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The Refractometer is used to measure the refractive index of a gemstone. The stone needs a flat and polished facet. Together with a contact liquid the stone is placed on the refractometer.
You read the refractive index on the scale visible trough the eyepiece. The examination is carried out on 4 or 8 different positions, through rotation of the stone. At the same time you rotate the polarising filter on top of the eyepiece. When encountering an anisotropic stone, this will produce a movement of the separation line between dark and bright. Note the lowest and highest value.
The correct interpretation of this results, reveals if the stone is isotrop or anisotrop. The anisotrop stones can be separated in two groups: uniaxe and biaxe. In most cases you can determine their optical character as either positiv or negative, in addition you determine the maximal double refraction (BR).
REFRACTOMETER
Be aware, that double refractive stones present one or more axes with a behavior like single refractive stones. In case of a doubt test more than one facet.
Each mineral has a typical refractive index due to its chemical composition and due to its crystallization structure. The refractometer allows to observe the optical properties and the structure of a gemstone. These properties are usually invisible to the naked eye. It allows to determine a great number of precious stones with precision.
Take care of your refractometer and this tool will serve you for a life time.
Make sure that the Anderson liquid does not dry on your glass prism. Sulfur crystals will appear and scratch the soft glass of your prism if you clean inappropriately. To clean correctly, apply some more Anderson liquid to dissolve the sulfur crystals, then clean lightly with a soft dust free tissue. Do not rub or apply pressure.
When you handle your gemstone with tweezers, do not place the gemstone directly onto the prism, place it next to the prism and slide the stone on top of it. Avoid scratching the glass with your tweezers.
Anderson Liquid
- The contact liquid usually delivered with a refractometer is of refractive index (RI) of 1.79.
- A contact liquid with an RI of 1.81 exists. This liquid is very corrosive and its toxicity is the reason why trading is forbidden in France and most of the European countries.
- The liquid with RI 1.74 is composed of " méthylène iodide " and not of great interest in gemology. Because to many stones can't be analyzed anymore.
Attention protect the liquid from temperature changes and protect from temperatures over 24°C. The sulfur starts to crystallize and the RI will drop. Close the bottle tight after use. Don't inhale or touch with your fingers, as all 3 liquids are toxic. Take extra precaution handling such toxic liquids.
1.720 - 1.720
1.720 - 1.720
1.720 - 1.720
1.720 - 1.720
constant
|
1.766 - 1.770
1.762 - 1.770
1.766 - 1.770
1.762 - 1.770
variable/constant
uniaxial -
BR 0.008
|
1.540 - 1.550
1.540 - 1.540
1.540 - 1.545
1.540 - 1.550
constant/variable
uniaxial +
BR 0.010
|
1.658 - 1.690
1.670 - 1.690
1.660 - 1.684
1.650 - 1.686
variable/variable
biaxial +/-
BR 0.032
|
isotropic / single refractive
|
anisotropic / double refractive
|
||
