallic bonding. This is the type of bonding that, not surprisingly, is typical in native metals such as silver, gold, and copper. Valence electrons in metallically bonded compounds are shared throughout the entire material (not simply between two atoms) and are “free” to move about.
Van der Waals bonding is another type of bonding found in nature but seldom in minerals. This form of bonding is quite weak and when present often defines cleavage planes, such as in the mineral graphite.
1.4 Physical Properties of Minerals
Each mineral has a distinct chemical composition and internal arrangement of atoms and bonding. Accordingly, every mineral will exhibit distinct physical properties.
Color is the most familiar of the physical properties and often what draws people to minerals and gemstones. In a simple sense, color is described as the outward appearance of mineral as observed by our eyes. It is a function of the nature of the incident light and its interaction with the mineral, including effects from transmission, reflection, refraction, scattering, and absorption of visible light. Minerals that display little to no absorption of visible light will appear white if light is scattered off the surface (as in kaolinite) or transparent if light is transmitted through the crystal (as in pure quartz). Despite being an easy to observe property, color is actually not a very good diagnostic property on its own. This is because many minerals can exhibit a range of colors depending on the impurities within them. This concept is developed in greater detail later, as it is critical to the world of gemstones.
Luster refers to how visible light interacts with the surface of a mineral. Minerals with metallic luster show strong reflection of light off their surfaces, as in the case of polished gold or the mineral pyrite (iron sulfide). Minerals with nonmetallic luster generally absorb at least some of the incident light in addition to reflection. Types of nonmetallic luster include vitreous, resinous, dull, earthy, pearly, greasy, silky, and adamantine.
Streak refers to the color of a mineral after it has been ground along the surface of a ceramic or porcelain streak plate. The process of grinding the mineral into finer particles results in a more even display of a mineral’s color under incident light. Streak is often more diagnostic for minerals than color.
Habit describes the common ways that a mineral crystallizes into macroscopic