Gem Refractometer – What It Is and How It Works

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 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 to three decimal places, 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:

• If one shadow edge remains nearly stationary, while the other edge may shift slightly, then your gemstone is uniaxial.

• If both shadow edges shifts irregularly as you rotate the gem, and the seperation between the two edges changes quite noticably, then your gem is biaxial.

3. What is the birefrinfence of the gem?

We can now calculate the birefringence of the doubly refractive gemstone, which is the difference in value between the higher and the lower RI.

1. Look into the eyepiece with your head kept still, and slowly rotate the gem about 30 degrees at a time.

2. After each rotation, record both RI values to three decimal places.

3. Once the gem has been rotated a full 360 degrees, review your recorded data.

4. Identify the highest RI value and the lowest RI value observed. Subtract the lowest from the highest.

5. The result, typically a small number between 0.003 and 0.287, is the birefringence of the gemstone.

4. Using the Polarizer

If at any point the two RI values become very close and difficult to distinguish, the polarizer can be used to help separate and identify the edges more clearly.

The polarizer is stored on the upper cover of the refractometer. To use it, simply unscrew the polarizer and place it over the eyepiece. While looking through the eyepiece with your head kept still, slowly rotate the polarizer. With the polarizer in place, only one of the two shadow edges will be visible at a time. As you rotate the polarizer, the visible edge will gradually fade while the other edge becomes visible. This alternation occurs every 90 degrees of rotation.

At this point, you have acquired all the information that can be obtained using a refractometer:

1. The optical properties of the gemstone: singly or doubly refractive, and if doubly refractive, whether it is uniaxial or biaxial.

2. The refractive index or indices.

3. For doubly refractive stones, the birefringence.

Next, simply compare your results with the reference data sheet to help identify the gemstone.

Step 3: Cleaning Up

 

 

 

 

A Little Bit About Us - Lindian Technology

Founded in 1995, Lindian Technology is a leading developer, manufacturer, and distributor of refractometers and optical instruments based in Fujian, China. Our instruments are crafted from the highest-quality materials to ensure both durability and precision. We take pride in delivering only the finest instruments and are committed to advancing the standards of optical measurement in China. With a global distribution network, Lindian Technology provides reliable instruments to customers all over the world.

 

If you are interested in learning more about our RHG-181 gem refractometer, Click Here.

To view our full range of gemological instruments, including the gemological polarimeter and gem magnifier, Click Here.

If you need assistance regarding our products, making a purchase, or anything at all, Contact Us.

 

 

Reference Data Sheet