Gemstone Geographic Origin Determination with special reference to Blue Sapphires

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Lareef A. Samad B.Sc (Hons)

Criteria used in determining country of origin of blue sapphires

Identification of inclusions is the first criterion used by gemologists in determining the country of origin of a blue sapphire.The presence of inclusions that are unique to certain sapphire producing regions, can be easily used in identifying the source, eg. the presence of needle-like pargasite or tourmaline crystals, would without any doubt indicate Kashmiri origin of the sapphire and the presence of uranpyrochlore inclusions would indicate Cambodian or southern Vietnam origin.
Apart from inclusions gemologists also use other criteria in determining the possible source of a sapphire. These include the use of optical properties, growth characteristics and chemical composition.

Optical properties used in origin determination

An optical property that imparts a soft velvety appearance to blue sapphires caused by the presence of silk, might indicate a Kashmiri origin, prompting the gemologists to look for other conclusive evidences for Kashmiri origin. Fluorescence is another optical property that may provide a clue to the origin of a gem. Sapphires originating from most of the world's sources are inert to long and short wave ultraviolet light except Sri Lankan sapphires that fluoresce regularly in U-V light. The presence of fluorescence is one indication of the sapphires origin, although not all Sri Lankan stones fluoresce.

 

Growth characteristics used in origin determination of sapphires


Growth characteristics such as the way crystals grow in a deposit can also provide clues to a sapphire's origin. Growth patterns as observed under a microscope can be diagnostic, eg.tight optical growth patterns are found regularly in Madagascan sapphires, less regularly in Sri Lankan and Myanmar stones, and not at all in Kashmir sapphires. Likewise color-zoning in sapphires can also give an indication as to their origin. Eg.Sapphire crystals from Antsiranana Province in Madagascar often display blueviolet, greenish blue, and greenish yellow zones within the same crystal. Color-zoning in Kashmir sapphires show distinct zones of alternating blue and milky-white.

 

Determination of chemical composition of sapphires and identifying trace elements found in and on the surface of sapphires


Other useful tools that may be helpful in determining the country of origin of a sapphire are spectrophotometry and energy-dispersive X-ray fluorescence (EDXRF), that can unravel the chemical make-up of a sapphire by identifying the elements of which the sapphire is composed of and even detecting various trace elements found in and on the surface of the sapphire. Most sapphires would show the same basic composition, but different concentrations of trace elements that would give a clue from which deposit the sapphire originated. Eg. Some Cambodian sapphires are the exact look-alike of Kashmir sapphires and are often touted as Kashmir sapphires. However, spectrophotometric and EDXRF analysis show that Cambodian sapphires have very high concentrations of the trace element iron compared to Kashmir sapphires that typically have very low iron levels.

 

How geographic origin of a given sapphire is determined ?

Gemstone geographic origin determination is a science that is still in its infancy and not yet fully developed, despite the advances in science and technology and the availability of modern tools and equipment for gathering relevant information, such as the Ultra Violet-Visible-Near Infrared spectrometer (UV-ViS-NIR) , the Raman spectroscope, Photo-Acoustic Spectroscopy (PAS), Fourier Transform Infrared Spectrometer (FT-IR), Energy-dispersive X-ray Fluorescence (ED-XRF), Laser-induced Breakdown Spectroscope (LIBS), Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA-ICP-MS), and the Scanning Electron Microscope (SEM). The geographic origin of a given sapphire is determined by collecting all relevant data on the sapphire, such as the nature of the inclusions, optical properties, growth characteristics if any, the nature and concentration of trace elements etc. and comparing it with the properties of a reference sapphire collection, in a database maintained by the Laboratory, containing a sufficiently high number of samples from commercially relevant mining areas and deposits worldwide which include reference stones from exhausted mines or deposits where production has ceased.

 

Limitations of the methods employed


Such an approach will be in order, as long as the given sapphire comes from an established mine whose existence is known and from which samples had been collected previously for the reference database. However, problems can arise when the given sapphire originates from a new deposit, from a previously unknown mine, from which geologists have not had the opportunity to collect samples. In such cases there have been instances when two different laboratories have issued widely differing geographic origin reports for the same sapphire, or a sapphire had been assigned a geographic origin that has a direct bearing on its market value, such as the blue sapphires from Elahera, Sri Lanka, misidentified as Kashmir blue sapphires, when the deposit was first discovered.
Mis-identification of the origin of a gemstone appears to be the unfortunate consequence of the lack of funding and research, in updating the reference database maintained by gem-testing laboratories, by visiting and collecting samples and relevant geological information from new deposits and mines, as well as new samples from traditional sources, such as Burma and Sri Lanka. However, given the fact that new sources and deposits are being discovered around the world on a regular basis, investigating the physical, chemical, optical, and other properties such as inclusion characteristics, of the new material collected, and updating the reference database maintained by the laboratory might prove to be a very difficult and challenging task.

 

Two broad geological origins of corundum - Basaltic (Magmatic) and non-Basaltic (Metamorphic)


Sapphires and Rubies,which are varieties of the mineral Corundum (Aluminium oxide), can have two broad geological origins - Basaltic or magmatic and non-basaltic or metamorphic. Blue sapphire deposits from Australia, Cambodia, Thailand, China (Shandong), Rwanda etc. are basaltic in origin, and those from Kashmir, Burma, Sri Lanka, Madagascar and Tanzania are non-basaltic or metamorphic. Most laboratories can easily distinguish between a sapphire of basaltic origin and non-basaltic origin.

 

Difficulties encountered when trying to distinguish between deposits of identical geological origin

However, after establishing the geological origin of the sapphire, difficulties arise when trying to distinguish between deposits of identical geological origin, eg. between Kashmir, Burma and Sri Lanka sapphires, or between Madagascar, Tanzania and Sri Lanka sapphires all of non-basaltic origin, or between Australia, Cambodia and Thailand sapphires of basaltic origin. Such difficulties arise mainly because of the partial or total overlap of characteristics between sapphires of identical geological origin. Where the character is unique, without any overlap identification is simple and straightforward, eg. presence of pargasite or tourmaline crystals unique only to some Kashmir sapphires,and the presence of uranpyrochlore inclusions unique only to Cambodian and Vietnam sapphires.

 

A comprehensive approach identifying inclusions, analysing chemical composition and trace elements, and study of spectral properties used when charaters are similar and overlapping


Where most of the characters are similar and overlapping as in some Sri Lankan and Kashmir sapphires or Sri Lankan and Madagascar sapphires, without a unique diagnostic character, a comprehensive approach is applied, such as identification and description of inclusions, analysis of chemical composition and trace elements, and spectral properties. The evaluation and interpretation of the observed features, combined with additional advanced analysis, may allow a laboratory to reach a conclusion on the geographic origin of the gemstones. Advanced analysis used to distinguish between Sri Lankan and Madagascar sapphires by the Gemological Association of All Japan (GAAJ) is LA-ICP-MS analysis, which showed that minute amount of heavy elements such as Sn (tin) and Ta (tantalum) were under detection limit in the stones from Sri Lanka, while several to several dozens ppm of those elements were detected in the stones from Madagascar, a significant difference that can be used to distinguish between the two.

 

Factors that can complicate Geographic origin determination


However, despite advances in analytical methods only a success rate of around 80% has been achieved in the relatively new field of Geographic Origin Determination, according to C. R. "Cap" Beesley of the American Gemological Laboratories (AGL) of New York. AGL maintains one of the largest reference databases in the world, consisting data of over 10,000 samples collected from gem mines and deposits situated around the world. Factors that can complicate Geographic Origin Determination are :- 1) Heat treatment that can alter the chemistry of gemstones (Eg. the distinctive color zoning seen in sapphires from Andranondambo, Madagascar, becomes significantly less distinct when the stones are heated); 2) Non-uniformity of characteristics of sapphires even from different areas of the same deposit 3) Discovery of new locations bringing hitherto unknown new materials into the market; 4) the need to maintain a sample of stones that remains current. eg. data collected from one of the best sample populations of Ceylon blue sapphires 20 years ago may not be valid today, and has to be continuously updated from time to time.

 

Laboratories that issue Geographic-origin/Country-of-origin reports on sapphires despite only 80% success rate


Inspite of all the difficulties, some of the renowned laboratories that issue Geographic-origin/Country-of-origin reports on sapphires are the American Gemological Laboratories (AGL), Gubelin Gem Lab (GGL), Swiss Gemmological Institute (SSEF), and the American Gem Trade Association (AGTA) Gemological Testing Center. However, these labs concentrate mainly on sapphires originating from non-basaltic deposits, such as Kashmir, Burma, Sri Lanka, Madagascar, Tanzania etc. whose sapphires command premium prices at international markets and not on sapphires originating from basaltic deposits, such as Rwanda, Australia, Cambodia, Thailand, Shandong [China], etc., where it is of little commercial interest to know the locality.

 

Labs that do not issue Geographic/Country-of-origin reports as a failure rate of 20% is considered too high and unacceptable


Since Geographic/Country-of-origin of a gemstone is based more on expert opinion than on conclusive evidence, there is a tendency for labs to differ occasionally, when issuing reports on the same sapphire. Sometimes when too many ambiguities arise, the labs may issue a tentative opinion or no opinion at all. All these difficulties come within the failure rate or negative identification of 20%.
A failure rate of 20% is considered too high and unacceptable by many renowned laboratories such as the Gemological Institute of America (GIA) and Professional Gem Sciences (PGSL) Inc. founded by Thomas and Myriam Tashey. Hence, such labs do not issue Geographic/Country-of-origin reports on any gemstone.

 

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