What Causes Fuzzy Refractometer Readings?

Fuzzy refractometer readings may be the result of poorly faceted gems, the wrong amount of refraction liquid, and even stone size. Learn more here.

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HomeLearning CenterGemologyQuestions About Gems and JewelryWhat Causes Fuzzy Refractometer Readings?
Question:I'm having trouble with fuzzy refractometer lines. And that's when I can see lines at all. Sometimes the lines disappear when I put on the magnifier. The problem is worse when I'm trying to measure the refractive index of a small gemstone. I'm just learning to use the refractometer. Can someone tell me what I'm doing wrong?
Can the small size of a gemstone contribute to fuzzy refractometer reading?
"Hoard," photo by Daniel Crookston. Licensed under CC By 2.0.

Answer: Several factors can contribute to fuzzy refractometer readings, including the quality of the gem cut, the amount of refraction liquid on the hemicylinder, and even the size of the gem.

How to Avoid Getting Fuzzy Refractometer Lines

In order to get a good refractometer reading, the stone you're checking must have perfectly flat and well-polished facets. If it doesn't, this will cause a blurry band to appear.

You must also use the correct amount of refraction fluid. (Editor's note: consult this article on refraction liquids commonly used by gemologists as well as this list of common household liquids for more information). Just a tiny drop is enough. Too much or too little will give you fuzzy refractometer readings.

Examples of gem images on the refractometer scale
Examples of gem images on the refractometer scale.

Make sure you wipe the window clean before and after using the refractometer. Also, be sure to use a soft tissue. Don't use the ones that have conditioners in them. Get the cheapest store brand you can.

One last note: sometimes you have to move your head up and down or left and right to get a good look at the line. If you see a good line without magnification, you should also be able to locate it with the magnifier in place. It takes a lot of practice to get good consistent RI readings!

Joe Volkel, GG

Fuzzy Readings May be Normal

I assume you're using a light-reflecting refractometer. These have inherent limitations, since you're reflecting light at the intersection of the facet and the lens. Getting a fuzzy refractometer reading is quite normal. With smaller stones, obviously, the image is smaller. Therefore, less light is returned. I believe you may be expecting more from your refractometer than it can deliver.

Jim V.H.

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