GO 340 Gemstones & Gemology

Emporia State University
Susan Ward Aber

www.emporia.edu/earthsci/amber/go340/chemical.htm

Crystal Chemistry

An introduction to crystal chemistry helps in understanding what gems are made of and how this affects gemstone environments and associations. Most gemstones are minerals, that are inorganic crystal treasures found in the natural world. Some gemstones are not minerals but are organically derived, such as amber and jet. Inorganic gems are crystalline, while organic gems are non-crystalline so described as amorphous, or without the crystalline form. Although chemical and crystalline properties are intertwined, all gemstones have a definite chemical composition. This lecture will focus on explaining chemical considerations of gemstones. Then the following lecture will concentrate on the crystalline nature of gemstones.

Composition

Jet is a bituminous coal sufficiently durable to be polished and valued as a gemstone. Diamond is another more durable and valuable gemstone known to, and owned by, many more people than jet. Structurally, jet is an amorphous rock, while diamond is crystalline mineral. Chemically, jet and diamond both contain carbon. Jet is a petrified version of biogenic debris that is plant remains, which collected in water-saturated environments such as swamps or bogs, that were compressed under the weight of accumulated sand, silt, and clay sediments to form what is considered a rock today. In contrast, diamond is a mineral composed of carbon atoms that formed deep underground under extremely high temperatures and pressures. Jet or coal is carbon plus additional chemical compounds; diamond is carbon in a native state. The composition of coal varies, specifically in the amount of carbon and volatile materials such as water, carbon dioxide and methane. Bituminous is only one variety of coal. The composition of diamond does not vary in the amount of carbon and, while it may contain trace amounts of other elements which provide color, it does not contain chemical compounds, but rather only the element carbon. Regardless of the organic or inorganic status of gems, all have a chemical story to tell.

A mineral is a macroscopically homogeneous solid, that grows in a symmetrical form, as a result of the regular geometrical arrangement of atoms, ions, and molecules. A well-developed mineral showing symmetry through its external form is referred to as a crystal, specifically a euhedral crystal. The crystal is a symmetrical or characteristic grouping of atoms within a mineral with a chemical composition expressed as a chemical formula, that is written in a shorthand manner. For example, the chemical formula of amethyst, a purple variety of quartz, is SiO2. Si is the shorthand abbreviation for silicon and O stands for oxygen. This chemical formula is derived from quantitative chemical analysis, that shows the amount of each type of atom or element present. The silicon and oxygen come together geometrically to form quartz.

Quantitative chemical testing of crystals can be destructive. Today, the composition of a gem is often determined with an electron microscrope or visible, ultraviolet, and infrared spectroscopy. These specialized instruments can detect elements, even in trace amounts. It is important to determine the chemical make-up because it aids in identification and classification, as well as in distinguishing natural from synthetic materials and and in detecting the agent responsible for the color.

Atoms and Ions

Atoms are the units composing all matter, designated as the smallest subdivisions that retain the characteristics of elements. Atoms, composed of protons, neutrons, and electrons, are arranged in the periodic table according to the number of protons. The atoms with lowest number of protons is on the left and highest on the right across the rows (e.g., http://www.webelements.com/index.html). With some exceptions, the atoms or elements that are found in gemstones are some of the most abundant in Earth's crust, such as silicon and oxygen.

A crystal grows in a regular array of atoms, or groups of atoms, coming together in a stacking arrangement built around a unit cell, that defines its crystalline structure. Atoms combine, or are held together, through atomic bonding, of which there are five types. Two of the most important in gem minerals are ionic [electron exchange] and covalent [ electron sharing]. Regardless of bond type, crystals must have a charge balance, which means negative charge shall be compensated with an equal amount of positive charge. When atoms loose or gain electrons they form ions, which become positively or negatively charged particles. These ions are termed cation (+) and anion (-). Thus, in the quartz example above, silicon has an oxidation state of Si4+ that combines with oxygen or O2-, and for a balanced charge two oxygens are needed per one silicon or SiO2. For more see the compounds - http://www.webelements.com/compounds/silicon), the web element essentials - http://www.webelements.com/silicon/, and the geology - http://www.webelements.com/silicon/geology.html from WebElements; and http://www.webmineral.com/data/Quartz.shtml from the Mineralogy Database of 4,442 minerals.

Characteristic gem properties are tied to chemical compositions. The fast-moving electrons in atoms are in energy levels or orbital shells around the atomic nucleus. The orbital shells farthest from the nucleus are incompletely filled thus electron movement between energy levels accounts for optical properties such as color, fluorescence, and phosphorescence. Electrons in the outermost shell are the valence electrons In the most stable configuration, these shells are filled. The outer shells are filled by gaining or losing electrons, creating positive and negative charges or cations and anions.

Some of the symbols for elements found in the most common crystals are shown below. It would be useful to memorize these shorthand symbols [shown in the chemical compositions of gemstones]. In the next section you can see how these atomic symbols come together for the chemical formula of gemstones.

Si - silicon Al - aluminum
O - oxygen Mg - magnesium
Fe - iron Ti - titanium
B - boron Li - lithium
Be - beryllium Cu - copper
Na - sodium K - potassium
Ca - calcium F - fluorine
Cr - chromium Mn - manganese
Zn - zinc Pb - lead
C - carbon Ag - silver
Au - gold Pt - platinum

Classification

Minerals are arranged into groups based on dominant anion (nonmetal) or anionic group. Minerals within the same chemical group have similar chemical properties, origins, occurrences, and physical properties. Examples of the different chemical or mineral classifications and the corresponding gemstone are shown below.

Chemical Classifications
Chemical Class Anion or Anionic Group An Example
Silicates Silicon and Oxygen Tourmaline, (Mg,Fe)2 SiO4
Oxides Oxygen Corundum, Al2O3
Carbonates Carbon and Oxygen Rhodochrosite, MnCO3
Native Elements One element, such as Carbon Diamond, C
Sulfides Sulfur Sphalerite, ZnS
Halides Halogen ions, such as Fluorine Fluorite, CaF2
Phosphates Phosphorus and Oxygen Apatite, Ca5(PO4)3 (F,Cl,OH)
Sulfates Sulfur and Oxygen Gypsum, CaSO4 2H2O

Chemical Bonding

The forces that bind atoms, ions, or ionic groups together in crystalline solids are electrical. The force type and intensity are responsible for the physical and chemical properties of minerals. The stronger the bond the harder the crystal, and higher the melting point. The high hardness of diamond is because of the strong electrical bonding forces linking the carbon atoms. These electrical forces holding inorganic minerals together are chemical bonds, such as: ionic, covalent, van der Waals, metallic, hydrogen, or some combination.

Ionic Substitution, Solid Solution, Exsolution

The solution or melt in which the mineral crystallizes can contain many elements not primary to the chemical composition. Such additional elements can be present in the crystal structure in minute amounts, partially substituting for a major element within the mineral. This ionic substitution can cause color, such as the chromium present in emerald (variety of beryl) creating green and the iron present in aquamarine (also a variety of beryl) creating blue. When ionic substitution is extensive, it is termed solid solution. Substitution is common if the ionic radius of normal ion versus substituted ion differs by less than 15% assuming the overall neutral charge of the mineral is maintained. An example is with the olivine group of minerals, where forsterite is a magnesium silicate, and fayalite is an iron silicate. The iron and magnesium substitute for one another because they have like charges and similar ionic radii size. "With no iron, forsterite is colorless, but with increasing iron the mineral darkens, going from light-toned olive green to dark green to black in fayalite" (Hurlbut and Kammerling, 1991, p. 30). The gem variety of olivine, peridot, has 10% of the magnesium of forsterite replaced by iron.

Exsolution is responsible for adularescence and asterism in gemstones. When minerals crystallize at high temperatures, high internal thermal energy allows for less stringent space requirements thus ionic substitution is extensive (Hurlbut and Kammerling, 1991, p. 30). When the mineral cools, the poorly fitting ions are forced to migrate through the crystal structure and a type of unmixing occurs. For example, a potassium-rich feldspar, called orthoclase, can tolerate sodium replacement of potassium at high temperatures, but forces these ions to migrate forming small localized areas of a sodium-rich feldspar, called albite. These pockets of albite intertwined with orthoclase result in an optical phenomenon called adularescence, which is an overall shimmery blue-white glow as well as localized flashes of color. This exsolution interaction gives the schimmer or adularescence phenomenon to moonstone.

An example of asterism is found in corundum referred to as star ruby and star sapphire. The aluminum and oxygen of corundum can accomodate titanium substituting for aluminum in the crystal structure. Upon slow cooling, the titanium reacts with the oxygen producing needle-like crystals of the mineral rutile. The hexagonal crystal structure of corundum constrains the rutile crystals to orient 60 degrees to one another and, if enough are present when the stone is cut en cabochon (a smooth convex top) perpendicular to the long c-axis direction, the star or asterism will result (Hurlbut and Kammerling, 1991, p. 30). Some corundum with titanium can be heat-treated and slowly cooled to enhance the asterism, while some corundum is heated and cooled rapidly to reduce the star effect and improve the transparency of the gem.


Milky white adularescence
of moonstone.


Star ruby showing asterism.
Image taken from the Mineral and Gemstone Kingdom.

The material for this section came primarily from:

  • Hurlbut, C. S., & Kammerling, R. C. (1991). Gemology. NY: John Wiley & Sons, Inc.

Optional and fun information on chemical elements:

Return to the Syllabus or move on to the next lecture.

This page originates from the Earth Science department for the use and benefit of students enrolled at Emporia State University. For more information contact the course instructor, S. W. Aber, e-mail: saber@emporia.edu Thanks for visiting! Webpage created: 1999; last update: January 14, 2009.

Copyright 1999-2009 Susan Ward Aber. All rights reserved.