In gemmological refractometry, gemstone refractometers determine the refractive index (physical abbreviation (n) on the surface of cut stones via the total reflection.
How do refractometer work? Gemstones have different visual characteristics due to their crystal structure and chemical composition, some of which can be determined with the gemstone refractometer. Refractometers are generally only used for polished stones. The measurement of rought stone is only possible, if they have a perfect plane crystal face or a small fat surface that has been ground. The devices can be used to directly determine the specific refraction of a polished stone, which is a characteristic feature. The specific value can be directly read off a visible scale in the refractometer.
The expert user also uses the device to determine information on birefringence, optical axes or optical orientation.
Function of gemstone refractometer
The examination involves placing a stone in the refractometer on a small glass surface and illuminating it at an angle from below. The reflected light beam is projected onto a scale which can be seen through the eyepiece of the refractometer. The refractive index obtained can be immediately identified by means of a light-dark boundary. It is essential to have the right lighting to make the lightdark boundary appear sharp.
In the ideal case, no normal sunlight or lamp light is used, but unicoloured, i.e. monochromatic, light. This is usually yellow sodium light with the wavelength of the sodium D line, which is 589 nm.
Do you want to read more?
If you are looking for more specialist information, order the white paper “gemstone refractometry”. “. For example, this publication describes the following examination methods:
- Measurement of single refracting (isotropic) stones
- Measurement of double refracting (anisotropic) materials with two mit two light-dark boundary
- Measurement of polished precious and gemstones
- Distinguishing between emerald, spinel, glass and tourmaline
- Measurement of polished stones with a curved surface
- Example: Examination with the “distant vision” method or “spot” method
- Measurement of rough stones with a smal facets (window) cut into them
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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