The Gemology , even science of gems or short Gemology (occasionally also Gemnologie ) who deals as a branch Mineralogy exclusively as gemstones ( gemstones used) minerals and varieties as well as their syntheses and imitations. Gemstone science has nothing to do with the esoteric meaning of stones, medicine and the like.
There are three questions to be answered when performing a gemological examination of a stone:
- Which stone is it? On a mineralogical basis, gemstones are recorded in the areas of origin, sites and properties and their determination is made on this basis.
- Is it a natural or synthetic stone? Some stones can be made artificially, e.g. B. a ruby, sapphire or an emerald.
- Is the stone treated or not? Artificial changes in properties (treatment) are carried out on gemstones for the purpose of improving the color or purity.
All analyzes are carried out using non-destructive methods, as gemology often works with first-class gemstones. Unlike in geology, no specimen can be used for analysis. Instead, the stones have to be identified based on their optical, physical, chemical and crystallographic properties. Both quantitative data and qualitative data can be obtained. So z. B. a gemstone can be classified according to the crystal systems, or a refractive index can be read.
In practice, the expertise of a neutral laboratory can only be obtained in special cases. Analysis and assessment, for example in the gemstone trade, must be done on site if possible. Professional gemologists and gemstone buyers therefore use mobile laboratories that combine all the necessary equipment in a transport case. So-called travel laboratories even have their own power supply so that they are independent of an external infrastructure. They are also suitable for gemological expeditions.
The title gemstone specialist is an additional qualification for goldsmiths , silversmiths or gemstone cutters . Corresponding experts from the banking and pawn loan industry also attend corresponding advanced training courses from the national gemological societies.
The international center for gemologists is Amsterdam in the Netherlands . Probably the most renowned institution is the Gemological Institute of America .
Optical analysis methods in gemology
Gemstones are mostly transparent minerals. Optical examination methods therefore play an important role in non-destructive analysis. After the first inspection with a magnifying glass, inclusions and surface properties are examined microscopically. Gemological microscopes are stereomicroscopes with a small to medium magnification (total magnification <10 to approx. 100). They must have a great depth of field as three-dimensional objects are examined. Viewing the object must be possible in the 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 when viewed for a long time. For certain examinations, for example to detect imitations, gemstones are immersed in an immersion liquid and examined microscopically in transmitted light. Further optical elements such as polarization filters can be arranged in front of and behind the stone.
A simple optical examination is the determination of the refractive index . Every mineral has a specific value that depends on the material and the crystal structure. Gemstones with a high refractive index are usually particularly brilliant - a diamond, for example, 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, autonomous instruments that can be easily carried around. The examination takes place in the light of the sodium D-line (589 nm), which is filtered out of the daylight with a sodium filter. Because of the high refractive indices of gemstones, the test item is placed on the prism of the refractometer with a drop of a high-index so-called Anderson solution (e.g. diiodomethane ), which extends the measuring range.
In the case of optically anisotropic gemstones such as sapphire or ruby, a polarization filter is placed on the refractometer's eyepiece in order to measure the different refractive indices of the birefringent stones. With a gemstone refractometer, most stones with a smooth or round polished surface can be assessed. It is the professional tool of choice for securely identifying synthetic and other low-cost stones.
A polariscope is used to examine birefringent stones more closely . When rotating the analyzer, birefringence and stress anomalies are easily seen. A conoscope lens is arranged between the object and the analyzer to measure the optical axis . The position of the optical axis allows conclusions to be drawn about the crystal structure. This also allows viewing through a dichroscope , where tensions in the crystal structure become visible.
In addition to the refraction measurement, spectroscopy provides information about the material of a gemstone . Based on the absorption lines in the spectrum of the light that shines through the stone, its components can be determined. Instruments that work with an Amici straight vision prism are used because 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. Artificial and falsified stones can be recognized. For example, natural sapphires show no fluorescence, while synthetic and heat-treated sapphires, on the other hand, show clear luminous phenomena in ultraviolet radiation of different wavelengths.
- ↑ Library at the Max Planck Institute - Geosciences, Mineralogy (GEO 411 Gemnology (Gemstone Studies ) )
- ↑ Gem stories , instructions for use for gemological tools by Hubert Heldner
- ^ UV Fluorescence as a Gemological Tool
- ^ Mineralienatlas: Gemmologie
- Wilhelm F. Eppler: Practical Gemmology . 6th edition. Rühle-Diebener-Verlag, Stuttgart 1999.
- Rudolf Graubner: Lexicon of geology, minerals and rocks . Emil Vollmer Verlag GmbH, Munich 1980, ISBN 3-87876-327-1 (keyword: gemstone studies ).
- Peter G. Read: Gemmology . 3. Edition. Elsevier / Butterworth-Heinemann, Amsterdam et al. 2005, ISBN 0-7506-6449-5 , p. 10 .
- Walter Schumann: Precious stones and gemstones. All species and varieties in the world. 1600 unique pieces . 13th, revised and expanded edition. BLV Verlags GmbH, Munich et al. 2002, ISBN 3-405-16332-3 , p. 10 .