Mineral Magnetism: The Forgotten Property

Zandrite is a synthetic glass with color change properties. It can be manufactured to show different color changes, even green to red, like alexandrite. However, among other differences in optical and physical properties, zandrite has a moderate to strong magnetic reaction, while alexandrite is inert. 7.80-ct zandrite, 14 mm round. Photos courtesy of liveauctioneers.com and Advantage Auction Company.

A Confirmative Test, Not a Determinative Test

Some time ago, I became interested in gemstone magnetism. While researching this property, I found that some gemology schools had little to say on the subject. (Many gemology books fail to list magnetic attraction as a gemstone property at all). Most think magnetism testing isn't reliable enough to use with any success in gemstone identification. I disagree with that position. However, I think this property should be used as a confirmative test only, not a determinative one.

Gemstone magnetism is a very complex subject, so I decided to test this property in the simplest of ways in order to find an easy way to measure it. I hope someday there will be an accurate and easy way to measure magnetic susceptibility or magnetic characteristics in gemstones. Until then, we'll have to use what's available to us as front-line gemologists. I also hope more research will be conducted on this subject.

Equipment for Testing Gemstone Magnetism

I was unable to find any instruments designed to measure the magnetic properties of gems. I did find that Dr. D.B. Hoover, FGA, FGAA (Hon), and Bear Williams, CG, had researched and actually put together an instrument that was able to measure the attraction of gemstones with a magnet and scale. They presented their findings and described the instrument in their 2007 paper, Magnetic Susceptibility for Gemstone Discrimination. While they had excellent results with their setup, I found it complicated and difficult to use outside the lab.

Sylvia M. Gumpesberger had tested the magnetic attraction of gemstones using a string and magnet as well as other methods, including the "direct method" of pulling stones across a low-friction surface with a magnet. In 2006, she published a paper, Magnetic Separation of Gemstones, which included a table of results using the direct method. The tested gems were listed as responsive to the magnet, non-responsive, or both. No information was provided regarding strength or a way to measure it.

Since I couldn't find any gemological equipment for testing gemstone magnetism, I decided to try Gumpesberger's methods. I began to test and experiment with stones and recorded the information. The results of these tests are listed below in the Gemstone Magnetism Reaction Table.

A Quick Gem Separation

Gumpesberger's methods aren't the most scientific way to conduct tests, but in many cases I've found these simple tests will help make fast and fairly accurate identifications of some faceted gems and rough. They can also be used to differentiate between similar looking gemstones.

Take red garnet, ruby, and red spinel, for example. While these gems can look nearly identical, they can quickly be separated with a simple magnet test that can be performed just about anywhere, including gem shows and pawn shops.

Types of Magnetic Behavior

Two types of magnetic behaviors interest gemologists. The first is a gemstone that is attracted to a magnet. The second is a gemstone that is a natural magnet, one that attracts iron to it. Lodestone, for example, is a natural magnet with very strong magnetism.

In this article, I refer to the first type, those stones attracted or repulsed by a magnet. I have only tested loose gemstones. I found that my test results didn't always agree with the results of other researchers. This would seemingly support the argument that magnetism is an unreliable property for testing. However, I still believe it can help identify gemstones. The different results may be due to non-standardized equipment.

Notes on Magnets and Gems

I used the string-and-magnet method for my testing. I found that different types, sizes, and strengths of magnets and different types and sizes of string gave different results with this test.

Neodymium Cylinder Magnets

After researching and testing many different magnets, I found that the very small neodymium cylinder magnets, NdFeB, grade N42 with a plating/coating of Ni-Cu-Ni (nickel), gave the most accurate results. These are also listed as the world's strongest magnets.

Different Magnet Sizes for Different Gem Sizes

From reading Hoover and Williams, I found that the magnet needed to be smaller than the gemstone table to get accurate readings. This allows you to actually get the surface of the gemstone in the magnetic field. Instead of using one large, strong magnet, I gathered different size magnets and used the one that would best fit inside the stone's table. This would make the trials more accurate and consistent. I recommend that other researchers use the same type string, magnet, and wax as I did.

Inclusions

Keep in mind that inclusions can play a part in the final results. Large inclusions can react to the magnet, which could lead to a false test result.

Gem Sizes and Cuts

I have tested thousands of gemstones with this method. These gemstones came from many different dealers and locations. Many of the gemstones were over 70 years old. I tried to use the same cut and size stone when possible. However, because of the limited availability of gemstones, I used whatever cut, species, and variety I had available to test. I found this one of the easiest and fastest tests to perform on a gemstone. I have tested stones as small as 3 mm and stones up to 10 carats with this method as well, so it works on a wide range of stone sizes.

How to Build Your Gemstone Magnetism Equipment

Here you'll find the instructions and list of materials you'll need to construct the simple test instrument shown below.

Materials Needed

• 5" to 6" length of extremely thin Coats & Clark transparent nylon thread #D.47.
• Neodymium cylinder magnets, NdFeB, grade N42 extra strong magnets in the following sizes: 1/16″ diameter x ¼" thick, 1/8″ diameter x ½" thick, 3/16″ diameter x  ½" thick, ¼" diameter x ½" thick. (I have found that the 1/8″ and ¼" size magnets will handle most of my tests).
• Clear dental wax, which can be purchased at many drug stores or online.

Building Instructions

Take a 1/8″ pinch of the wax and press it around the end of the nylon thread. Attach the other end of the sewing thread to the top of your desk or table and let it hang freely down 6". I like to use a third hand and leave the thread attached. I remove the wax after testing to keep it clean and save it for future tests.

How to Test for Gemstone Magnetism

This test will show the reaction, or lack thereof, of a gemstone to a magnet without the need for a lab or sensitive equipment. You can then grade the type of reaction. When used along with other gemological tests, this technique can help you properly identify a gemstone.

As in any gemological test, always clean the stone and check it with a loupe before testing. Highly included stones are suspect.

• Take a stone and attach the stone's girdle to the wax. (Make sure your hands and the stone are clean before handling the wax. Dirt and other debris can get in the wax and this can cause an attraction to the magnet. If this happens, replace the wax).
• Hang the stone so that its longest dimension is vertical.
• Let the thread settle and stop moving. Make sure there's no wind moving the gemstone.
• Use a magnet that will fit inside the table and slowly move it towards the stone's table with the face of the magnet parallel to the stone's table. (Don't let the magnet touch the stone or sewing thread).
• Watch the movement of the stone as you approach it with the magnet. (Stones with a very high attraction to the magnet will move toward the magnet as it gets within ¼" of the stone. You may need to bring the magnet to within 1/16″ of the stone to get a reaction from weaker stones).

Magnetic Reactions

Possible reactions include:

• Attraction, also known as paramagnetism.
• Repulsion, also known as diamagnetism. Some gemstones are repulsed by the magnet.

For grading purposes, I used the same measurements of movement as the gemstones that are attracted to the magnet.

Test Gems with Notable Magnetic Reactions

First, try testing one of the stones that has more attraction to the magnet to see what distinct reactions look like. Good gems to try include almandine-pyrope garnet, spessartite garnet, or peridot. Next, test emerald or amethyst. These stones are inert, so you shouldn't see any movement when you introduce a magnet.

Grading Gemstone Reactions To Magnetism

Grade the movement by the scale above and record it. The more you use the test, the better feel you'll get for the type of movements you'll observe, which will improve your accuracy.

Keep Your Testing Consistent

For the most accurate and consistent readings, always use the same type and length thread and a magnet that fits inside the stone's table.

It's very important to check the wax with the magnet while it's hanging on the string to make sure it hasn't picked up anything that could attract the magnet and cause an inaccurate reading. To test it, attach a stone that you know is inert. If you get a reaction, replace the wax. If the result is questionable, try the test again using another inert test stone.

I recommend that you keep a gemstone that isn't attracted to the magnet as a test stone. A 6 mm round, colorless, cubic zirconia works well.

Gemstone Magnetism Reaction Table

Notes

These are the results of my own tests. If a gemstone isn't listed above, I haven't tested it.

Synthetic gemstones may show different reactions due to different dopants used for colors.

Keep in mind that the attraction of a gemstone to a magnet isn't exact. You may find that some stones will have a different reaction from those listed. I encourage others who use this test to share the results of the different species and varieties they analyze.

Further Research

I believe that we, as gemologists, need to keep an open mind and use every tool and resource available to help us in our work. It will take every resource we have today to identify new synthetics and enhancements, so the more tests we have in our arsenal, the better prepared we'll be. Try the test. I feel you'll find it very helpful.

One way to make gemstone magnetism testing more reliable is through equipment standardization. Currently, the magnets described in this article have approximately 11 different strengths commonly available. There are also many different brands and types of thread and wax. Without standardization of these items, results may vary dramatically from one tester to another, as well from my results.

I accept corrections, clarifications, and constructive criticisms that help improve the accuracy of the information in this article.

Additional Reading on Gemstone Magnetism

Editor's note: In 2008, Dr. D.B. Hoover, FGA, FGAA (Hon), Bear Williams, C. Williams, FGA, and C. Mitchell, FGA, published Magnetic susceptibility, a better approaching to defining garnets, which explores using magnetic susceptibility to identify the chemical composition of garnets.

A 2011 article by T. Pradat and J. P. Gauthier also concluded that gemstone magnetism testing with neodymium magnets can help make quick separations. However, standard tests are still needed for positive identifications. The authors also compiled a table of gemstones and reactions. You can read the entire article (in the original French or through Google Translate).

The "Magnetism in Gemstones" website by Kirk Feral contains a great deal of information on magnetic reactions as well as a Magnetic Susceptibility Index.

Health Warning

Keep neodymium magnets away from people with pacemakers or similar medical aids. Strong magnetic fields can affect their operation. Keep these magnets away from credit cards and computers. Keep these magnets away from children, as serious health problems may occur if they swallow them. Strong magnets can cause injury, pinch fingers, or break if they slam together. Always exercise caution when using them.

Richard C. Massingill, PG MG

View All Articles