Logan Sapphire, 423 carats, Sri Lanka. Among largest-known faceted blue sapphires and set as a brooch. Photo by S.W. Aber, 4/2009, taken at Smithsonian National Museum of Natural History |
GO 340 Gemstones & Gemology
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Logan Sapphire, 423 carats, Sri Lanka. Among largest-known faceted blue sapphires and set as a brooch. Photo by S.W. Aber, 4/2009, taken at Smithsonian National Museum of Natural History |
GO 340 Gemstones & Gemology
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Gemology was first taught in the U.S. as a course for geology majors at the Colorado School of Mines from 1909-1913. The professor, Dr. G. Montague Butler, who was teaching the class left the school in 1913 and no one took his place! In 1916, the University of Michigan began teaching Gems & Gem Materials, which continued for many years. In 1920, Columbia University offered a gem class and by 1930 the University of Southern California was teaching classes that would lead to the founding of the Gemological Institute of America by that California teacher, Robert M. Shipley. Mr. Shipley (1887-1978) had 16 years of retail jewelry experience and 2 years of concentrated study in Europe, culminating in graduation from the Gemmological Association of Great Britian. He developed and integrated GIA into the business world of the US and Canada, with teaching the first GIA class in 1931. He was also the founder of the American Gem Society. Shipley retired in 1952 and Richard T. Liddicoat, Jr. took over as director of GIA. He had a master's degree in mineralogy and expanded the Institute to have resident and outreach programs worldwide. While many universities may offer a course or two on gemstones and gemology, GIA's primary purpose is to provide an authoritative, complete, and reliable education to students on gem identification, grading and appraising, stone repairing and designing, bead stringing, marketing and sales. GIA is a non-profit institution, funded by tuition fees, book and magazine publications, manufacture and gem grading and testing instruments, and laboratory fees for services offered to jewelers and the public.
In addition to GIA, other educational opportunities with gemstones exist. Paris Junior College, Paris, Texas offers a gemology certificate program, while Dr. Jill Banfield, a mineralogist, teaches a course, Gems and Precious Stones, from the University of California at Berkeley, although she originally developed the course at the University of Wisconsin, Madison. Neither school offers a formal gem program. The University of Texas at Austin has a class on gem cutting, which was featured in an August 2005 Lapidary Journal article that can be read online, www.lapidaryjournal.com/feature/aug05.cfm. While some courses exist, offerings from universities and colleges are limited. Gem societies and industries have taken the lead in education; for example, the American Gem Society offers courses and Jewelers of America have an educational arm as well. The International Colored Gemstone Association sponsors educational opportunities through industry, while other organizations outside the U.S. exist too. The Canadian Institute of Gemmology, Gemmological Association of Great Britain, Escola de Gemmologia in Spain, and the Swiss Gemmological Institute all have classes and materials online. So many opportunities so little time...
Websites with educational opportunities include:
Websites with general gem career information include:
Gemology Gemology is the distinct science of gems, most closely related to mineralogy, which is a branch of the geosciences that draws from chemistry, physics, and geology. Physics and chemistry help to relate the physical, chemical, and optical properties of minerals; geology creates the understanding of how the gem material formed and where to prospect. Gemology is not only concerned with the study of gem materials, but also gem testing and evaluation methods, cutting and polishing, synthetically manufactured gems, precious metals and alloys, grading, marketing, and sales.
Gemstone is a collective term for all objects used with ornamental stones for personal adornment that possesses beauty, durability, and stability. Beauty, a quality that varies among individuals, can be based on one or any combination of: color or lack of color, luster, transparency, or enhanced optical properties due to cutting and fashioning by the lapidary. Durability is the resistance of the gemstone damage and dependent upon physical properties, such as hardness and tenacity. Tenacity is the resistance to bending and breaking, while hardness measures the resistance to scratch. Diamond cannot be scratched by any other mineral and therefore, it is a gem with high hardness; diamond has low tenacity, in part due to perfect cleavage. Nephrite jade is relatively soft for a gem, easily scratched by dust in the air, but it is composed of fibrous crystals that resist breaking and therefore, it is tough or has high tenacity. Stability refers to the gem's ability to retain color inspite of heat, light, or chemical assaults.Of the 4,000+ known minerals, 70 possess qualifications for gemstones, but of these, only approximately 20 are commonly encountered (Hurlbut & Kammerling, 1991, p. 3). These minerals include: diamond, corundum (ruby, sapphire, star ruby and sapphire), beryl (emerald, aquamarine, morganite, goshenite, golden or heliodor), chrysoberyl (cat's eye & alexandrite), spinel, topaz, zircon, tourmaline (indicolite, rubellite, schorl, elbaite), garnet group (almandite/almandine, rhodolite, pyrope, grossular/tsavorite, spessartine, uvarovite), quartz: crystalline (rock crystal, amethyst, citrine, cairngorm or smoky, rose, aventurine, tiger's-eye, rutilated), quartz: cryptocrystalline (chrysoprase, carnelian, sard, bloodstone, agate, onyx, jasper, agatized or petrified wood), olivine peridot (chrysolite), jadeite jade, tremolite-actinolite or nephrite jade, spodumene (kunzite, hiddenite), feldspar group (microcline amazonite, labradorite, orthoclase moonstone, oligoclase sunstone), turquoise, lapis-lazuli, and opal.
Factors other than beauty, durability, and stability, that effect the value of a gemstone include: rarity, demand or fad, and portability. The rare gem, or one with limited available quantities, is considered desirable by those who can possess it. Rarity can lead to increased demand and prices, which can lead to new discoveries that will bring down the price. Demand and gem value fluctuate with current fads or fashion. Dark red garnets from Bohemia were very popular in the nineteenth century, but these jewelry pieces are not in demand today. Amber and turquoise jewelry was popular at the beginning of the twentieth century, fell out of favor, and has experienced a resurgance of popularity at the end of this century. Lastly, gemstones are valued for their portability, which necessitates a small volume, high value commodity.
Two traditional modifiers of gemstones, precious and semi-precious, are mostly discouraged from usage today as they imply one is more beautiful and valuable than another. In part, these designations began as a result of taxation on imported goods, where the highest taxes were paid on diamond, ruby, sapphire, and emerald, and other gems were taxed at a lower rate. Schumann (1997) stated the semi-precious stone designation was derogatory, or believed to denote a gem of lesser value and hardness, but he promoted the term, colored precious stone, as a new trade term used to include all colored and noncolored gems except for diamond (p. 10). All material that fits the definition is referred to as gemstones or gems and subdivided into diamonds and colored stones or colored precious stones.
Gem naming
Gems were named on the basis of color (prase for its green color), place of occurrence (agate for a Sicilian river), mysterious properties (amethyst believed to protect from drunkenness), a special characteristic (moonstone for a whitish-blue schiller effect), or to honor the discover or person (smithsonite for James Smithson). The gem and jewelry trade sometimes assigned names to promote sales, even if it was misleading (e.g., smoky topaz is in fact smoky quartz and not topaz). See Schumann (1997) for some common misleading names (p. 12-13). Today new gem names must be presented for evaluation by the Commission on New Mineral Names of the International Mineralogical Association (Schumann, 1997, p. 12). On an international scale, the Confederation Internationale de la Bijorterie, Joaillerie, Orfevrerie des diamants, perles, et pierres or CIBJO (International Association of Jewelry, Silverwares, Diamonds, Pearls, and Stones) oversee gem definitions and trade customs.
Mineralogy is the science or study of minerals including crystalline structure and chemical composition, mineral origin, occurrence, and associations. A mineral is naturally occurring homogeneous solid with a definite, but not fixed, chemical composition, characteristic crystalline structure, usually of an inorganic origin.
- Naturally occurring rules out products made in laboratory. Even though synthetic gem material has identical chemical, physical, and optical properties to the natural, they are not termed minerals or gems, but rather synthetics or created stones.
- Homogeneous is of one thing or cannot be physically subdivided into anything simpler.
- Solid implies material that is neither liquid or gaseous, for example ice is a mineral but water and steam are not.
- Chemical composition and crystalline structure refer to the atoms or ions that are arranged in a regular array in three-dimensions. Quartz has a fixed chemical composition, SiO2, arranged in a hexagonal pattern and referred to as a pure substance. The mineral group, olivine, has a definite but not fixed composition of (Mg, Fe)2SiO4. A magnesium silicate, Mg2SiO4, is at one extreme, while the iron silicate, Fe2SiO4, is at the other extreme. The iron and magnesium are close enough in size to substitute for one another in the crystal lattice. The more magnesium-rich olivine is known as forsterite and if it is gem quality, termed peridot.
- A characteristic crystalline structure means there is an internal structural framework in which atoms are arranged in a geometric pattern.
- Inorganic origin has been a necessary component in past definitions of minerals. However, according to Nickel (1995) a mineral is an element or chemical compound ...formed as a result of geological processes (p. 689). Nickel goes on to clarify
...biogenic substances are chemical compounds produced entirely by biological processes without a geological component (e.g., urinary calculi, oxalate crystals in plant tissues, shells of marine molluscs, etc.) and are not regarded as minerals. However, if geological processes were involved in the genesis of the compound, then the product can be accepted as a mineral.Thus, modern shells of marine molluscs are not mineral but calcite and aragonite constitutents that result from compaction of these marine organisms in limestones are considered mineral.
Mineraloid
"Crystalline" restricts the material defined as mineral and mostly excludes liquid and organic material. As such, naturally occurring material that is lacking a portion of the mineral definition criteria can be called mineraloid. Opal is a mineraloid because the crystalline structure is not well defined, whereas modern marine shell material and kidney stones are mineraloid because they are biogenic substances or organic in origin. Some gem material, such as pearl and mother-of-pearl, hold a kind of dual citizenship, called mineral by some and mineraloid by others. Although most pearls are created by bivalve mollusks, some are formed in caves as calcium carbonate precipitates around bat bones or some other small grains on the floor of the cave.
Classification scheme for minerals and gem materials
The system used by gemologists to classify gems includes: group, species, variety, organics, and synthetics (Hurlbut & Kammerling, 1991, pg. 5).
- The group is two or more chemically related gem species that have similar properties and structures (e.g., feldspars or garnets).
- The inorganic species are defined by chemical composition and crystalline structure (e.g., albite feldspar or tsavorite garnet, corundum or beryl).
- Organic gem material species are divided on composition and can include pearl, coral, shell, amber, jet (coal), and ivory.
- Species are further subdivided into varieties (e.g., moonstone albite feldspar, ruby and sapphire corundum, or emerald and aquamarine beryl). These classifications can be based on color, color distribution, transparency, optical phenomena, or any combination.
- Examples of subdivisions based on color are: the red variety of corundum is termed ruby, the blue variety is sapphire, and the many other colors of gem quality corundum are termed fancy sapphires, with varieties designated by color (e.g., green sapphire, pink sapphire). The mineral species beryl is found in a deep green variety, emerald; a medium blue variety, aquamarine; a pink variety, morganite; a colorless or "white" variety, goshenite; a yellow variety called golden beryl or heliodor; and a red variety, red beryl.
- Examples of subdivisions based on color distribution are onyx and agate. Onyx is a black and white, fine-grained variety of chalcedony, a microcrystalline quartz. Agate has many color possibilities with curved or angular banding.
- Transparency, or the ability of the material to transmit light, can be highly transparent in colorless quartz, medium transparency or translucent as in the reddish chalcedony called carnelian, or a low transparency, that is nearly opaque, in the red variety of quartz called jasper.
- Gem material can also be subdivided based on optical phenomenon. One example is asterism or the star effect in star ruby, shown by intersecting bands of light on a curved surface.
Gems In the U.S., the term "gem" can only be used for natural material, not synthetics. The high value of gems and a demand for perfection lead to synthesizing materials by the end of the nineteenth century. Corundum and spinel were the first gem materials to be created large enough for faceting. Rutile, quartz, alexandrite, and opal are more synthesized gem materials to follow. Industrial grade diamonds were first synthesized in 1953 and gem quality by the 1970s. When the lab created material is identical to the optical, physical, and chemical properties of its natural counterpart it is called synthetic. Lab created materials that mimic natural gems, that is resemble but not contain identical properties, are termed imitations or simulants. Common imitations are lab created glass and plastics, produced to resemble a great number of different gemstones. Another example of an imitation is synthetic cubic zirconia or CZ, originally marketed as a diamond simulant. CZ resembles diamond but does not have the same properties as diamond. A lab created material could be both a synthetic and imitation depending on how the material is marketed, for example purple corundum could be created and termed synthetic purple sapphire or used to resemble purple quartz and termed an amethyst imitation or simulant. Visit the USGS webpage on Synthetic or Simulant from the United States Geological Survey (USGS), http://minerals.usgs.gov/minerals/pubs/commodity/gemstones/sp14-95/synthetic.html.
Highly Recommended Readings!
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: saber@emporia.edu Thanks for visiting! Webpage created: November 15, 2000; last update: January 11, 2010.
Copyright 1999-2010 Susan Ward Aber. All rights reserved.
Gemstone
Factors effecting value
Mineralogy and Mineral