Refractometer Catagories

How to Use a Refractometer to Identify Gemstones

[ 2014-8-29 ]

A refractometer measures the refraction of light by an object. Refraction is the "bending" of light as it passes through some material and its measurement is called a refractive index. Refractometers are commonly used to identify gemstones because gemstones of the same color frequently have other optical properties that are very different. A refractometer can accurately identify gemstones by measuring a variety of these properties and comparing the test results to known values.


  1. Clean the stone and place a single stone on the glass prism. Use the largest polished facet for faceted stones. Otherwise use the most polished surface. Slide the selected surface of the stone carefully into the liquid with your fingers. Close the cover.
  2. View the stone through the refractometer under white or monochromatic light, typically with a special pen light. Move the light source to the back of the refractometer and point it through the viewing lens. You will know the light is pointed correctly when you can see it through the front of the lens.
  3.  Look through the viewing lens so you can see the numbered scale. Move your head up and down a few times to find the position that shows the best separation between the light and dark areas. This line of separation indicates the refractive index of the stone.
  4. Use a polarizing filter. Many stones have more than one refractive index, depending on the polarization of the light they are viewed with. Place the polarizing filter over the viewing lens.
  5. Rotate the polarizing filter to find the position that provides the highest refractive index and record this value. Rotate the filter to locate the position with the lowest refractive index . If the refractive index changes, the stone is doubly refractive. The difference between the maximum and minimum refractive index is the stone's birefringence. This value is frequently helpful in identifying the stone.

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