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Best way to calibrate a refractometer

I just bought a new Fable refractometer. I tested it on synthetic corundum that I have and it looks like it’s off about .04.

What is the best way to calibrate it?

Do they calibrate it? the one I have I don’t remember it’s brand name… but never needed to calibrate. Is it a digital one… Can you share a picture please.

The refractometer doesn’t need to be calibrated. That is done at the factory. I was hoping to know if there was a gemstone that has such a pronounced reading that I would know if my refractometer is correct or if it is off a little…

“Standard” Synthetic spinel will give 1.727 sharp.

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Thank you. I have synthetic spinels and synthetic corundum in my collection. Thanks for the tip.

I will have to do some checking, as I always thought that quartz was the most invariant gem…try that, also. You should be closer than 0.04…this is why I always tell folks that the Chinese refractometer is OK, just get a return privilege and check it on some quartz as soon as you get it. You also need a yellow filter or single wavelength light source to be sure you have an exact reading. Your idea of where 589 nm is may be off, but it shouldn’t be off by 0.04. If using white light, you are reading at the yellow, right? -royjohn

My refractometer comes with its own internal light source. I checked it against some synthetic corundum and spinel and it gave an RI reading for both those stones, so I imagine that the refractometer is working properly.

The light source of the refractometer can run on AA batteries as well as being plugged into the wall so it is very convenient. It also has a scale that goes to two (2) decimal places.

Yes, OK, so it has a light source, but is the light source white light or monochromatic yellow light. If you have a monochromatic light source or a yellow filter, you see only a shadow edge (or two shadow edges for a birefringent gem), whereas if you are reading with white light, you see a small rainbow and have to read the yellow portion to get an exact reading. So which is it? -royjohn

It’s a yellow light, so all I get to see is the shadow edge.

OK, so you should have a scale divided into tenths…1.40, 1.41, 1.42, etc., so you should be able to read to about 0.002 or 0.0025. Quartz is birefringent and should read 1.544 and 1.553. So if you try a few samples of clear quartz, amethyst and/or citrine and don’t get that value, your instrument is off by whatever you are reading as the error for quartz. You can still use the instrument, but you’ll have to correct your readings by whatever error you read. -royjohn

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Thank you very much for the tip… Now I just have to find a nice piece of quartz or citrine to use.

A sodium light source provides the “standard” wavelength of yellow light yielding sharp Refractometer readings. Scientifically measured Refractive Indexes for minerals and gems are usually derived from this standard wavelength, or from other referenced standard wavelengths. If it was worth the investment of a Refractometer, and accuracy of the readings are the goal, purchase a sodium vapor lamp intended for this purpose. A yellow filter is somewhat useful, but not as accurate. Most ‘casual’ users of Refractometers lack the knowledge how a Refractometers works and the understanding of Optical Crystallography to begin to use one to its potential. I built a Refractometer years ago as a high school Science Fair project which does not require Refractive Index Oil! Can you figure out how it worked? Hint: I made the heart of the Refractometer with a Lee Faceting machine. The Lynchburg Gem and Minerals Society recently posted an introduction to crystallography on YouTube titled “Crystallography by Dave Woolley” which is the background preparation for a lecture I hope to make at the United States Faceters Guild’s, “Franklin Faceters Frolic” in Franklin, North Carolina at the end of July. Look for the lecture to be posted in the USFG’s newsletter in the near future, titled “Useful Crystallography”. The YouTube video and follow up article are to assist with increase successes in faceting, and with the background understand of Optical Crystallography for the use of a Refractometer.

For gemology a yellow led is more then enough. You simply have no use for better accuracy. A sodium lamp is pure overkill, waiting for it to get heated up, and pricewise. Why on earth go for it??

Thank you CitroenDave!

I’m actually fascinated by crystallography… I just haven’t had the time or patience to read more about it… When I first got interested in gemology I was working in Tanzania in telecom and all I was doing back then was studying more telecom engineering, photography and whatever time I had left was for gemology. I was lucky that my boss’s passion for gemology let him teach me a thing or two, introduced me to various gem brokers in the country where I purchased various rubies, sapphires and tanzanite with his assistance.

My personal refractometer already has a built in sodium lamp. It doesn’t have any area where you can attach a flashlight or other light source. It also has a second light I can turn on if the standard light is insufficient.

It’s a very decent refractometer.

In all honesty I have no idea how you would go about building a refractometer let alone one that doesn’t require any oil to use. I would definitely like to learn more about that.

If you have any links i would be grateful to learn more about this.

I’m here to learn since I know so little. A little knowledge is a dangerous thing.

With what little knowledge I have i have used to build a small business when I lost my job running a company (it went bankrupt with all the lockdowns)… So now I am using all my connections and knowledge for this endeavor.

I just purchased a smoky quartz to check on my refractometer.

I’ve been using synthetic corundum and spinel to see if my refractometer olives accurate readings and it appears that it does.

At this point I need to pick an online gemology program to fill in the many holes in my knowledge and to be able to ask experts questions that come up.

The online course here is the most affordable, but I’m not sure if you actually get to ask questions to an actual genologist as part of the course.

It looks like you can with GIA and the GEM A online courses, so I have a lot more research to do.

Thanks for the info.