Refractometer

The ultimate test is at the bottom of this page for those who have been here before.

General Information

What is it?: This handy little piece of equipment tests a gemstone's ability to slow down or bend light. The amount of slowing down it does to a beam of light is called the refractive index, which is a number assigned to a scale we will see in a minute. And while some gemologists think that a refractometer is used just for gemstone identification, truth is that long before it was used for the identification of gemstones, the refractometer was used to set prescriptions on eye-glasses, and (get ready for this) to tell when grapes are ready to be picked to make wine...based on the refractive index of the grape juice. For our purposes we will work with the idea of gemstones...although I know a lot of gemologists who work on their own to test a lot of wine. But enough of that...on to the study of the refractometer.

How does it work?: The refractometer works by the use of a glass hemicylinder inside that has a flat surface extending into the body of the refractometer as shown above. When light is introduced into the back of the unit, it travel through the glass hemicylinder and up to the gemstone, where the stone is in optical contact with the glass by use of a refractive index liquid. This liquid keeps air from between the stone and the glass hemicylinder and allows light to travel at the same speed through the glass, gets refracted by the gemstone, and then back through the glass hemicylinder to the viewing lens. The amount of slowing down or bending of the light that the gemstone does to the light beam, causes a light and dark area joined by a thin green line as shown below. This green line is the refractive index reading of the gemstone.

How do I use it to get a refractive index? You must use caution when placing a gemstone on the glass hemicylinder because the glass is very soft and will scratch easily. Once scratched it is very difficult to get it repaired. Here is the method you should use:

N Warning!: The refractive index liquid used in a refractometer is highly toxic and should not be used in a closed room. The liquid should be washed off immediately if you get is on your skin.

Once you have the stone in place...move your light source, such as a pen light...to the back of the refractometer and focus the beam inside the back until you can see light inside the view lens. then......

Some stones are single refractive and will give only one reading. But most will be double refractive and give two or more readings. Here is who you determine if a stone is single or double refractive using the refractometer:

N Remember: The glass used in the refractometer's hemicylinder is very soft and will scratch easily. It is very expensive to polish or replace. So take great care when sliding stones onto or off of it, and take equal care when cleaning the residual RI liquid off of the glass surface. This is IMPORTANT!

Spot Method of Testing Cabochon Cut Gemstones

You will invariably come across gemstones with no nice tables to test on your refractometer, but will instead have a rounded top cabochon. For this you can use your refractometer also. Just place the tiniest spot of RI liquid on your metal base plate and gently slide the cabochon over the liquid and onto your glass hemicylinder. Then, move your head away from the viewing lens and slide your head up and down to be able to see the small dot of a reading you will have on your scale. It will not be a nice, even dark to light and very distinct line like faceted stones. But will rather be a small spot showing up on your reading scale. You need to move your head up and down to see the dot turn from dark to light and back, and then find a place where the spot is half light and half dark. This will be your approximate refractive index reading for the gemstone. This is called the spot reading method for testing. Here are some examples:

Here is what you will see as you move your head up and down...sort of like bobbing for apples but instead you are bobbing for an RI reading.....OK bad joke, I know. But its late as I write this part....

This is what you are looking for. The spot at the far left is totally dark and is below the true RI of this gemstones. The reading is between 1.65 and 1.70. In the middle is a half dark/half light circle with a faint green line in between at approximately 1.74-1.75. At the right is a circle at the point of the arrow that is filled with light. This is too high and above the proper RI of the stone. The spot method reading for this gemstone is 1.74-1.75. Can you guess what it is? Yes, chrysoberyl. This is the cat's eye chrysoberyl seen one the Chrysoberyl page here in The Gemstones section of YourGemologist.com

For those who have requested that the comparison of YourGemologist Refractometer and the Duplex II refractometer be placed back for discussion, here it is:

The Ultimate Test for YourGemologist Refractometer, or

How to make a sodiumD light source when the electricity has gone out.....

Below we are going to show that our YourGemologist Refractometer is the handiest refractometer to have on the road. Because what can you do when the lights go out and you have no batteries for your hand torch? Well, use a candle with some table salt. And there is no need to buy one of the expensive sodiumD light sources from the big gemological equipment companies, you can make your own using the same table salt and a candle.

Here's how it all works: Table salt is sodium chloride. So by putting some table salt in your candle you will be burning the sodium in the salt thereby created the wavelength's of a sodium light source. May not be near as good of a scientific explanation as it could be...but it works so who cares.

Take a small candle and some regular table salt

Next, place the candle behind YourGemologist Refractometer like this:

Just be careful not to get too close to the flame...now...

Drop your salt into the flame and prepare to test your stones...

Here is what I got using some of the refractometer units sent in to the office for testing, please note that using a candle you can see quite nicely. However, trying to take digital photographs with a candle is another story. So while the readings may seem a bit fuzzy they were very clear using this homemade sodiumD light when viewing in person.....

Tourmaline

Peridot

Ruby...showing the 1.76 to 1.77 readings.

With the initial problems we had with the refractive index fluid, I thought it might be nice to prove to everyone that the RI liquid was indeed the problem. And that YourGemologist Refractometer could hold its own....even by candle light. In fact, these photographs were taken with two of the units sent in for testing...just to prove this point. And all were taken by candle light with salt to make a sodiumD light source.

My wife thought I was totally nuts for trying this.....

This is the basic use of the refractometer. You will find all of the refractive index numbers for the various gemstones in the The Gemstones section of YourGemologist.com.

I prefer not to have to mess up the photographs with a bunch of copyright notices so please respect the copyright for the sake of everyone else who visits this site.

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