The gemological refractometer is an optical tool used to measure the refractive index of gemstones, a key property for identifying and verifying them. In this article, we explain what a refractometer is, its main applications,how to use one, and how to choose the right one for your needs.
What is a Gemological Refractometer?
The gemological refractometer, or simply gem refractometer, is an essential instrument for identifying and analyzing gemstones. It measures the refractive index (RI) of a gem, one of the most reliable properties for distinguishing different materials. Each gemstone bends light in a specific way, expressed as its RI value and related optical behaviors. With this device, users can determine whether a stone is singly or doubly refractive, uniaxial or biaxial, and can calculate its birefringence. By comparing these measurements with known reference data, users can accurately identify a gem and determine whether it is natural or synthetic. Reliable RI testing ensures correct valuation and builds trust in the gemstone trade.
Gemological refractometers come in slightly different materials, shapes, and sizes, but their main parts and functionality are largely consistent across manufacturers and brands. Using our RHG-181 gem refractometer as an example, the main components and their functions are shown below.
How to Use a Gemological Refractometer Properly?
To properly use a gem refractometer, you will need four things: a gem to be tested, the refractometer, RI fluid (or contact fluid), and a light source. Our product comes with everything except for the gem.
Step 1: Preparation
1. Place the refractometer on a flat, even surface with the upper cover (1) open.
2. Apply a small drop of RI fluid onto the prism. The drop should be around 1 to 2 mm in diameter.
3. Carefully place the gemstone onto the prism, taking care not to scratch the glass.
4. Direct the light source into the sodium light filter (3) on the back of the refractometer.
Note: For refractometers without a sodium light filter, a sodium light source with a precise wavelength of 589 nm should be used for the most accurate results. The RHG-181 gem refractometer includes a sodium color filter, which means any white LED light can be used.
The RI fluid is toxic and must be handled with care. We recommend using a dropper, which is included in our product packaging.
Step 2: Observation
Look into the eyepiece — the field of view should be brightly lit. You will see an RI scale ranging from 1.30 to 1.81. One or two shadow edges may appear, which are faint dividing lines between the light and dark areas. The point where the shadow edge intersects the scale is your RI reading. You may need to adjust the position of your head to see the shadow edges more clearly.
To identify your gemstone, you should then ask the following questions in order:
1. Is the gem singly or doubly refractive?
Slowly rotate the gemstone in any direction while continuing to look through the eyepiece. If you see only one shadow edge and it does not move, the gemstone is singly refractive. If at any point you see two shadow edges that separate and come together again as you rotate, the gemstone is doubly refractive.
Note: For doubly refractive stones, the shadow edge at the higher RI value is typically fainter than the lower-RI edge. When the two edges are very close together, they can be hard to distinguish, so look carefully.
For singly refractive gemstones, your work is complete. Simply record the RI value of the stone and compare it with the reference data sheet (provided at the end of this article) to identify the gem. RI values are typically noted to three decimal places, while the refractometer scale only provides two. Therefore, you should make your best estimate of the third decimal place.
For doubly refractive gemstones, we still need to determine whether the stone is uniaxial or biaxial, and then calculate its birefringence.
2. Is the gem uniaxial or biaxial?
After confirming that a gemstone is doubly refractive, the next step is to determine whether it is uniaxial (one optic axis) or biaxial (two optic axes).
Slowly rotate the gem while looking through the eyepiece, and observe the movement of the two shadow edges:
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