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The Refractometer: How To Use a Refractometer to Identify Rocks, Minerals and Gemstones

how to use a refractometer

There are a number of different instruments available to help you in gemstone identification. But once you learn how to use it, a refractometer on its own can often provide you with enough information for accurate identification of many different types of gemstones.

Related: Guide to Best Lapidary Equipment for Beginners

What Is a Refractometer?

The refractometer is considered by some to be the most important of all gem-testing instruments. Refractometers are generally fairly small, portable sized instruments that are simple to carry with you and use whether you’re out in the field or in a store.

Refractometers measure the angle at which light rays bend (refract) as they travel through a substance. This measurement provides a numerical reading from a scale that you can see while looking through the eyepiece of the refractometer.

But you might think that the refractometer is only useful on gemstones that are transparent or translucent, that’s not the case. What’s most beneficial about using a refractometer to identify gemstones is that they provide you with information about the opaqueness of stones. In other words, stones that you cannot see through, such as jade, opal and lapis.

What Is a Refractometer Used For?

The most common, and primary use of the refractometer is to measure the angle which light travels through the gemstone bends or refracts. This angle measurement is called the “refractive index“.

Refractive Index: is a value calculated from the ratio of the speed of light in a vacuum to that in a second medium of greater density. The refractive index variable is most commonly symbolized by the letter n or n’ in descriptive text and mathematical equations.

The refractive index is usually different and unique for each type of gemstone. It’s these unique index measurements that make using a refractometer extremely valuable for accurately identifying gemstones.

How Much Do Refractometers Cost?

Refractometers that are used for industrial type purposes are very expensive.

The good news, however, is that for those of us who need a refractometer for mineral and gemstone identification as hobbyists or smaller commercial operations, refractometers are much more affordable.

You could expect to find that these refractometers to cost from anywhere between $100 and $500, depending on what your needs are.

The more expensive range will include digital refractometers.

How To Use a Refractometer

The first, and most important thing to keep in mind when using a refractometer is making sure that you have a good source of light. What you need is a source of:

  1. White Light (halogen lamp)
  2. Monochromatic yellow light (filters out all light except yellow)

There are lamps available that are able to provide you with both types of light sources. Many refractometers have a light source built, so no need to purchase an external light source for these.

The refractometer has a scale that shows the refractive index of a stone. The scale can measure as low as 1.35 and as high as 1.80, which is more than enough scale for most gemstones.

  1. Carefully place a small drop of refractive index liquid on the hemicylinder.
  2. Place the stone on the hemicylinder
  3. Place a white light in front of the refractometer
  4. Look through the eyepiece and note the measurement on the scale
  5. Repeat the process using a yellow light to determine if single or double refracting stone

How To Care For a Refractometer

The hemicylinder, which is the glass part that you place the stone on, can scratch very easily. If this glass scratches, it’s possible that you will not be able to get accurate readings from your refractometer.

If the contact liquid dries or evaporates, it can leave behind little crystals on the hemicylinder. To remove these, simply moisten the crystals by adding a small drop of the liquid, and then carefully wipe the glass.

If you wipe the hemicylinder before dissolving the crystals, you risk scratching the glass as the crystals are abrasive.

If you know that the refractometer will not be in use for quite some time, then you can apply a small amount of Vaseline on the surface of the hemicylinder. This will help prevent tarnishing over time which can affect the functionality of the refractometer.

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