Gemology
Encyclopedia
Science
Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...
dealing with natural and artificial gem
Gemstone
A gemstone or gem is a piece of mineral, which, in cut and polished form, is used to make jewelry or other adornments...
s and gemstone
Gemstone
A gemstone or gem is a piece of mineral, which, in cut and polished form, is used to make jewelry or other adornments...
s. It is considered a geoscience and a branch of mineralogy
Mineralogy
Mineralogy is the study of chemistry, crystal structure, and physical properties of minerals. Specific studies within mineralogy include the processes of mineral origin and formation, classification of minerals, their geographical distribution, as well as their utilization.-History:Early writing...
. Some jewelers
Jewellery
Jewellery or jewelry is a form of personal adornment, such as brooches, rings, necklaces, earrings, and bracelets.With some exceptions, such as medical alert bracelets or military dog tags, jewellery normally differs from other items of personal adornment in that it has no other purpose than to...
are academically trained gemologists and are qualified to identify and evaluate gems.
Background
Rudimentary education in gemology for jewelers and gemologists began in the nineteenth century, but the first qualifications were instigated after the National Association of Goldsmiths of Great Britain (NAG) set up a Gemmological Committee for this purpose in 1908. This committee matured into the Gemmological Association of Great Britain (also known as Gem-A), now an educational charity and accredited awarding body with its courses taught worldwide. The first US graduate of Gem-A's Diploma Course, in 1929, was Robert Shipley who later established both the Gemological Institute of AmericaGemological Institute of America
The Gemological Institute of America, or GIA, is a nonprofit institute dedicated to research and education in the field of gemology and the jewelry arts. Founded in 1931, GIA's mission is to protect all buyers and sellers of gemstones by setting and maintaining the standards used to evaluate...
and the American Gem Society
American Gem Society
The American Gem Society is a trade association of retail jewelers, independent appraisers, suppliers, and selective industry members, which was founded in 1934 by Robert M. Shipley, who also founded the...
. There are now several professional schools and associations of gemologists and certification programs around the world.
The first gemological laboratory serving the jewellery trade was established in London in 1925, prompted by the influx of the newly developed 'cultured pearl' and advances in the synthesis of rubies and sapphires. There are now numerous Gem Labs around the world requiring ever-more-advanced equipment and experience to identify the new challenges - such as treatments to gems, new synthetics and other new materials.
Gemstones are basically categorized based on of their crystal structure
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
, specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
, refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
, and other optical properties, such as pleochroism
Pleochroism
Pleochroism is an optical phenomenon in which a substance appears to be different colors when observed at different angles with polarized light.- Background :Anisotropic crystals will have optical properties that vary with the direction of light...
. The physical property of "hardness" is defined by the non-linear Mohs scale of mineral hardness
Mohs scale of mineral hardness
The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German geologist and mineralogist Friedrich Mohs and is one of several definitions of hardness in...
.
Gemologists study these factors while valuing or appraising cut and polished gemstones. Gemological microscopic study of the internal structure is used to determine whether a gem is synthetic or natural by revealing natural fluid inclusions
Fluid inclusions
thumb|250px|Trapped in a time capsule the same size as the diameter of a human hair, the ore-forming liquid in this inclusion was so hot and contained so much dissolved solids that when it cooled, crystals of halite, sylvite, gypsum, and hematite formed. As the samples cooled, the fluid shrank more...
, and included partially melted exogenous
Exogenous
Exogenous refers to an action or object coming from outside a system. It is the opposite of endogenous, something generated from within the system....
crystals to demonstrate evidence of heat treatment to enhance colour.
The spectroscopic analysis of cut gemstones also allows a gemologist to understand the atomic structure and identify its origin as it is a major factor in valuing a gemstone.
For example, a ruby
Ruby
A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum . The red color is caused mainly by the presence of the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires...
from Burma will have definite internal and optical activity variance as compared to a Thai ruby.
When the gemstones are in a rough state, the gemologist studies the external structure; the host rock and mineral association; and natural and polished colour. Initially, the stone is identified by its colour, refractive index, optical character, specific gravity, and examination of internal characteristics under magnification.
General identification of gems
Gem identification is basically a process of elimination. Gemstones of similar color undergo non-destructive optical testing until there is only one possible identity. Any single test is indicative, only. For example, the specific gravity of ruby is 4.00, glass is 3.15-4.20, and cubic zirconia is 5.6-5.9. So, one can easily tell the difference between cubic zirconia and the other two; however, there is overlap between ruby and glass.And, as with all naturally occurring material(s), no two gems are identical. The geological environment they are created in influences the overall process so that although the basics can be identified the presence of chemical "impurities" and substitutions along with structural imperfections vary thus creating "individuals".
Identification by refractive index
One test to determine the gem's identity is to measure the refraction of light in the gem. Every material has a critical angleCritical angle
Critical angle can refer to:*Critical angle the angle of incidence above which total internal reflection occurs*Critical angle of attack, in aerodynamics; the angle of attack which produces the maximum lift coefficient...
, at which point light is reflected back internally. This can be measured and thus used to determine the gem's identity. Typically, this is measured using a refractometer
Refractometer
A refractometer is a laboratory or field device for the measurement of an index of refraction . The index of refraction is calculated from Snell's law and can be calculated from the composition of the material using the Gladstone-Dale relation....
although it is possible to measure it using a microscope.
Identification by specific gravity
Specific gravitySpecific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
, also known as relative density, varies depending upon the chemical composition and crystal structure type. Heavy liquids with a known specific gravity are used to test loose gemstones.
Specific gravity is measured by comparing the weight of the gem in air with the weight of the gem suspended in water.
Identification by spectroscopy
This method uses a similar principle to how a prismPrism (optics)
In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...
works to separate white light into its component colors. A gemological spectroscope is employed to analyze the selective absorption of light in the gem material. Essentially, when light passes from one medium to another, it bends. Blue light bends more than red light. Depending on the gem material, it will adjust how much this light bends. Coloring agents or chromophores show bands in the spectroscope and indicate which element is responsible for the gem's color.
Institutes, laboratories, schools and publications
- Institutes and laboratories
- American Gem SocietyAmerican Gem SocietyThe American Gem Society is a trade association of retail jewelers, independent appraisers, suppliers, and selective industry members, which was founded in 1934 by Robert M. Shipley, who also founded the...
- AGS - Canadian Gemmological Association - CGACanadian Gemmological Association"The Canadian Gemmological Association is a Canada-wide professional organization which has set standards in the practice of gemmology....
- Canadian Institute of GemmologyCanadian Institute of GemmologyThe Canadian Institute of Gemmology is a non-profit, post-secondary gemological educational institution in Canada. The C.I.G. is the largest career training institution for gemmologists and gem technologists in Canada...
- Cigem - European Gemological Laboratory (USA)European Gemological Laboratory (USA)The European Gemological Laboratory or EGL USA is an independent gemstone grading organization. Originally the EGL USA was a part of the European EGL which was founded in 1974. In 1986 EGL USA became independent of the European organization, and today is not affiliated with other EGL laboratories....
- EGL USA - Gemmological Association of Great BritainGemmological Association of Great BritainThe Gemmological Association of Great Britain, also known as Gem-A, is an international gemmology education and qualifications body based in the UK.-History:...
- Gem-A - Gemological Institute of AmericaGemological Institute of AmericaThe Gemological Institute of America, or GIA, is a nonprofit institute dedicated to research and education in the field of gemology and the jewelry arts. Founded in 1931, GIA's mission is to protect all buyers and sellers of gemstones by setting and maintaining the standards used to evaluate...
- GIA - Gemmology Institute of Southern Africa - GISA
- Gemological Science International
- International Gemological InstituteInternational Gemological InstituteInternational Gemological Institute is a diamond, colored stone and jewelry certification organization. IGI is headquartered in Antwerp and has offices in New York City, Hong Kong, Mumbai, Bangkok, Tokyo, Dubai, Tel Aviv, Toronto, Los Angeles, Kolkata, New Delhi, Surat, Chennai, Thrissur,...
- IGI - Swiss Gemmological InstituteSwiss Gemmological InstituteThe is a major laboratory of gemmology located in Basel, Switzerland. It is a part of the Schweizerische Stiftung für Edelstein Forschung . It was founded on an independent basis, by trade organisations, in 1974. It soon began to expand, offering its independent services to the global gemstone...
- SSEF - International School of GemologyInternational School of GemologyThe International School of Gemology , founded in 2004 by Robert James FGA, GG, started as an Allied Gem Tutorial centre for the Gemmological Association of Great Britain...
- ISG - gci gemological centersGCI Universal Gemological LaboratoriesUniversal Gemological Laboratories is a modern gemological laboratory and a college for educational services. GCI founded in 1998. The laboratory’s main headquarter is situated right in the heart of the Israel Diamond Exchange in Ramat Gan, the largest in the world offering the greatest selection...
- GCI - Indian Institute of Gemmology - IIG
- Gemmological Institute of IndiaGemmological Institute of India-Description:The Gemmological Institute of India is a non-profit school in Mumbai that conducts courses on gemmology. It was founded in April 1971 by the Gem and Jewellery Exporters' Association, Bombay. It offers diplomas for gemmology and diamond grading, and courses on jewellery design, pearls...
- GII
- American Gem Society
- Publications
- Gems & GemologyGems & GemologyGems & Gemology is a quarterly scientific journal published by the Gemological Institute of America. Each issue is devoted to research on diamonds, gemstones, and pearls. Topics include geographic sources, imitations and synthetics, treatments, and identification techniques...
- Gems & Gemology