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How to Use a Gemstone Refractometer

A gemstone refractometer is a specialized instrument used to identify precious and semi-precious gemstones by measuring their refractive index (RI), or capacity to hold and bend light. To determine if a gemstone is single refractive, yielding one RI reading, or double refractive, giving two readings, rotate the stone placed inside the refractometer. Most gemstones yield a double RI reading, with the difference between them calculated as the birefringence. Using a refractometer is relatively simple; it involves placing the gemstone and small drops of special RI or contact liquid inside the device, shining light through the back and viewing the RI reading from the view lens.

Things You'll Need

Gemstone
Light source
Refractometer
RI liquid

Instructions

- 1
  Open the refractometer and place several small drops of RI or contact liquid on the metal base surrounding the glass lense. The RI liquid expels air and establishes optical contact between the stone and the glass lens.
- 2
  Place the stone gently on top of the RI liquid drops. Slowly slide the stone onto the glass lense or hemicylinder. Close the lid to block out extraneous light.
- 3
  Shine your light source through the back of the refractometer. A light source can be a pen light or a small flashlight.
- 4
  Read the RI results on the viewing lens. Refractive Index is indicated by one or more small green lines marking a certain number. If two green lines appear, the stone is double refractive. In this case, calculate the numerical difference between the two number readings to determine the stone's birefringence. For best results, rotate the stone inside the device several times.
- 5
  Compare RI and birefringence values corresponding to the type of gemstone analyzed by use of established value charts. These value charts may vary by institution.
- 6
  Clean the stone and refractometer after each use. This prevents scratching or liquid residue from interfering with future readings.

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