Magnetism in Gemstones
An Effective Tool and Method for Gem Identification
Almandine Garnet Absorption Spectrum (image courtesy of John S. Harris)
Hoover Magnetic Susceptibility Balance
(assembled by Kirk Feral)
© Kirk Feral 2009, All Rights Reserved. These materials may be duplicated for educational purposes only. No part of this website may be duplicated or distributed for profit, for commercial purposes, or for posting to another website without the expressed written consent of the copyright holder.
Let's Keep it Simple
Theoretical and technical details about magnetism can quickly become overwhelming, but gem testing with a hand-held magnet is simple. We’re just observing gemstone responses. Knowing which types of metals are present in a gem, and which valence states, is not necessary for gem identification. Anyone can enjoy using magnets to separate and identify gems without understanding gemstone chemistry or the physics of magnetic responses.
Although precise quantitative measurements of magnetism requires a Hoover balance, a hand-held magnetic wand is actually a more sensitive instrument. This is because a magnetic wand uses a much larger magnet with a greater total pulling force. Very weak levels of magnetism that cannot be detected by the tiny magnets used in a Hoover balance can be detected with a magnetic wand when we use the Floatation Method (see page 2 of How to Use a Magnet). The magnetic wand is a reliable tool for determining the approximate level of magnetic susceptibility of any gem, which is all we need for basic gem identification. The responses we observe (inert, weak, moderate, strong, drag, pick-up) provide a wealth of information for separating and identifying gemstones.
Spectroscopy in Relation to Magnetism
Spectroscopy and the study of magnetism in gemstones are 2 sides of the same coin. Hand-held spectroscopes used for gem identification detect the presence of metal ions that cause color, just as magnets do. The bands of absorption seen with a spectroscope are indicators of exactly the same metal ions that cause magnetic attraction. The locations of the black absorption bands along the light spectrum provide clues about which particular metals are present within a gem.
In many cases, gems that show prominent absorption patterns also show a significant degree of magnetic attraction. As you might expect, gemstones that show no visible absorption pattern also tend to show little or no attraction to a magnet. The strong absorption spectrum shown below is due to high iron content in Almandine Garnet.
Quantitative Measurements of Magnetism
In addition to magnetic response, we can measure the precise degree of magnetic attraction that a gemstone has. We find that many gemstones have ranges of magnetic susceptibility that are characteristic for a particular gem species or variety, just as gems have characteristic ranges for refractive index, specific gravity, fluorescence and absorption spectra. For example, synthetic Ruby has a magnetic response range of Inert to Moderate and a magnetic susceptibility (SI) range of SI <0-87 (from less than zero to 87), while Almandine Garnet shows a Pick-up response and has a magnetic susceptibility range of SI 1926-3094.
We measure magnetic susceptibility (SI) with a Hoover Magnetic Susceptibility Balance, a new instrument developed by geophysicist and gemologist Dr. D.B Hoover, along with Bear Williams (see the 2007 article Magnetic Susceptibility for Gemstone Discrimination). This instrument, which integrates a compound microscope base with a digital gem scale, simply measures the loss of weight of a gem as it is pulled upward from a digital scale by the magnetic field of a small neodymium magnet. Quantitative measurements are recorded as SI (International System of Units) X 10(-6), which represents the degree of magnetism in terms of the ratio of the induced magnetic field of the gem to the inducing field of the magnet. The SI ranges for most gems are listed in Kirk Feral's Magnetic Susceptibility Index for Gemstones.
With a spectroscope, we are better able to detect metals that are strong chromophores such as chromium (Cr3+) and cobalt (Co2+) than we can with a magnetic wand. But a magnet allows us to detect weak chromophores such as iron (Fe3+) and manganese (Mn2+) at lower concentrations than are detectable with a spectroscope. Without our magnetic wand, more sophisticated instrumentation such as a spectrometer are required to detect low levels of iron and manganese. You can find more information about spectroscopy in the book A Student's Guide to Spectroscopy by Colin Winter (2003) OPLspectra.com, The Spectroscope and Gemmology by Basil Anderson, and at John Harris' gem spectroscopy website gemlab.co.uk.
Even though the concept of the Hoover balance was published in 2008, the advantages of this important instrument have not yet caught on, and no such balances are currently in use in gem research labs or student training facilities anywhere. The balance is not commercially available, but anyone with a little ingenuity can learn to assemble and use a Hoover balance by referring to the descriptive article in the Journal of Gemology titled Magnetic Susceptibility: A Better Approach to Defining Garnets authored by Hoover, Williams and associates at Stone Group Laboratories.
Hand-held OPL Teaching Spectroscope
Only manganese is responsible for the strong magnetism of this gem, but magnetic testing alone cannot reveal this. Magnetic susceptibility only reflects the collective magnetism of all the metal ions within the gem. A hand-held spectroscope can help us identify which specific metals are present. An electronic spectrometer is a more sensitive instrument that permits a more detailed analysis of the metallic components, their relative concentrations and even their valence states.
In our lab, a UV-Vis-NIR spectrometer (Ultraviolet, Visible spectrum, and Near-Infrared) helps us identify specifically which metals within a gem are responsible for the color and/or magnetism. When we look at an absorption spectrum represented as a spectrometer graph like the one below, we can see that the peaks on the graph correspond to the black lines and bands seen with a spectroscope. These peaks represent the wavelengths or areas of the light spectrum that are absorbed by specific metal ions. Gems that do not contain paramagnetic metals show no peaks. Manganese and copper are clearly indicated in the graph below for green Paraiba Tourmaline. We have determined that the low concentrations of copper ions (Cu2+) found in Paraiba Tourmalines are not magnetically detectable.
A Magnet is Lowered
to the Table Facet of a Gem
The Hoover balance is the only instrument that allows us to take precise quantitative measurements of magnetism in faceted gemstones. Magnetometers such as CGS meters and Kappa meters used today in mineralogical research to measure paramagnetism in soils, rocks and minerals are more sensitive than the Hoover balance. But magnetometers are not suitable for testing gemstones because the varying shapes of faceted gems affect magnetic readings. Cut gems and even rough crystals would have to be crushed or ground into powder in order for accurate measurements to be taken.
Fortunately, the the ingenious design of the Hoover Balance makes gem shape irrelevant as long as the surface of the magnet pole can fit within the surface area of a polished facet or flat gem surface. Any stone with a smooth flat surface that is 2mm or more in diameter can be measured for magnetic susceptibility, providing the weight of the stone does not exceed the capacity of the digital scale.
Excerpt from the Magnetic Susceptibility Index Showing Magnetic Response
Ranges and Magnetic Susceptibility (SI) Ranges for Gemstones
Click this link to continue on: How to Use a Magnet for Gem Identification
Use the Magnetic Susceptibility Index:
Every gem species and variety has a characteristic response or range of magnetic response listed on the Magnetic Susceptibility Index, and the response we observe when we apply a magnetic wand to a gem can be checked against that Index to help us identify the gem.
That's all there is to it. Once you start using magnetic testing routinely, you'll remember what magnetic responses to expect from most major gemstones without having to refer to the Index.
You may find it helpful to refer to the SI column on the Index to get a better sense of the differences in the degree of magnetism shown by different types of gemstones. Because SI (magnetic susceptibility) measurements are quantitative, differences between gem species and varieties can be seen more precisely in this column.
As an alternative to the Index on this website, those who own the Gemology Tools Professional (GTPro) software program can find magnetic responses listed on the Gemstone Database. GTPro incorporates all the magnetic responses listed in our Index, and regularly adds any updates. The 2014 or later online edition of the Handbook of Gemmology is another teaching aid/reference book that includes information about magnetic testing derived from our website gemstonemagnetism.com. The Handbook incorporates portions of our Magnetic Susceptibility Index on Appendix K, including magnetic response ranges and magnetic susceptibility (SI) ranges.
It is also of interest that the geographic origin or geologic conditions of some gemstones such as Sapphire and Peridot can in some cases be inferred by measuring the magnetic susceptibility. Different locations can be associated with different ranges of magnetic susceptibility.
Testing with a Magnetic Wand
Green Paraiba Tourmaline (Mozambique)
How Magnetic Are Gems?
To illustrate how magnetic allochromatic gems are relative to each other, we constructed the graph shown below. The bar graph shows the highest SI values measured for 20 transparent allochromatic gemstones colored by metallic impurities. Gem types are presented from least magnetic (Andalusite, SI 26) to most magnetic (yellow Tourmaline, SI 512) allochromatic gem. The height of each bar represents the maximum magnetic susceptibility measured for each species or color variety. Garnets and other idiochromatic gems are not included in this graph.
Comparing Magnetic Susceptibilities of 20 Allochromatic Gemstones
Often there are two or more metallic chromophores mixed within a single gem, but a magnet cannot be used to distinguish between them. For example, Paraiba Tourmalines such as the green gem from Mozambique shown below often contain both copper and manganese impurities. The color of this copper-bearing gem is due to copper ions (Cu2+, blue color) mixing with manganese ions (Mn2+, yellow color) to create green color (blue + yellow = green). This Tourmaline shows a very strong magnetic response. Is the strong magnetism due to a high concentration of paramagnetic copper ions, or is it primarily due to the presence of manganese? Could iron (Fe2+) or even a rare earth metal also contribute to the green color and strong magnetic response of this gem?
There is much to learn through the study of gemstone paramagnetism in relation to spectroscopy, but this is an area of gemological research that has yet to be pursued by gemologists. You can find an online spectrometry database showing absorption graphs for many gems at Dr. George Rossman's Spectroscopy Database for Minerals.
Absorption Spectrum Showing Manganese and Copper in a Green Paraiba Tourmaline
As a practical example of the usefulness of quantitative measurements using a Hoover Balance, any Garnet gem can be identified at the species level by simply measuring its magnetic susceptibility. This is a more precise identification than what is possible with a spectroscope.
If we add refractive index measurements to these magnetic susceptibility measurements, we can go much further with the identification of Garnet and determine the major chemical composition for individual gems. A single Garnet gem can contain two or more paramagnetic metals. We can use the RIMS method (Refractive Index, Magnetic Susceptibility) developed by Hoover to infer what percentage of the total magnetic susceptibility is caused by one metal relative to another i.e. how much of the magnetism is due to divalent iron (Fe2+), how much to trivalent iron (Fe3+), and how much to manganese (Mn2+).
The RIMS method of plotting refractive index and magnetic susceptibility measurements of Garnets onto a graph also allows us to identify gems at the variety level, not just the species level, and also provides a means to estimate the composition of an individual Garnet in terms of percentages of the 3 primary end-member species within the gem. See the Garnet section for more detailed information about how this method can be used to identify Garnets and determine Garnet composition.
LWUV Fluorescence: Strong due to Manganese
Magnetic Response: Inert
LWUV Fluorecence: Moderate due to Rare Earth Metals
Magnetic Response: Weak due to Rare Earth Metals
Iron (Fe2+ and Fe3+) can quench fluorescence when concentrations are above trace amounts (more than 0.1%). Natural Ruby provides a good example of fluorescence-quenching. Synthetic Ruby is practically iron-free and always fluoresces bright red due to a high concentration of chromium, which can also cause weak magnetic attraction. The intensity of fluorescence in synthetic Ruby can be orders of magnitude greater than in natural Ruby. Chromium is also the chromophore and activator in natural Ruby. Fluorescence can be strong in natural Ruby, but some gems show only weak or moderate fluorescence, and others can appear inert to long wave UV light.
Weaker fluorescence in natural Rubies occurs not only becasue natural Rubies often contain less chromiun than synthetics, but also because they contain a significant amount of iron (Fe3+), which can quench fluorescence. For the same reasons, synthetic Emeralds that are colored primarily by chromium (rather than by vanadium) tend to be more fluorescent than natural Emeralds colored primarily by chromium. It follows that synthetic Rubies are on average less magnetic than natural Rubies. Increasing concentrations of iron in natural Rubies (and in natural Emeralds colored by chromium) is associated with greater magnetism and lower fluorescence.
LWUV Fluoresence: Strong due to Chromium
Magnetic Response: Weak due to Chromium
LWUV Fluorescence: Inert
Magnetic Response: Moderate due to Iron & Chromium
Chrome Pyrope Garnet gems are dark red and can contain serveral times more chromium (4%-8% chromium oxide by weight) than the reddest Rubies, but Chrome Pyrope gems do not fluoresce. This is primarily because Chrome Pyrope has much higher iron content than Ruby. The amount of fluorescence-quenching iron (Fe2+) in these Garnets typically causes a Drag response to a magnetic wand. The lack of fluorescence in Chrome Pyrope might also be partly due to a phenomenon known as concentration quenching, which in this case involves an over-abundance of chromium. A high saturation of an activator that normally causes fluorescence can in some cases reduce or eliminate the fluorescence (Roberts, Manuel, 1994).
A good example of concentration quenching is found in Uvarovite Garnet, which is an idiochromatic gem colored dark green by high chromium content. Whether in druse form or as large individual crystals, Uvarovite does not fluoresce. This Garnet species is quite magnetic, but not due to iron or manganese. Large individual Uvarovite crystals show a Drag response to a magnetic wand due entirely to an exceptionally high concentration of chromium (up to 27% chromium oxide by weight). This level of chromium inhibits all fluorescence. Concentration quenching may also be the reason manganese (Mn2+) does not cause fluorescence in idiochromatic gems such as Rhodochrosite and Rhodonite, which both show a Pick-up magnetic response due to high manganese.
Fluorescence in Relation to Magnetism
Some gems fluoresce or emit a glow of color when ultraviolet light is applied. Often these fluorescent colors are quite striking. To examine fluorescence, we use a long wave ultraviolet (LWUV) flashlight (365nm-375nm wavelength) or a blue laser (405nm), and short wave ultraviolet (SWUV) lamp (254nm). We also use photoluminescence spectroscopy to help us determine the causes of fluorescence in individual gems. With one exception, all photos of fluorescence shown below were taken under long wave UV light.
Our studies reveal that fluorescence is almost always restricted to allochromatic gemstones, which are less magnetic than idiochromatic gems. The primary cause of the fluorescence is metal ion impurities. Fluorescence often involves the same paramagnetic transition metals and rare earth metals that can cause magnetic attraction, absorption spectra. Chelsea filter reactions and gem color. When these metals cause fluorescence, they are referred to as activators.
Metallic activators are often found only in trace amounts (less than 0.1% by weight) within gems, and although they can often be detected via fluorescence, concentrations are usually too low to induce any visible magnetic attraction when the flotation method is used. As examples, the light green Columbian Emerald and dark green Chrome Tourmaline pictured below both show green body color and pink to red fluorescence due to chromium. Because both gemstones have a low concentration of chromium (likely less than 0.4%) and contain no magnetically detectable iron (under 0.1%), they are magnetically inert.
Cobalt in trace amounts creates blue daylight color and pink fluorescence in synthetic blue Spinel, which is magnetically inert. Cobalt, along with iron, also contributes to blue body color in natural blue Spinels, and can create pink/red fluorescence in rare gems that contain little iron and a relatively high proportion of cobalt. All natural blue Spinel gems are weakly to strongly magnetic due to iron, while the cobalt component is very rarely magneticaly detectable. The rare Cobalt Spinel from Viet Nam pictured below derives its pure blue body color and strong pink fluorescence from cobalt. This gem shows weak magnetic attraction when floated, likely due to a small amont of the cobalt activator in conjunction with a small amount of iron.
LWUV Fluorescence: Strong due to Chromium
Magnetic Response: Moderate due to Iron
LWUV Fluorescence: Inert
Magnetic Response: Large Crystals Drag
Chrome Pyrope Garnet
LWUV Fluorescence: Inert
Magnetic Response: Drags
LWUV Fluorescence: Strong due to Iron (Fe3+)
Magnetic Response: Inert
The activator manganese (Mn2+) can cause orangey pink fluorescence in colorless Spodumene and pink Spodumene, also known as Kunzite. Both are diamagnetic. Manganese (Mn2+) can also create yellow/green fluorescence in synthetic Spinel. The blue-green synthetic Spinel pictured below (right) fluoresces green and is moderately magnetic due entirely to a high concentration of the activator manganese. A combination of manganese and cobalt serve as dopants in this gem. Magnanese contributes the yellow color component, while cobalt contributes the blue component, resulting in the blue-green or "mint" daylight color shown below.
Synthetic Green Spinel
LWUV Fluorescence: Strong due to Manganese
Magnetic Response: Moderate due to Manganese
LWUV Fluorescence: Strong due to Chromium Magnetic Response: Inert
The most common activator that causes fluorescence in gems is chromium, which induces pink to red fluorescence under long wave UV light. As a coloring agent (chromophore), chromium can produce either red or green color under visible light. Chromium is highly fluorescent, and slight gem fluorescence can occur even in daylight. The relatively high concentrations of chromium found in some high-chromium Ruby and Emerald gems can induce weak magnetic attraction.
But the concentrations of chromium and other activators such as magnanese in gemstones are usually lower than what can be detected with a magnet, and often the concentrations are so low that the activators/chromophoes produce no gem color in visible light. As an example, in a number of allochromatic gemstones, chromium ions can cause bright pink fluorescence under LWUV light without producing any red or green gem color in daylight. Two examples are colorless Sapphire, which is diamagnetic (inert), and yellow Sapphire, which is weakly to moderately magnetic due to iron (Fe3+).
Fluorescence: Strong due to Chromium
Magnetic Response: Inert
Various rare earth metals are responsible for fluorescence in a number of gems, as in the blue Apatite pictured below (left), which fluoresces pink. Blue Apatite is inert to weakly magnetic due to these rare earth activators. On rare occasions, uranium acts as an activator in gems, causing green fluorescence, as it does in some Opals. Uranium is quite magnetic, but due to very low concentrations of this activator, uranium Opals are magnetically inert. Even iron can be an activator of fluorescence (as per the Online Database of Fluorescent Minerals). Iron (Fe3+) in trace amounts induces pink fluorescence in yellow Scapolite, which is a magnetically inert mineral that derives its yellow body color from color centers.
LWUV Fluorescence: Strong due to Cobalt
Magnetic Response: Weak due to Cobalt & Iron
Fluorescence: Strong due to Chromium
Magnetic Response: Inert
At times, more than one activator may be responsible for fluorescence within a single gem, as we already demonstrated with synthetic Spinel. As another example, the weakly magnetic colorless “Leuco” Grossular Garnet pictured below fluoresces orangey pink due to a combination of chromium and manganese. Chromium acts as an activator and not a chromophore in this natural gemstone, causing pink fluorescence under long wave UV light. The manganese activator reveals itself as orange fluorescence when the gem is viewed with a blue laser, and as yellowish green fluorescence under short wave UV light.
Both chromium and manganese are present in concentrations too low to contribute to the weak magnetic attraction of this gem. The very low magnetic susceptibility we detect with a magnetic wand is due entirely to a small amount of cryptic iron (Fe3+), which is neither an activator nor a chromophore.
Fluorescence in the colorless Grossular Garnet above can easily be analyzed with a spectrometer when we use a UV light source rather than incandescent light. This is called photoluminescent spectroscopy, and it can help us identify which metal activators are causing fluorescence. With this type of spectroscopy, we examine the light that is transmitted by activators rather than the light that is absorbed by chromophores. On the transmission spectrum graph below for the colorless Grossular Garnet, the long wave UV transmission peaks of manganese in the yellow/orange portion of the light spectrum, as well as peaks indicating chromium transmission in the red portion of the spectrum, are clearly evident.
Daylight LWUV Fluorescence Blue Laser Fluorescence SWUV Fluorescence
due to Chromium due to Manganese due to Manganese
Colorless Grossular Garnet
Photoluminescent Transmission Spectrum for Colorless Grossular Garnet