GO 340 Gemstones & Gemology
Some gems fall inbetween strictly organic and inorganic such as pearl. The origin of pearl is clearly organic because this gem is harvested from the organism. The shell or exoskeleton of South Sea mollusk is shown in the image below. However, pearls can form in caves, far from the oysters living in the sea. Pearls are composed of calcium carbonate, and a pearl forms in consecutive rings that layer around a central nucleus. In the case of the image shown, the oyster secretes the calcium carbonate around an irritant, which could be a parasite drilling in through the shell or grain of sand unfortunately sucked in with other nutrients when the oyster was feeding. In contrast, cave pearls are also composed of calcium carbonate that dissolves from limestone and is re-precipitated around a nucleus of tiny fragments such as bat bones. Cave pearls may lack the outer glowing luster and orient produced by the oyster, but otherwise, the chemical composition and crystalline structure is identical.
Silver-lipped Pearl Oyster Shells are from the South Sea species, Pinctada maximus, which is the largest pearl-producing oyster known. The pearls shown are termed baroque because of the irregular shape that results from calcium carbonate or nacre collecting in pools around the irritant. These two are cultured and 38 and 33 mm (1.5 and 1.3 inches) in diameter. Pearls were a gift to the Smithsonian from Paspaley Pearls Pry. Ltd., Northwest Australia. Photo by S.W. Aber, 4/2009, taken at Smithsonian National Museum of Natural History.
Polished rough shapes of Jet
Photo by S.W. Aber 2009; Tucson Gem Shows.
|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 or plant remains, collected in water-saturated environments such as swamps or bogs and 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 underextremely 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 has a symmetrical, characteristic
grouping of atoms within a mineral coupled with a chemical composition expressed as a chemical formula. The formula can be written as symbols using a shorthand code. 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
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 IonsAtoms 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.
Amethyst and rock quartz column. Photo by S.W. Aber 2008; Tucson Gem Shows.
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 the Mineralogy Database, http://www.webmineral.com/data/Quartz.shtml, is now listing, as of December 31, 2009, the newest revised number of 4,714 individual mineral species as 4,714 according to the IMA (www.webmineral.com/); this is up from the previous count of 4,442 minerals (2010).
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|
|Chemical Class||Anion or Anionic Group||An Example|
|Silicates||Silicon and Oxygen||Tourmaline, (Mg,Fe)2 SiO4|
|Carbonates||Carbon and Oxygen||Rhodochrosite, MnCO3|
|Native Elements||One element, such as Carbon||Diamond, C|
|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|
|All photos, except apatite, were photographed at Smithsonian National Museum of Natural History. Photos by S. W. Aber 2009.|
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
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
The material for this section came primarily from:
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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: firstname.lastname@example.org Thanks for visiting! Webpage created: 1999; last update: August 30, 2012.Copyright 1999-2012 Susan Ward Aber. All rights reserved.