Gemology

Gemology is the science dealing with natural and artificial gemstone materials. It is a specific interdisciplinary branch of mineralogy. Some jewelers (and many non-jewelers) are academically trained gemologists and are qualified to identify and evaluate gems.

History

Rudimentary education in gemology for jewellers 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 as an Education Committee for this purpose in 1908. The committee emerged as a distinct branch of NAG (named the Gemmological Association) in 1931, shortly after the incorporation of the Gemological Institute of America (GIA). In 1938 the branch was renamed as the Gemmological Association of Great Britain, before being incorporated in 1847. The organisation is 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 then established both the Gemological Institute of America and the American Gem Society. There are now several professional schools and associations of gemologists and certification programs around the world.

The first gemological laboratory serving the jewelry 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 laboratories 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.

Background

It is often difficult to obtain an expert judgement from a neutral laboratory. Analysis and estimation in the gemstone trade usually have to take place on site. Professional gemologists and gemstone buyers use mobile laboratories, which pool all necessary instruments in a travel case. Such so-called travel labs even have their own current supply, which makes them independent from infrastructure. They are also suitable for gemological expeditions.

Gemstones are basically categorized based on their crystal structure, specific gravity, refractive index, and other optical properties, such as pleochroism. The physical property of "hardness" is defined by the irregular Mohs scale of mineral hardness.

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 or partially melted exogenous crystals that are evidence of heat treatment to enhance color.

The spectroscopic analysis of cut gemstones also allows a gemologist to understand the atomic structure and identify its origin, which is a major factor in valuing a gemstone. For example, a ruby from Myanmar (Burma) will have definite internal and optical activity variance from 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 color. Initially, the stone is identified by its color, refractive index, optical character, specific gravity, and examination of internal characteristics under magnification.

Scientific aspects

In mineralogy, an inclusion is any material trapped within a rock or mineral, and they are crucial for its identification in almost all cases. They tell us how it was created, when it was created, and whether it is natural or synthetic.

Gemology encompasses many aspects developed in a variety of ways throughout history. Deposits have historically been discovered by chance, and scientific knowledge has emerged after their exploitation.

Europe purchased precious stones from distant countries, and information about these deposits was reported by travelers and scientists in numerous documents. Legends were alluded to, such as the case of diamonds, which were said to lie in inaccessible valleys and were captured by birds to make them easier to collect. Another important true account was by Marco Polo, recorded by Tavernier (1675) on his trip to Turkey, Persia, and India, where a multitude of gems were described, as well as the Indian mines of Golconda, where they all came from.

Physical properties of gems

For a piece to qualify as a gem it must present the following three qualities:

beauty

durability

rarity

In their identification and evaluation, the properties of color, brightness and purity also play an important role.

Classical Gemology

Characterized by the study of the optical properties of minerals. A book was written about this knowledge in 1978, based on the work of Kurt Nassau. Currently, color is determined through chromatic concordances. Knowledge of the physical, optical, and beauty properties of gems has allowed the application of scientific criteria to improve their appearance.

The best-known example is the brilliant-cut diamond design. The round diamond cut was developed by Marcel Tolkowsky in 1919. This type of cut achieves a mathematically calculated balance between the brilliance and fire of this gemstone, based on its optical properties. It is calculated so that every ray of light entering from the crown (top) is reflected and returned to the observer, maximizing brilliance. At the same time, the exit angle produces color dispersion, creating what is known as fire.

The evolution of carving has paralleled scientific and technical development, moving from simply polished forms to the current faceted cut, to brilliant cut, and to computer design where the results can be seen before the specimens are carved.

Gemological instruments

Gemologists use a variety of tools and equipment which allow for the accurate tests to be performed in order to identify a gemstone by its specific characteristics and properties.

These include:

Corrected 10× loupe

Microscope

Refractometer

Polarising filter

Magnifying eyepiece

Contact liquid for RI (refractive index) up to 1.81

Polariscope

Optic figure sphere

Dichroscope

Spectroscope (handheld or desktop)

Penlight

Tweezers

Stone cloth

Color filter

Immersion cell

Ultraviolet lamp

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 nearly always only indicative. 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.

As with all naturally occurring materials, 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 create "individuals".

Identification by refractive index

One test to determine the gem's identity is to measure the refraction of light in the gem. Essentially, when light passes from one medium to another, it bends. Blue light bends more than red light. How much the light bends will vary depending on the gem mineral.

Every material has a critical angle, above 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, although it is possible to measure it using a microscope.

Identification by specific gravity

Specific gravity, 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 prism 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. Coloring agents or chromophores show bands in the spectroscope and indicate which element is responsible for the gem's color.

Identification by inclusions

Inclusions can help gemologists to determine whether or not a gemstone is natural, synthetic or treated (i.e. fracture-filled or heated).

Identification by flaws and striations

During the Verneuil process for synthesizing gems, a fine crushed material is heated at extremely high temperatures. The powdered gem mineral is then melted (or a metallic mixture directly burned in an oxygen flame) the residue of which then drips through a furnace onto a boule. The boule where the corundum or spinel cools down and crystallizes, spins and thus causes the curved striations, which are diagnostic for a lab-created gem: Natural corundum does not show curved striations.

Likewise, natural stones, particularly beryl minerals, show small flaws – short planar cracks where the direction of the crystalline orientation in the gem abruptly changes. The natural formation of gemstones tends to layer the minerals in regular crystalline sheets, whereas many synthetically produced gems have an amorphous structure, like glass. Synthetics made by the Verneuil process either do not show flaws at all, or if any flaws are present, show curvy, undulating surfaces rather than flat ones.

Optical analysis methods in gemmology

Gemstones are usually transparent minerals. Optical examination methods therefore play an important role in non-destructive analysis. After initial observation with a magnifying glass, inclusions and surface properties are examined microscopically. Gemmological microscopes are stereomicroscopes with low to medium magnification (total magnification <10 to approx. 100). They must have a large depth of field, as three-dimensional objects are examined. The object must be viewed in dark field, transmitted light, and reflected light. A cold light source is used as the light source, which does not increase the temperature of the stone even after prolonged observation. For certain examinations, e.g. to identify imitations, gemstones are immersed in an immersion liquid and examined microscopically in transmitted light. Additional optical elements such as polarizing filters can be arranged in front of and behind the stone.

A simple optical test is the determination of the refractive index. Every mineral has a specific value which depends on the material and the crystal structure. Gemstones with a high refractive index are usually particularly brilliant - for example, a diamond has a refractive index of 2.4, that of quartz is around 1.55. The refractive index can be measured easily and precisely with a refractometer. Optical gemstone refractometers are compact, standalone instruments that are easy to carry. The test is carried out in the light of the sodium D line (589 nm), which is filtered out of daylight with a sodium filter. Because of the high refractive indices of gemstones, the test piece is placed on the prism of the refractometer with a drop of a highly refractive so-called Anderson solution (e.g. diiodomethane), which extends the measuring range.

For optically anisotropic gemstones such as sapphire or ruby, a polarizing filter is placed over the refractometer's eyepiece to measure the different refractive indices of the birefringent stones. A gem refractometer can assess most stones with smooth or round polished surfaces. It is the professional tool of choice for reliably identifying synthetic and other low-priced stones.

A polariscope is used for the more detailed examination of doubly refractive stones. By rotating the analyzer, birefringence and stress anomalies are easily visible. To measure the optical axis, a conoscope lens is placed between the specimen and the analyzer. The position of the optical axis allows conclusions to be drawn about the crystal structure. This also allows observation through a dichroscope, which reveals stresses in the crystal structure. 

In addition to refraction measurements, spectroscopy provides information about a gemstone's material. The stone's components can be determined based on the absorption lines in the spectrum of light passing through it. Instruments that use an Amici straight-view prism are used for this purpose, as they have very compact dimensions.

In addition to the optical behavior of gemstones in visible light, their observation in ultraviolet light of different wavelengths also plays a role. This can help identify artificial and counterfeit stones. For example, treated and synthetic diamonds often fluoresce strikingly in pink, yellow, or neon green. 

Institutes, laboratories, schools, and publications

Institutes

American Gem Society – AGS

Asian Institute of Gemological Sciences – AIGS

Canadian Gemmological Association – CGA

Canadian Institute of Gemmology – CIG

Gemological Science International – GSI

Gemmological Association of Australia – GAA

Gemmological Association of Great Britain – Gem-A

Gemological Institute of America – GIA

German Gemmological Association – DGemG

Guild Education Gemology

Institut National de Gemmologie – ING

International School of Gemology – ISG

Hoge Raad voor Diamant – HRD

International Gemological Institute – IGI

Italian Gemmological Institute – IGI

Institute of Gem Trading – IGT

Gemmological Institute of India – GII

Center of Gemological of Indonesia – COG

Swiss Gemmological Institute – SSEF

World Gem Foundation

Commercial laboratories

Advanced Gem Testing Laboratory – AGTL

American Gemological Laboratories – AGL

European Gemological Laboratory – EGL

Gübelin Gem Lab – GGL

Guild Gem Laboratories

Laboratoire français de gemmologie – LFG

Professional gemstone testing laboratory – PGTL

Himalaya gem testing laboratory – HGTL

Universal Gemological Laboratories – GCI

Publications

The Journal of Gemmology

Gems & Gemology

Gemology Frontier

Gemology, law and economics

Gemologists specializing in appraisals must be familiar with regulations, the relationship between the appraiser and the auditor, valuation bases, special property values, special-purpose appraisals, estimates and off-market valuations, specific securities legislation and practices, and applicable procedural law.

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