Refractometry allows the specific refraction of light (refractive index) to be determined directly in polished stones. Since the refractive index is a material constant that depends on the chemical composition of a substance, it provides information about important characteristics feature of a gemstone. The determination with a special gemstone refractometer is an easy-to-use method. With this method and a high-quality refractometer, for example, the authenticity and quality of a stone can be assessed. The gemstone refractometer is therefore part of the basic equipment of a gemological and gemstone laboratory.
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EMERALD, SPINEL, GLASS OR TOURMALINE?
A dark green, faceted stone is available for testing. It could be an emerald or synthetic green spinel, green glass, green tourmaline or chrome diopside. The stone to be tested must have a well-polished surface of at least 1-2 mm². For measurment, a special contact liquid is applied between the stone and the measuring surface (for physical reasons). The stone is then placed on the measurment prism, close the cover of the gemstone refractometer and read the refraction value.
When observing without moving polarising filter, the refractometer shows only a shadow line by chance. Two shadow lines appear when the stone is rotated on the measuring surface. It is clear that this is an anisotropic stone, amorphous glass or the cubic, synthetic spinel is immediately eliminated through the shadow lines. The maximum value is at n = 1.65, the minimum at n = 1.62. Therefore, emerald (n = 1.56-1.58) is not an option for the stone measured.
The measured values speak for a completely different stone. The expert can also see that the stone is optically uniaxial, – that is, trigonal, hexagonal or tetragonal, – because when the stone is rotated, one of the shadow lines moves while the other remains constant. This information points to a very specific mineral as the identity for the ornamental stone you are looking for. It is a popular collector’s stone, some of which are even more valuable than diamonds.
If you want to know which stone our example shows and if you are looking for more technical information, please order the white paper “Gemmstone Refractometer”. In the specialist publication, the refractometer examination of the “distant vision method” or “spot method” is explained in detail. The whitepaper was produced in collaboration with Prof. Dr. Jochen Schlueter wrote. He is a graduate mineralogist and responsible for the display collection of the Mineralogical Museum in Hamburg.
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Here you can order the “Gemstone Refractometer” whitepaper free of charge, in which many examination methods are described. Simply enter your contact details here and we will send you our detailed white paper as a PDF on the subject of “Gemstone Refractometers” by email.